Patent Publication Number: US-2019186366-A1

Title: Jet Engine with Fuel Injection Using a Dielectric of a Resonator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application hereby incorporates by reference U.S. Pat. Nos. 5,361,737; 7,721,697; 8,783,220; 8,887,683; 9,551,315; 9,624,898; and 9,638,157. The present application also hereby incorporates by reference U.S. Patent Application Pub. Nos. 2009/0194051; 2011/0146607; 2011/0175691; 2014/0283780; 2014/0283781; 2014/0327357; 2015/0287574; 2017/0082083; 2017/0085060; 2017/0175697; and 2017/0175698. In addition, the present application hereby incorporates by reference International Patent Application Pub. Nos. WO 2011/112786; WO 2011/127298; WO 2015/157294; and WO 2015/176073. Further, the present application hereby incorporates by reference the following U.S. Patent Applications, each filed on the same date as the present application: “Plasma-Distributing Structure in a Resonator System” (identified by attorney docket number 17-1501); “Magnetic Direction of a Plasma Corona Provided Proximate to a Resonator” (identified by attorney docket number 17-1502); “Fuel Injection Using a Dielectric of a Resonator” (identified by attorney docket number 17-1505); “Jet Engine Including Resonator-based Diagnostics” (identified by attorney docket number 17-1506); “Power-generation Turbine Including Resonator-based Diagnostics” (identified by attorney docket number 17-1507); “Electromagnetic Wave Modification of Fuel in a Jet Engine” (identified by attorney docket number 17-1508); “Electromagnetic Wave Modification of Fuel in a Power-generation Turbine” (identified by attorney docket number 17-1509); “Jet Engine with Plasma-assisted Combustion” (identified by attorney docket number 17-1510); “Jet Engine with Fuel Injection Using a Conductor of a Resonator” (identified by attorney docket number 17-1511); “Jet Engine with Fuel Injection Using a Conductor of At Least One of Multiple Resonators” (identified by attorney docket number 17-1513); “Jet Engine with Fuel Injection Using a Dielectric of At Least One of Multiple Resonators” (identified by attorney docket number 17-1514); “Plasma-Distributing Structure in a Jet Engine” (identified by attorney docket number 17-1515); “Power-generation Gas Turbine with Plasma-assisted Combustion” (identified by attorney docket number 17-1516); “Power-generation Gas Turbine with Fuel Injection Using a Conductor of a Resonator” (identified by attorney docket number 17-1517); “Power-generation Gas Turbine with Fuel Injection Using a Dielectric of a Resonator” (identified by attorney docket number 17-1518); “Power-generation Gas Turbine with Plasma-assisted Combustion Using Multiple Resonators” (identified by attorney docket number 17-1519); “Power-generation Gas Turbine with Fuel Injection Using a Conductor of At Least One of Multiple Resonators” (identified by attorney docket number 17-1520); “Power-generation Gas Turbine with Fuel Injection Using a Dielectric of At Least One of Multiple Resonators” (identified by attorney docket number 17-1521); “Plasma-Distributing Structure in a Power Generation Turbine” (identified by attorney docket number 17-1522); “Jet Engine with Plasma-assisted Combustion and Directed Flame Path” (identified by attorney docket number 17-1523); “Jet Engine with Plasma-assisted Combustion Using Multiple Resonators and a Directed Flame Path” (identified by attorney docket number 17-1524); “Plasma-Distributing Structure and Directed Flame Path in a Jet Engine” (identified by attorney docket number 17-1525); “Power-generation Gas Turbine with Plasma-assisted Combustion and Directed Flame Path” (identified by attorney docket number 17-1526); “Power-generation Gas Turbine with Plasma-assisted Combustion Using Multiple Resonators and a Directed Flame Path” (identified by attorney docket number 17-1527); “Plasma-Distributing Structure and Directed Flame Path in a Power Generation Turbine” (identified by attorney docket number 17-1528); “Jet engine with plasma-assisted afterburner” (identified by attorney docket number 17-1529); “Jet engine with plasma-assisted afterburner having Resonator with Fuel Conduit” (identified by attorney docket number 17-1530); “Jet engine with plasma-assisted afterburner having Resonator with Fuel Conduit in Dielectric” (identified by attorney docket number 17-1531); “Jet engine with plasma-assisted afterburner having Ring of Resonators” (identified by attorney docket number 17-1532); “Jet engine with plasma-assisted afterburner having Ring of Resonators and Resonator with Fuel Conduit” (identified by attorney docket number 17-1533); “Jet engine with plasma-assisted afterburner having Ring of Resonators and Resonator with Fuel Conduit in Dielectric” (identified by attorney docket number 17-1534); and “Plasma-Distributing Structure in an Afterburner of a Jet Engine” (identified by attorney docket number 17-1535). 
    
    
     BACKGROUND 
     Resonators are devices and/or systems that can produce a large response for a given input when excited at a resonance frequency. Resonators are used in various applications, including acoustics, optics, photonics, electromagnetics, chemistry, particle physics, etc. For example, electromagnetic resonators can be used as antennas or as energy transmission devices. Further, resonators can concentrate a large amount of energy in a relatively small location (for example, as in the electromagnetic waves radiated by a laser). 
     Aircraft, including jets, can be used to transport cargo and/or passengers from one location to another at high velocities. By providing thrust using a jet engine or a propeller, aircraft can generate lift based on Bernoulli&#39;s principle. One way of powering a jet engine or a propeller includes combusting hydrocarbon fuel. 
     SUMMARY 
     In a first implementation, a system is provided. The system includes a combustion chamber of a jet engine. In addition, the system includes a radio-frequency power source and a resonator. The resonator is electromagnetically coupled to the radio-frequency power source and has a resonant wavelength. The resonator includes (i) a first conductor, (ii) a second conductor, and (iii) a dielectric between the first conduct and the second conductor. The resonator is configured such that, when the resonator is excited by the radio-frequency power source with a signal having a wavelength proximate to an odd-integer multiple of one-quarter of the resonant wavelength, the resonator provides at least one of a plasma corona or electromagnetic waves. The system also includes a fuel conduit configured to couple to a fuel source and having a fuel outlet for expelling fuel into a combustion zone of the combustion chamber. A portion of the fuel conduit is arranged proximate to the dielectric. 
     In a second implementation, a method is provided. The method includes providing a plasma corona in a combustion chamber of a jet engine by exciting a resonator with a signal having a wavelength proximate to an odd-integer multiple of one-quarter of a resonant wavelength of the resonator. The resonator includes (i) a first conductor, (ii) a second conductor, and (iii) a dielectric between the first conductor and the second conductor. The method also includes moving fuel from a fuel source into the combustion chamber of the jet engine by way of a fuel conduit such that the plasma corona causes combustion of the fuel. A portion of the fuel conduit is arranged proximate to the dielectric. 
     In a third implementation, a method is provided. The method includes providing electromagnetic waves by exciting a resonator with a signal having a wavelength proximate to an odd-integer multiple of one-quarter of a resonant wavelength of the resonator. The resonator includes (i) a first conductor, (ii) a second conductor, and (iii) a dielectric between the first conductor and the second conductor. The method also includes moving fuel from a fuel source into a combustion chamber of a jet engine by way of a fuel conduit. A portion of the fuel conduit is arranged proximate to the dielectric such that the fuel moving through the fuel conduit is exposed to the electromagnetic waves, thereby pre-treating fuel within the fuel conduit so as to provide pre-treated fuel. 
     Other implementations will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a cross-sectional view of an internal combustion engine. 
         FIG. 1B  illustrates an isometric view of an example quarter-wave coaxial cavity resonator (QWCCR) structure, according to example implementations. 
         FIG. 1C  illustrates a cutaway side view of a QWCCR structure, according to example implementations. 
         FIG. 1D  illustrates a cross-sectional view of a QWCCR structure, according to example implementations. 
         FIG. 1E  is a cross-sectional illustration of an electromagnetic mode in a QWCCR structure, according to example implementations. 
         FIG. 1F  is a cross-sectional illustration of an electromagnetic mode in a QWCCR structure, according to example implementations. 
         FIG. 1G  is a plot of a quarter-wave resonance condition of a QWCCR structure, according to example implementations. 
         FIG. 2  illustrates a system that includes a coaxial resonator, according to example implementations. 
         FIG. 3A  illustrates a system that includes a coaxial resonator, according to example implementations. 
         FIG. 3B  illustrates a system that includes a coaxial resonator, according to example implementations. 
         FIG. 4A  illustrates a system that includes a coaxial resonator, according to example implementations. 
         FIG. 4B  illustrates a controller, according to example implementations. 
         FIG. 5  illustrates a cutaway side view of a QWCCR structure connected to a fuel pump and a fuel tank, according to example implementations. 
         FIG. 6  illustrates a cross-sectional view of an example coaxial resonator connected to a direct-current (DC) power source through an additional resonator assembly acting as a radio-frequency (RF) attenuator, according to example implementations. 
         FIG. 7  illustrates a cross-sectional view of an example coaxial resonator connected to a DC power source through an additional resonator assembly acting as an RF attenuator, according to example implementations. 
         FIG. 8  illustrates an aircraft having a jet engine, according to example implementations. 
         FIG. 9  illustrates a jet engine, according to example implementations. 
         FIG. 10A  illustrates a combustor, according to example implementations. 
         FIG. 10B  illustrates a combustor, according to example implementations. 
         FIG. 10C  illustrates a combustor, according to example implementations. 
         FIG. 10D  illustrates a combustor, according to example implementations. 
         FIG. 10E  illustrates a combustor, according to example implementations. 
         FIG. 10F  illustrates a combustor, according to example implementations. 
         FIG. 11  illustrates a partial view of a combustor, according to example implementations. 
         FIG. 12  illustrates air flow paths through a combustor, according to example implementations. 
         FIG. 13  illustrates a jet engine including an afterburner, according to example implementations. 
         FIG. 14A  illustrates a cutaway side view of a resonator, according to example implementations. 
         FIG. 14B  illustrates a cutaway side view of a resonator, according to example implementations. 
         FIG. 15  illustrates a cross-sectional view of a resonator, according to example implementations. 
         FIG. 16  illustrates multiple cross-sectional view of a resonator, according to example implementations. 
         FIG. 17A  illustrates a cutaway side view of a resonator, according to example implementations. 
         FIG. 17B  illustrates a cross-sectional view of a resonator, according to example implementations. 
         FIG. 18  is a flow chart depicting operations of a representative method, according to example implementations. 
         FIG. 19  is a flow chart depicting operations of a representative method, according to example implementations. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods, devices, and systems are presently disclosed. It should be understood that the word “example” is used in the present disclosure to mean “serving as an instance or illustration.” Any implementation or feature presently disclosed as being an “example” is not necessarily to be construed as preferred or advantageous over other implementations or features. Other implementations can be utilized, and other changes can be made, without departing from the scope of the subject matter presented in the present disclosure. 
     Thus, the example implementations presently disclosed are not meant to be limiting. Components presently disclosed and illustrated in the figures can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated in the present disclosure. 
     Further, unless context suggests otherwise, the features illustrated in each of the figures can be used in combination with one another. Thus, the figures should be generally viewed as components of one or more overall implementations, with the understanding that not all illustrated features are necessary for each implementation. 
     In the context of this disclosure, various terms can refer to locations where, as a result of a particular configuration, and under certain conditions of operation, a voltage component can be measured as close to non-existent. For example, “voltage short” can refer to any location where a voltage component can be close to non-existent under certain conditions. Similar terms can equally refer to this location of close-to-zero voltage (for example, “virtual short circuit,” “virtual short location,” or “voltage null”). In examples, “virtual short” can be used to indicate locations where the close-to-zero voltage is a result of a standing wave crossing zero. “Voltage null” can be used to refer to locations of close-to-zero voltage for a reason other than as result of a standing wave crossing zero (for example, voltage attenuation or cancellation). Moreover, in the context of this disclosure, each of these terms that can refer to locations of close-to-zero voltage are meant to be non-limiting. 
     In an effort to provide technical context for the present disclosure, the information in this section can broadly describe various components of the implementations presently disclosed. However, such information is provided solely for the benefit of the reader and, as such, does not expressly limit the claimed subject matter. Further, components shown in the figures are shown for illustrative purposes only. As such, the illustrations are not to be construed as limiting. As is understood, components can be added, removed, or rearranged without departing from the scope of this disclosure. 
     I. Overview 
     A resonator can be excited so as to establish a plasma corona and/or electromagnetic radiation. An example of such a resonator can include a center conductor and a larger, surrounding conductor, which could be separated by a dielectric insulator such as a ceramic material. In some implementations, the resonator can also be configured to provide fuel into a combustion environment or other type of environment in which fuel may be desired. 
     One way the resonator can provide fuel is through the dielectric insulator. For instance, the dielectric insulator can include one or more fuel conduits, through which fuel can pass. These conduits can terminate at one or more fuel outlets out of which the fuel can be expelled into the environment and/or into another portion of the resonator. 
     Using a resonator configured in this manner in a jet engine can be advantageous in a variety of ways. For example, the resonator can be used as a substitute or supplement for a separate fuel injector component, possibly even eliminating a need for such a component. As another example, passing the fuel through the dielectric insulator can expose the fuel to resonator-generated electromagnetic waves, which can result in a reformation of the fuel before the resonator provides the fuel into a combustion chamber of the jet engine and/or ignites the fuel. And in addition, outlets of the fuel conduit can be oriented towards the location where the resonator will excite a plasma corona, thereby providing the fuel proximate to the ignition source (particularly, toward/through the plasma corona), which can improve the resulting combustion. 
     II. Example Combustion 
     Igniters can be used to ignite a mixture of air and fuel (for example, within a combustion chamber of an internal combustion engine  101 , such as that illustrated in cross-section in  FIG. 1A ). For example, igniters can be configured as gap spark igniters, similar to an automotive spark plug. However, gap spark igniters might not be desirable in some applications and/or under some conditions. For example, a gap spark igniter might not be capable of igniting and initiating combustion of fuel mixtures that have fuel-to-air ratios below a certain threshold. Further, lean mixtures of fuel and air might have significant environmental and economic benefits by making combustion (for example, within a combustor or an afterburner) more efficient, and thus, using a gap spark igniter might preclude achieving such benefits. In addition, higher thermal efficiencies can be achieved by operating at higher power densities and pressures. However, using more energetic or powerful gap spark igniters reduces overall ignition efficiency because the higher energy levels can be detrimental to the gap spark igniter&#39;s lifetime. Higher energy levels might also contribute to the formation of undesirable pollutants and can reduce overall engine efficiency. 
     While gap spark igniters are described above, other types of igniters can generally include glow plugs (for example, in diesel-fueled internal combustion engines), open flame sources (for example, cigarette lighters, friction spark devices, etc.), and other heat sources. 
     A variety of fuels (for example, hydrocarbon fuels) can be combusted to yield energy within an internal combustion engine, within a power-generation turbine, within a jet engine, or within various other applications. For example, kerosene (also known as paraffin or lamp oil), gasoline (also known as petrol), fractional distillates of petroleum fuel oil (for example, diesel fuel), crude oil, Fischer-Tropsch synthesized paraffinic kerosene, natural gas, and coal are all hydrocarbon fuels that, when combusted, liberate energy stored within chemical bonds of the fuel. Jet fuel, specifically, can be classified by its “jet propellant” (JP) number. The “jet propellant” (JP) number can correspond to a classification system utilized by the United States military. For example, JP-1 can be a pure kerosene fuel, JP-4 can be a 50% kerosene and 50% gasoline blend, JP-9 can be another kerosene-based fuel, JP-9 can be a gas turbine fuel (for example, including tetrahydrodimethylcyclopentadiene) specifically used in missile applications, and JP-10 can be a fuel similar to JP-9 that includes endo-tetrahydrodicyclopentadiene, exo-tetrahydrodicyclopentadiene, and adamantane. Other forms of j et fuel include zip fuel (for example, high-energy fuel that contains boron), SYNTROLEUM® FT-fuel, other kerosene-type fuels (for example, Jet A fuel and Jet A-1 fuel), and naphtha-type fuels (for example, Jet B fuel). It is understood that other fuels can be combusted as well. Further, the fuel type used can depend upon the application. For example, jet engines, internal combustion engines, and power-generation turbines may each burn different types of fuels. 
     When fuel (for example, hydrocarbon fuel) interacts with electromagnetic radiation, the fuel can change chemical composition. For example, when hydrocarbon fuel interacts with (for example, is irradiated by) microwaves, some of the hydrogen atoms can be ionized and/or one or more hydrogen atoms can be liberated from a hydrocarbon chain. The processes of liberating hydrogen within fuel, ionizing hydrogen within fuel, or otherwise changing the chemical composition of fuel are collectively referred to in the present disclosure as “reforming” the fuel. Reforming the fuel can include exciting the hydrocarbon fuel at one or more of its natural resonant frequencies (for example, acoustic and/or electromagnetic resonant frequencies) to break one or more of the carbon-hydrogen (or other) bonds within the hydrocarbon chain. When hydrogen within a hydrocarbon fuel becomes ionized and/or is liberated from the hydrocarbon chain, the resulting hydrocarbon fuel can require less energy to burn. Thus, a leaner fuel/air mixture that includes reformed fuel can achieve the same output power (for example, within a combustion chamber of a jet engine or a power-generation turbine) as compared to a more rich fuel/air mixture that includes non-reformed fuel, since the reformed fuel can combust more quickly and thoroughly. Analogously, when comparing equal fuel-to-air ratios, less input energy can be required to combust a mixture that includes reformed fuel when compared to a mixture that includes non-reformed fuel. 
     In addition to reforming fuels, electromagnetic radiation can alter an energy state of fuel and/or of a fuel mixture. In an example implementation, altering the energy state of fuel can include exciting electrons within the valence band of the hydrocarbon chain to higher energy levels. In such scenarios, raising the energy state can also include reorienting polar molecules (for example, water and/or polar hydrocarbon chains) within a fuel/air mixture due to electromagnetic fields applying a torque on polar molecules. Reorienting polar molecules can result in molecular motion, thereby increasing an effective temperature and/or kinetic energy of the molecule, which raises the energy state of fuel. By raising the energy state of fuel, the activation energy for combustion of the fuel can be reduced. When the activation energy for combustion is reduced, the energy supplied by the ignition source can also be decreased, thereby conserving energy during ignition. 
     Presently disclosed are ignition systems with resonators (for example, QWCCR structures) that use both RF power and DC power. The presently disclosed RF ignition systems provide an alternative to other types of igniters. For example, the QWCCR structure can be used as an igniter (for example, in place of an automotive gap spark plug) in the internal combustion engine  101 . Such RF ignition systems can excite plasma (for example, within a corona). If an igniter is configured as one of the RF ignition systems presently disclosed, then more efficient, leaner, cleaner combustion can be achieved. Such increased combustion efficiency can be achieved at decreased air pressures and temperatures when compared with a gap spark igniter (for example, if the RF ignition system is used in a jet engine). Further, such increased combustion efficiency can be achieved at higher air pressures and temperatures when compared with a gap spark igniter. It is understood throughout this disclosure that where reference is made to “RF” or to microwaves, in alternate implementations, other wavelengths of electromagnetic waves outside of the RF range can be used alternatively or in addition to RF electromagnetic waves. 
     As described above, RF ignition systems can excite plasma. Plasma is one of the four fundamental states of matter (in addition to solid, liquid, and gas). Further, plasmas are mixtures of positively charged gas ions and negatively charged electrons. Because plasmas are mixtures of charged particles, plasmas have associated intrinsic electric fields. In addition, when the charged particles in the mixture move, plasmas also produce magnetic fields (for example, according to Ampere&#39;s law). Given the electromagnetic nature of plasmas, plasmas interact with, and can be manipulated by, external electric and magnetic fields. For example, placing a ferromagnetic material (for example, iron, cobalt, nickel, neodymium, samarium-cobalt, etc.) near a plasma can cause the plasma to be attracted to or repelled from the ferromagnetic material (for example, causing the plasma to move). 
     Plasmas can be formed in a variety of ways. One way of forming a plasma can include heating gases to a sufficiently high temperature (for example, depending on ambient pressure). Additionally or alternatively, forming a plasma can include exposing gases to a sufficiently strong electromagnetic field. Lightning is an environmental phenomenon involving plasma. One application of plasma can include neon signs. Further, because plasma is responsive to applied electromagnetic fields, plasma can be directed according to specific patterns. Hence, plasmas can also be used in technologies such as plasma televisions or plasma etching. 
     Plasmas can be characterized according to their temperature and electron density. For example, one type of plasma can be a “microwave-generated plasma” (for example, ranging from 5 eV to 15 eV in energy). Such a plasma can be generated by a QWCCR structure, for example. 
     III. Example Resonator 
     An example implementation of a QWCCR structure  100  is illustrated in  FIGS. 1B-1D . As illustrated, the QWCCR structure  100  can include an outer conductor  102 , an inner conductor  104  with an associated electrode  106 , a base conductor  110 , and a dielectric  108 . Also as illustrated, the QWCCR structure  100  can be shaped as concentric circular cylinders. The inner conductor  104  can have radius ‘a’, the outer conductor  102  can have inner radius ‘b’, and the outer conductor  102  can have outer radius ‘c’, as illustrated in cross-section in  FIG. 1D . In alternate implementations, the QWCCR structure  100  can have other shapes (for example, concentric ellipsoidal cylinders or concentric, enclosed, elongated volumes with square or rectangular cross-sections). The inner conductor  104 , the outer conductor  102  (or just the inner surface of the outer conductor  102 ), the electrode  106 , and the base conductor  110  can be made of various conductive materials (for example, steel, gold, silver, platinum, nickel, or alloys thereof). Further, in some implementations, the inner conductor  104 , the outer conductor  102 , and the base conductor  110  can be made of the same conductive materials, while in other implementations, the inner conductor  104 , the outer conductor  102 , and the base conductor  110  can be made of different conductive materials. Additionally, in some implementations, the inner conductor  104 , the outer conductor  102 , and/or the base conductor  110  can include a dielectric material coated in a conductor (for example, a metal-plated ceramic). In such implementations, the conductive coating can be thicker than a skin-depth of the conductor at a given excitation frequency of the QWCCR structure  100  such that electricity is conducted throughout the conductive coating. 
     As illustrated, an electrode  106  can be disposed at a distal end of the inner conductor  104 . The electrode  106  can be made of a conductive material as described above (for example, the same conductive material as the inner conductor  104 ). For example, the electrode  106  can be machined with the inner conductor  104  as a single piece. In some implementations, as illustrated, the base conductor  110 , the outer conductor  102 , the inner conductor  104 , and the electrode can be shorted together. For example, the base conductor  110  can short the outer conductor  102  to the inner conductor  104 , in some implementations. When shorted together, these components can be directly electrically coupled to one another such that each of these components is at the same electric potential. 
     Further, in implementations where the base conductor  110 , the outer conductor  102 , and the inner conductor  104  (including the electrode  106 ) are shorted together, the base conductor  110 , the outer conductor  102 , and the inner conductor  104  (including the electrode  106 ) can be machined as a single piece. In addition, the electrode  106  can include a concentrator (for example, a tip, a point, or an edge), which can concentrate and enhance the electric field at one or more locations. Such an enhanced electric field can create conditions that promote the excitation of a plasma corona near the concentrator (for example, through a breakdown of a dielectric, such as air, that surrounds the concentrator). The concentrator can be a patterned or shaped portion of the electrode  106 , for example. The electrode  106 , including the concentrator, can be electromagnetically coupled to the inner conductor  104 . In the present disclosure and claims, the electrode  106  and/or the concentrator can be described as being “configured to electromagnetically couple to” the inner conductor  104 . This language is to be interpreted broadly as meaning that the electrode  106  and/or the concentrator: are presently electromagnetically coupled to the inner conductor  104 , are always electromagnetically coupled to the inner conductor  104 , can be selectively electromagnetically coupled to the inner conductor  104  (for example, using a switch), are only electromagnetically coupled to the inner conductor  104  when a power source is connected to the inner conductor  104 , and/or are able to be electromagnetically coupled to the inner conductor  104  if one or more components are repositioned relative to one another. For example, the electrode  106  can be “configured to electromagnetically couple to” the inner conductor  104  if the electrode  106  is machined as a single piece with the inner conductor  104 , if the electrode  106  is connected to the inner conductor  104  using a wire or other conducting mechanism, or if the electrode  106  is disposed sufficiently close to the inner conductor  104  such that the electrode  106  electromagnetically couples to one or more evanescent waves excited by the inner conductor  104  when the inner conductor  104  is connected to a power source. 
     As illustrated in  FIG. 1C , the electrode  106  and/or a concentrator of the electrode  106  can extend beyond the distal end of the outer conductor  102  and/or the distal end of the dielectric  108 . In alternate implementations, the electrode  106  and/or a concentrator of the electrode  106  can be flush with the distal end of the outer conductor  102  and/or the distal end of the dielectric  108 . In alternate implementations, the electrode  106  and/or a concentrator of the electrode  106  can be shorter than the outer conductor  102 , such that no portion of the electrode  106  and/or concentrator is flush with the distal end of the outer conductor  102  and no portion extends beyond the distal end of the outer conductor  102 . The QWCCR structure  100  can be excited at resonance, in some implementations. The resonance can generate a standing voltage quarter-wave within the QWCCR structure  100 . If the concentrator, the distal end of the outer conductor  102 , and the distal end of the dielectric  108  are each flush with one another, the electromagnetic field can quickly collapse outside of the QWCCR structure  100 , thereby concentrating the majority of the electromagnetic energy at the concentrator. In still other implementations, the distal end of the outer conductor  102  and/or the distal end of the dielectric  108  can extend beyond the electrode  106  and/or a concentrator of the electrode  106 . The electrode  106  can effectively modify the physical length of the inner conductor  104 , which can modify the resonance conditions of the QWCCR structure  100  (for example, can modify the electrical length of the QWCCR structure  100 ). Various resonance conditions can thus be achieved across a variety of QWCCR structures  100  by varying the geometry of the electrode  106  and/or a concentrator of the electrode  106 . 
     Further, as illustrated in  FIG. 1C , the base conductor  110  can be electrically coupled to the outer conductor  102  and the inner conductor  104 . In alternate implementations, the inner conductor  104  can be electrically insulated from the outer conductor  102  (rather than shorted together through the base conductor  110 ). 
     Plasmas (for example, plasma coronas generated by the QWCCR structure  100 ) can be used to ignite mixtures of air and fuel (for example, hydrocarbon fuel for use in a combustion process). Plasma-assisted ignition (for example, using a QWCCR structure  100 ) is fundamentally different from ignition using a gap spark plug. For example, efficient electron-impact excitation, dissociation of molecules, and ionization of atoms, which might not occur in ignition using gap spark plugs, can occur in plasma-assisted ignition. Further, in plasmas, an external electric field can accelerate the electrons and/or ions. Thus, using electric fields, energy within the plasma (for example, thermal energy) can be directed to specific locations (for example, within a combustion chamber). 
     There are a variety of mechanisms by which plasma can impart the energy necessary to ignite mixtures of air and fuel. For example, electrons can impart energy to molecules during collisions. However, this singular energy exchange might be relatively minor (for example, because an electron&#39;s mass is orders of magnitude less than a molecule&#39;s mass). So long as the rate at which electrons are imparting energy to the molecules is higher than the rate at which molecules are undergoing relaxation, a population distribution of the molecules (for example, a population distribution that differs from an initial Boltzmann distribution of the molecules) can arise. The molecules having higher energy, along with the dissociation and ionization processes, can emit ultraviolet (UV) radiation (for example, when undergoing relaxation) that affects mixtures of fuel and air. Further, gas heating and an increase in system reactivity can increase the likelihood of ignition and flame propagation. In addition, when the average electron energy within a plasma (for example, within a combustion chamber) exceeds 10 eV, gas ionization can be the predominant mechanism by which plasma is formed (over electron-impact excitation and dissociation of molecules). 
     Plasma-assisted ignition can have a variety of benefits over ignition using a gap spark plug. For example, in plasma-assisted ignition, a plasma corona that is generated can be physically larger (for example, in length, width, radius, and/or overall volumetric extent) than a typical spark from a gap spark plug. This can allow a more lean fuel mixture (also known as lower fuel-to-air ratio) to be burned once combustion occurs as compared with alternative ignition, for example. Also, because a larger energy can be energized in plasma-assisted ignition, stoichiometric ratio fuels can be combusted more fully, thereby creating fewer regulated pollutants (for example, creating less NO x  to be expelled as exhaust) and/or leaving less unspent fuel. 
     Dielectric breakdown of air or another dielectric material near the electrode  106  of the QWCCR structure  100  can be a mechanism by which a plasma corona is excited near the concentrator of the QWCCR structure  100 . Factors that impact the breakdown of a dielectric, such as dielectric breakdown of air, include free-electron population, electron diffusion, electron drift, electron attachment, and electron recombination. Free electrons in the free-electron population can collide with neutral particles or ions during ionization events. Such collisions can create additional free electrons, thereby increasing the likelihood of dielectric breakdown. Oppositely, electron diffusion and attachment can each be mechanisms by which free electrons recombine and are lost, thereby reducing the likelihood of dielectric breakdown. 
     As presently described, a plasma corona can be provided “proximate to” a distal end of the QWCCR structure  100 , the electrode  106 , and/or a concentrator of the QWCCR structure  100 . In other words, the plasma corona could be described as being provided “nearby” or “at” a distal end of the QWCCR structure  100 , the electrode  106 , and/or a concentrator of the QWCCR structure  100 . Further, this terminology is not to be viewed as limiting. For example, while the plasma corona is provided “proximate to” the QWCCR structure  100 , this does not limit the plasma corona from extending away from the QWCCR structure  100  and/or from being moved to other locations that are farther from the QWCCR structure  100  after being provided “proximate to” the QWCCR structure  100 . 
     When used to describe a relationship between a plasma corona and a distal end of the QWCCR structure  100 , a relationship between a plasma corona and the electrode  106 , a relationship between a plasma corona and a concentrator of the electrode  106 , or similar relationships, the term “proximate” can describe the physical separation between the plasma corona and the other component. In various implementations, the physical separation can include different ranges. For example, a plasma corona provided “proximate to” the concentrator can be separated from the concentrator (in other words, can “stand off from” the concentrator) by less than 1.0 nanometer, by 1.0 nanometer to 10.0 nanometers, by 10.0 nanometers to 100.0 nanometers, by 100.0 nanometers to 1.0 micrometer, by 1.0 micrometer to 10.0 micrometers, by 10.0 micrometers to 100.0 micrometers, or by 100.0 micrometers to 1.0 millimeter. Additionally or alternatively, a plasma corona provided “proximate to” the concentrator can be separated from the concentrator by 0.01 times a width of the plasma corona to 0.1 times a width of the plasma corona, by 0.1 times a width of the plasma corona to 1.0 times the width of the plasma corona, or by 1.0 times a width of the plasma corona to 10.0 times a width of the plasma corona. Even further, a plasma corona provided “proximate to” the concentrator can be separated from the concentrator by 0.01 times a radius of the concentrator to 0.1 times a radius of the concentrator, by 0.1 times a radius of the concentrator to 1.0 times a radius of the concentrator, or by 1.0 times a radius of the concentrator to 10.0 times a radius of the concentrator. 
     It is understood that in various implementations, the plasma corona can emit light entirely within the visible spectrum, partially within the visible spectrum and partially outside the visible spectrum, or completely outside the visible spectrum. In other words, even if the plasma corona is “invisible” to the human eye and/or to optics that only sense light within the visible spectrum, it is not necessarily the case that the plasma corona is not being provided. 
     IV. Mathematical Description of Example Resonator 
     In order for dielectric breakdown to occur, an electric field within the dielectric must be greater than or equal to an electric field breakdown threshold. An electric field generated by an alternating current (AC) source can be described by a root-mean-square (rms) value for electric field (E rms ). The rms value for electric field (E rms ) can be calculated according the following equation: 
     
       
         
           
             
               E 
               
                 rm 
                  
                 
                     
                 
                  
                 s 
               
             
             = 
             
               
                 
                   1 
                   
                     
                       T 
                       2 
                     
                     - 
                     
                       T 
                       1 
                     
                   
                 
                  
                 
                   
                     ∫ 
                     
                       T 
                       1 
                     
                     
                       T 
                       2 
                     
                   
                    
                   
                     
                       E 
                       2 
                     
                      
                     dt 
                   
                 
               
             
           
         
       
     
     where T 2 −T 1  represents the period over which the electric field is oscillating (for example, corresponding to the period of the AC source generating the electric field). As described mathematically above, the rms value for electric field (E rms ) represents the quadratic mean of the electric field. Using the rms value for electric field, an effective electric field (E eff ) can be calculated that is approximately frequency independent (for example, by removing phase lag effects from the oscillating electric field): 
     
       
         
           
             
               E 
               eff 
               2 
             
             = 
             
               
                 E 
                 
                   rm 
                    
                   
                       
                   
                    
                   s 
                 
                 2 
               
                
               
                 
                   v 
                   c 
                   2 
                 
                 
                   
                     ω 
                     2 
                   
                   + 
                   
                     v 
                     c 
                     2 
                   
                 
               
             
           
         
       
     
     where ω represents the angular frequency of the electric field (for example, 
     
       
         
           
             
               ω 
               = 
               
                 
                   2 
                    
                   π 
                 
                 
                   
                     T 
                     2 
                   
                   - 
                   
                     T 
                     1 
                   
                 
               
             
             ) 
           
         
       
     
     and v c  represents the effective momentum collision frequency of the electrons and neutral particles. The angular frequency (ω) of the electric field can correspond to the frequency of an excitation source used to excite the electric field (for example, the QWCCR structure  100 ). Using this effective electric field (E eff ), DC breakdown voltages for various gases (and potentially other dielectrics) can be related to AC breakdown values for uniform electric fields. For air, v c ≈5·10 9 ×p, where p represents the pressure (in torr). At atmospheric pressure (for example, around 760 torr) or above and excitation frequencies of below 1 THz, the effective momentum collision frequency of the electrons and neutral particles (v c ) will dominate the denominator of the fractional coefficient of E rms   2 . Therefore, an approximation of the rms breakdown field (E b ) can be used. The rms breakdown field (E b ), in V/cm, of a uniform microwave field in the collision regime can be given by: 
     
       
         
           
             
               E 
               b 
             
             = 
             
               
                 30 
                 · 
                 297 
               
                
               
                 ( 
                 
                   p 
                   T 
                 
                 ) 
               
             
           
         
       
     
     where T is the temperature in Kelvin. 
     An analytical description of the electromagnetics of the QWCCR structure  100  follows. 
     If fringing electromagnetic fields are assumed to be small, the lowest quarter-wave resonance in a coaxial cavity is a transverse electromagnetic mode (TEM mode) (as opposed to a transverse electric mode (TE mode) or a transverse magnetic mode (TM mode)). The TEM mode is the dominant mode in a coaxial cavity and has no cutoff frequency (ω c ). In the TEM mode (as illustrated in  FIG. 1E ), because neither the electric field nor the magnetic field have any components in the z-direction (coordinate system illustrated in  FIG. 1D ), the electric and magnetic fields can be written, respectively, as: 
     
       
         
           
             H 
             = 
             
               
                 
                   H 
                   ϕ 
                 
                  
                 
                   
                     a 
                     ^ 
                   
                   ϕ 
                 
               
               = 
               
                 
                   
                     I 
                     0 
                   
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     r 
                   
                 
                  
                 
                   cos 
                    
                   
                     ( 
                     
                       β 
                        
                       
                           
                       
                        
                       z 
                     
                     ) 
                   
                 
                  
                 
                   
                     a 
                     ^ 
                   
                   ϕ 
                 
               
             
           
         
       
       
         
           
             E 
             = 
             
               
                 
                   E 
                   r 
                 
                  
                 
                   
                     a 
                     ^ 
                   
                   r 
                 
               
               = 
               
                 
                   
                     V 
                     0 
                   
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     r 
                   
                 
                  
                 
                   sin 
                    
                   
                     ( 
                     
                       β 
                        
                       
                           
                       
                        
                       z 
                     
                     ) 
                   
                 
                  
                 
                   
                     a 
                     ^ 
                   
                   r 
                 
               
             
           
         
       
     
     where H is a phasor representing the magnetic field vector, E is a phasor representing the electric field vector, â v  represents a unit vector in the φ direction (labeled in  FIG. 1D ), â r  represents a unit vector in the r direction (labeled in  FIG. 1D ), β represents the wave number (canonically defined as 
     
       
         
           
             Q 
             = 
             
               
                 
                   
                     ω 
                     · 
                     U 
                   
                   
                     P 
                     L 
                   
                 
                 -&gt; 
                 U 
               
               = 
               
                 
                   
                     P 
                     L 
                   
                   · 
                   Q 
                 
                 ω 
               
             
           
         
       
     
     where λ is the wavelength), I 0  represents the maximum current in the cavity, V 0  represents the maximum voltage in the cavity, and z represents a distance along the QWCCR structure  100  in the z direction (labeled in  FIG. 1D ). 
     In various implementations, various electromagnetic modes of the QWCCR structure  100  can be excited in order to achieve various electromagnetic properties. In some implementations, for instance, a single electromagnetic mode can be excited, whereas in alternate implementations, a plurality of electromagnetic modes can be excited. For example, in some implementations, the TE 01  mode (as illustrated in  FIG. 1F ) can be excited. 
     Quality factor (Q) can be defined as: 
     
       
         
           
             
               β 
               = 
               
                 
                   2 
                    
                   π 
                 
                 λ 
               
             
             , 
           
         
       
     
     where ω is the angular frequency, U is the time-average energy, and P L  is the time-average power loss. Quality factor (Q) can be used to measure goodness of a resonator cavity. Other formulations of goodness measurement can also be used (for example, based on full-width, half-max (FWHM) or a 3 decibel (dB) bandwidth of cavity resonance). In some implementations, the quality factor (Q) can be maximized when the ratio of the inner radius of the outer conductor ‘b’ to the radius of the inner conductor ‘a’ is approximately equal to 4. However, it will be understood that many other ways to adjust and/or maximize quality factor (Q) are possible and contemplated in the present disclosure. 
     At resonance, the stored energy of the QWCCR structure  100  oscillates between electrical energy (U e ) (within the electric field) and magnetic energy (U m ) (within the magnetic field). Time-average stored energy in the QWCCR structure  100  can be calculated using the following: 
     
       
         
           
             U 
             = 
             
               
                 
                   U 
                   m 
                 
                 + 
                 
                   U 
                   e 
                 
               
               = 
               
                 
                   
                     1 
                     4 
                   
                    
                   
                     
                       ∫ 
                       vol 
                       
                           
                       
                     
                      
                     
                       μ 
                        
                       
                         
                            
                           H 
                            
                         
                         2 
                       
                     
                   
                 
                 + 
                 
                   ɛ 
                    
                   
                     
                        
                       E 
                        
                     
                     2 
                   
                 
               
             
           
         
       
     
     where μ is magnetic permeability and E is dielectric permittivity. By inserting the values for electric field and magnetic field from above, and integrating over the entire volume of the QWCCR structure  100 , the following expression can be obtained: 
     
       
         
           
             U 
             = 
             
               
                 
                   
                     ln 
                      
                     
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                   · 
                   λ 
                 
                 
                   64 
                    
                   π 
                 
               
                
               
                 ( 
                 
                   
                     μ 
                     · 
                     
                       I 
                       0 
                       2 
                     
                   
                   + 
                   
                     ɛ 
                     · 
                     
                       V 
                       0 
                       2 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     where b represents the inner radius of the outer conductor  102  of the QWCCR structure  100  (as illustrated in  FIG. 1D ), a represents the radius of the inner conductor  104  of the QWCCR structure  100  (as illustrated in  FIG. 1D ), and represents the wavelength of the source (for example, AC source) used to excite the QWCCR structure  100 . Because the magnetic energy at maximum is the same as the electric energy at maximum, μ·I 0   2  can be replaced with ε ·V 0   2 , thus resulting in: 
     
       
         
           
             U 
             = 
             
               
                 
                   
                     ln 
                      
                     
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                   · 
                   λ 
                 
                 
                   32 
                    
                   π 
                 
               
                
               
                 ( 
                 
                   ɛ 
                   · 
                   
                     V 
                     0 
                     2 
                   
                 
                 ) 
               
             
           
         
       
     
     Now, by equating the two above expressions for U, the following relationship can be expressed: 
     
       
         
           
             
               
                 
                   P 
                   L 
                 
                 · 
                 Q 
               
               ω 
             
             = 
             
               
                 
                   
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             b 
                             a 
                           
                           ) 
                         
                       
                       · 
                       λ 
                     
                     
                       32 
                        
                       π 
                     
                   
                    
                   
                     ( 
                     
                       ɛ 
                       · 
                       
                         V 
                         0 
                         2 
                       
                     
                     ) 
                   
                 
                 → 
                 
                   V 
                   0 
                 
               
               = 
               
                 
                   
                     32 
                      
                     
                       π 
                       · 
                       Q 
                       · 
                       
                         P 
                         L 
                       
                     
                   
                   
                     ω 
                     · 
                     ɛ 
                     · 
                     
                       ln 
                        
                       
                         ( 
                         
                           b 
                           a 
                         
                         ) 
                       
                     
                     · 
                     λ 
                   
                 
               
             
           
         
       
     
     Further, in recognizing that 
     
       
         
           
             
               ω 
               = 
               
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   f 
                 
                 = 
                 
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     c 
                   
                   λ 
                 
               
             
             , 
           
         
       
     
     where c is the speed of light; 
     
       
         
           
             
               
                 c 
                 = 
                 
                   
                     1 
                     
                       μ 
                       · 
                       ɛ 
                     
                   
                 
               
               ; 
               
                 and 
                  
                 
                     
                 
                  
                 η 
                  
                 
                   
                     μ 
                     ɛ 
                   
                 
               
             
             , 
           
         
       
     
     where η is the impedance of the dielectric between the inner conductor  104  and the outer conductor  102  of the QWCCR structure  100 , the following relationship for the peak potential (V 0 ) can be identified: 
     
       
         
           
             
               V 
               0 
             
             = 
             
               4 
                
               
                 
                   
                     η 
                     · 
                     Q 
                     · 
                     
                       P 
                       L 
                     
                   
                   
                     ln 
                      
                     
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     Given that electric field decays as the distance from the peak potential (V 0 ) increases, the largest value of electric field corresponding to the peak potential (V 0 ) occurs exactly at the surface of the inner conductor (for example, at radius a, as illustrated in  FIG. 1D ). Using the above equation for phasor electric field (E), the peak value of electric field (E a ) can be expressed as: 
     
       
         
           
             
               E 
               a 
             
             = 
             
               
                 
                   V 
                   0 
                 
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   a 
                 
               
               = 
               
                 
                   2 
                   
                     π 
                      
                     
                         
                     
                      
                     a 
                   
                 
                  
                 
                   
                     
                       η 
                       · 
                       Q 
                       · 
                       
                         P 
                         L 
                       
                     
                     
                       ln 
                        
                       
                         ( 
                         
                           b 
                           a 
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
     If the above peak value of electric field (E a ) meets or exceeds the above-described rms breakdown field (E b ), a dielectric breakdown can occur. For example, a dielectric breakdown of the air surrounding the tip of the QWCCR structure  100  can result in a plasma corona being excited. As indicated in the above equation for peak electric field (E a ), the smaller the radius a of the inner conductor  104 , the smaller the inner radius b of outer conductor  102 , the higher the quality factor (Q) of the QWCCR structure  100 , and the larger the time-average power loss (P L ), the more likely it is that breakdown can occur (for example, because the peak value of electric field (E a ) is larger). A larger excitation power can correspond to a larger time-average power loss (h) in the QWCCR structure  100 , for example. 
     The power loss (h) can include ohmic losses (P σ ) on conductive surfaces (for example, the surface of the outer conductor  102 , the surface of the inner conductor  104 , and/or the surface of the base conductor  110 , as illustrated in  FIG. 1C ), dielectric losses (P σ     e   ) in the dielectric  108 , and radiation losses (P rad ) from a radiating end of the QWCCR structure  100  (for example, the distal end of the QWCCR structure  100 ). Each of the conductors can have a corresponding surface resistance (R S ). The surface resistance (R S ) can be the same for one or more of the conductors if the corresponding conductors are made of the same conductive materials. The corresponding surface resistance for each conductor can be expressed as 
     
       
         
           
             
               
                 R 
                 S 
               
               = 
               
                 
                   
                     ω 
                     · 
                     
                       μ 
                       c 
                     
                   
                   
                     2 
                     · 
                     
                       σ 
                       c 
                     
                   
                 
               
             
             , 
           
         
       
     
     where μ c  is the magnetic permeability of the respective conductor and a, is the conductivity of the respective conductor. The power lost by each conductor can be calculated according to the following: 
     
       
         
           
             
               P 
               σ 
             
             = 
             
               
                 1 
                 2 
               
                
               
                 
                   ∫ 
                   A 
                 
                  
                 
                   
                     R 
                     S 
                   
                    
                   
                     
                        
                       
                         H 
                         // 
                       
                        
                     
                     2 
                   
                 
               
             
           
         
       
     
     where H //  is the magnetic field parallel to the surface of the conductor. Thus, the total power loss in all conductors can be represented by: 
     
       
         
           
             
               P 
               σ 
             
             = 
             
               
                 
                   P 
                   inner 
                 
                 + 
                 
                   P 
                   outer 
                 
                 + 
                 
                   P 
                   base 
                 
               
               = 
               
                 
                   
                     
                       R 
                       S 
                     
                     · 
                     
                       I 
                       0 
                       2 
                     
                   
                   
                     4 
                      
                     π 
                   
                 
                  
                 
                   [ 
                   
                     
                       λ 
                       
                         8 
                         · 
                         a 
                       
                     
                     + 
                     
                       λ 
                       
                         8 
                         · 
                         b 
                       
                     
                     + 
                     
                       ln 
                        
                       
                         ( 
                         
                           b 
                           a 
                         
                         ) 
                       
                     
                   
                   ] 
                 
               
             
           
         
       
     
     Further, if the dielectric  108  is an isotropic, low-loss dielectric, the dielectric  108  can be characterized by its dielectric constant (E) and its loss tangent (tan(δ e )), where the loss tangent (tan(δ e )) represents conductivity and alternating molecular dipole losses. Using dielectric constant (ε) and loss tangent (tan(δ e )), an effective dielectric conductivity (σ e ) can be approximately defined as: 
       σ e ≈ω·ε·tan(δ e )
 
     Based on the above, the power dissipated in the dielectric can be calculated according to the following: 
     
       
         
           
             
               P 
               
                 σ 
                 e 
               
             
             = 
             
               
                 
                   1 
                   2 
                 
                  
                 
                   
                     ∫ 
                     vol 
                   
                    
                   
                     
                       σ 
                       e 
                     
                      
                     
                       
                          
                         E 
                          
                       
                       2 
                     
                   
                 
               
               = 
               
                 
                   
                     
                       σ 
                       e 
                     
                     · 
                     η 
                     · 
                     
                       I 
                       0 
                       2 
                     
                   
                   
                     4 
                      
                     π 
                   
                 
                  
                 
                   ( 
                   
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             b 
                             a 
                           
                           ) 
                         
                       
                       · 
                       λ 
                     
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
     In order to combine all quality factors of the QWCCR structure  100  into a total internal quality factor (Q int ), the following relationship can be used: 
     
       
         
           
             
               Q 
               int 
             
             = 
             
               1 
               
                 ( 
                 
                   
                     Q 
                     inner 
                     
                       - 
                       1 
                     
                   
                   + 
                   
                     Q 
                     outer 
                     
                       - 
                       1 
                     
                   
                   + 
                   
                     Q 
                     base 
                     
                       - 
                       1 
                     
                   
                   + 
                   
                     Q 
                     
                       σ 
                       e 
                     
                     
                       - 
                       1 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     where Q inner   −1 , Q outer   −1 , Q base   −1 , and Q σ     e     −1  are the quality factors of the inner conductor  104 , the outer conductor  102 , the base conductor  110 , and the dielectric  108 , respectively. Using the above expression for quality factor (Q) in terms of time-average power loss (P L ), angular frequency (ω), and time-average energy (U), the following expression for internal quality factor (Q int ) can be determined: 
     
       
         
           
             
               Q 
               int 
             
             = 
             
               
                 ( 
                 
                   
                     
                       
                         R 
                         S 
                       
                       
                         2 
                         · 
                         π 
                         · 
                         η 
                       
                     
                      
                     
                       [ 
                       
                         
                           
                             ( 
                             
                               
                                 b 
                                 a 
                               
                               + 
                               1 
                             
                             ) 
                           
                           
                             
                               b 
                               a 
                             
                             · 
                             
                               ln 
                                
                               
                                 ( 
                                 
                                   b 
                                   a 
                                 
                                 ) 
                               
                             
                           
                         
                         + 
                         8 
                       
                       ] 
                     
                   
                   + 
                   
                     tan 
                      
                     
                       ( 
                       
                         δ 
                         e 
                       
                       ) 
                     
                   
                 
                 ) 
               
               
                 - 
                 1 
               
             
           
         
       
     
     Based on the definitions of the individual quality factors above, the individual contribution of the outer conductor quality factor (Q outer ) to the internal quality factor (Q int ) can be greater than the individual contribution of the inner conductor quality factor (Q inner ). Thus, to increase the internal quality factor (Q int ), a material with higher conductivity can be used for the inner conductor  104  than is used for the outer conductor  102 . Further, the base conductor  110  quality factor (Q base ) and the dielectric  108  quality factor (Q σ     e   ) can be unaffected by the geometry of the QWCCR structure  100  (both in terms of 
     
       
         
           
             b 
             a 
           
         
       
     
     and in terms of 
     
       
         
           
             
               
                 b 
                 λ 
               
               ) 
             
             . 
           
         
       
     
     The QWCCR structure  100  can also radiate electromagnetic waves (for example, from a distal, non-closed end opposite the base conductor  110 ). For example, if the QWCCR structure  100  is being excited by an RF power source (for example, a signal generator oscillating at radio frequencies), the QWCCR structure  100  can radiate microwaves from a distal end (for example, from an aperture of the distal end) of the QWCCR structure  100 . Such radiation can lead to power losses, which can be approximated using admittance. Assuming that the transverse dimensions of the QWCCR structure  100  are significantly smaller than the wavelength (λ) being used to excite the QWCCR structure  100  (in other words, a&lt;&lt;λ and b&lt;&lt;λ), the real part (G r ) and imaginary part (B r ) of admittance can be represented by: 
     
       
         
           
             G 
             ≈ 
             
               
                 4 
                 · 
                 
                   π 
                   5 
                 
                 · 
                 
                   
                     [ 
                     
                       
                         
                           ( 
                           
                             
                               ( 
                               
                                 b 
                                 λ 
                               
                               ) 
                             
                             
                               ( 
                               
                                 b 
                                 a 
                               
                               ) 
                             
                           
                           ) 
                         
                         2 
                       
                       - 
                       
                         
                           ( 
                           
                             b 
                             λ 
                           
                           ) 
                         
                         2 
                       
                     
                     ] 
                   
                   2 
                 
               
               
                 3 
                 · 
                 η 
                 · 
                 
                   
                     ln 
                     2 
                   
                    
                   
                     ( 
                     
                       b 
                       a 
                     
                     ) 
                   
                 
               
             
           
         
       
       
         
           
             
               B 
               r 
             
             ≈ 
             
               
                 
                   16 
                   · 
                   π 
                   · 
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             b 
                             λ 
                           
                           ) 
                         
                         
                           ( 
                           
                             b 
                             a 
                           
                           ) 
                         
                       
                       - 
                       
                         ( 
                         
                           b 
                           λ 
                         
                         ) 
                       
                     
                     ) 
                   
                 
                 
                   η 
                   · 
                   
                     
                       ln 
                       2 
                     
                      
                     
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                 
               
               · 
               
                 [ 
                 
                   
                     E 
                      
                     
                       ( 
                       
                         
                           2 
                            
                           
                             
                               b 
                               a 
                             
                           
                         
                         
                           1 
                           + 
                           
                             b 
                             a 
                           
                         
                       
                       ) 
                     
                   
                   - 
                   1 
                 
                 ] 
               
             
           
         
       
     
     where E(x) is the complete elliptical integral of the second kind. Namely: 
     
       
         
           
             
               E 
                
               
                 ( 
                 x 
                 ) 
               
             
             = 
             
               
                 ∫ 
                 0 
                 
                   π 
                   2 
                 
               
                
               
                 
                   
                     
                       1 
                       - 
                       
                         
                           x 
                           2 
                         
                         · 
                         
                           
                             sin 
                             2 
                           
                            
                           
                             ( 
                             θ 
                             ) 
                           
                         
                       
                     
                   
                   · 
                   d 
                 
                  
                 
                     
                 
                  
                 θ 
               
             
           
         
       
     
     Further, the line integral of the electric field from the inner conductor  104  to the outer conductor  102  can be used to determine the potential difference (V ab ) across the shunt admittance corresponding to the electromagnetic waves radiated. 
     
       
         
           
             
               
                 V 
                 ab 
               
                
               
                 | 
                 
                   
                     β 
                      
                     
                         
                     
                      
                     z 
                   
                   = 
                   
                     π 
                     4 
                   
                 
               
                
               
                 
                   ∫ 
                   
                     a 
                     -&gt; 
                     b 
                   
                 
                  
                 
                   E 
                   r 
                 
               
             
             = 
             
               
                 
                   V 
                   0 
                 
                  
                 
                   ln 
                    
                   
                     ( 
                     
                       b 
                       a 
                     
                     ) 
                   
                 
               
               
                 2 
                  
                 π 
               
             
           
         
       
     
     Using the potential difference (V ab ) across the shunt admittance corresponding to the electromagnetic waves radiated, the power going to radiation (P rad ) can be represented by: 
     
       
         
           
             
               P 
               
                 ra 
                  
                 
                     
                 
                  
                 d 
               
             
             = 
             
               
                 
                   1 
                   2 
                 
                  
                 
                   G 
                   r 
                 
                  
                 
                   V 
                   ab 
                   2 
                 
               
               = 
               
                 
                   
                     V 
                     0 
                   
                    
                   
                     
                       
                         
                           
                             π 
                             3 
                           
                            
                           
                             ( 
                             
                               b 
                               λ 
                             
                             ) 
                           
                         
                         4 
                       
                        
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 b 
                                 a 
                               
                               ) 
                             
                             2 
                           
                           - 
                           1 
                         
                         ] 
                       
                     
                     2 
                   
                 
                 
                   6 
                    
                   
                     
                       η 
                        
                       
                         ( 
                         
                           b 
                           a 
                         
                         ) 
                       
                     
                     4 
                   
                 
               
             
           
         
       
     
     In addition, using the potential difference (V ab ) across the shunt admittance corresponding to the electromagnetic waves radiated, the energy stored during radiation (U rad ) can be represented by: 
     
       
         
           
             
               U 
               
                 r 
                  
                 
                     
                 
                  
                 ad 
               
             
             = 
             
               
                 
                   1 
                   4 
                 
                  
                 
                   ( 
                   
                     
                       B 
                       r 
                     
                     ω 
                   
                   ) 
                 
                  
                 
                   V 
                   ab 
                   2 
                 
               
               = 
               
                 
                   
                     ɛ 
                      
                     
                         
                     
                      
                     
                       V 
                       0 
                       2 
                     
                      
                     
                       
                         λ 
                          
                         
                           ( 
                           
                             b 
                             λ 
                           
                           ) 
                         
                       
                        
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 b 
                                 a 
                               
                               ) 
                             
                             
                               - 
                               1 
                             
                           
                           + 
                           1 
                         
                         ] 
                       
                     
                   
                   
                     2 
                      
                     
                       π 
                       2 
                     
                   
                 
                 [ 
                 
                   
                     E 
                     ( 
                     
                       
                         2 
                          
                         
                           
                             b 
                             a 
                           
                         
                       
                       
                         1 
                         + 
                         
                           b 
                           a 
                         
                       
                     
                     ) 
                   
                   - 
                   1 
                 
                 ] 
               
             
           
         
       
     
     Based on the above, the overall quality factor of the QWCCR structure  100  (Q QWCCR ) can be described by the following: 
     
       
         
           
             
               Q 
               QWCCR 
             
             = 
             
               
                 ω 
                  
                 
                   ( 
                   
                     U 
                     + 
                     
                       U 
                       
                         ra 
                          
                         
                             
                         
                          
                         d 
                       
                     
                   
                   ) 
                 
               
               
                 
                   P 
                   inner 
                 
                 + 
                 
                   P 
                   outer 
                 
                 + 
                 
                   P 
                   base 
                 
                 + 
                 
                   P 
                   
                     σ 
                     e 
                   
                 
                 + 
                 
                   P 
                   
                     r 
                      
                     
                         
                     
                      
                     ad 
                   
                 
               
             
           
         
       
     
     If the energy stored during radiation (U rad ) is small compared with the energy stored in the interior of the QWCCR structure  100  (U), the radiation power (P rad ) can be treated similarly to the other losses. Further, the energy stored during radiation (U rad ) can be neglected in the above equation: 
     
       
         
           
             Q 
             ≈ 
             
               
                 ω 
                  
                 
                   ( 
                   U 
                   ) 
                 
               
               
                 
                   P 
                   inner 
                 
                 + 
                 
                   P 
                   outer 
                 
                 + 
                 
                   P 
                   base 
                 
                 + 
                 
                   P 
                   
                     σ 
                     e 
                   
                 
                 + 
                 
                   P 
                   
                     ra 
                      
                     
                         
                     
                      
                     d 
                   
                 
               
             
           
         
       
     
     Still further, the quality factor of the radiation component (Q rad ) can be described using the above relationship for quality factors: 
     
       
         
           
             
               Q 
               
                 ra 
                  
                 
                     
                 
                  
                 d 
               
             
             = 
             
               
                 
                   ω 
                    
                   
                       
                   
                    
                   U 
                 
                 
                   P 
                   
                     ra 
                      
                     
                         
                     
                      
                     d 
                   
                 
               
               = 
               
                 
                   3 
                    
                   
                     
                       ( 
                       
                         b 
                         λ 
                       
                       ) 
                     
                     4 
                   
                    
                   
                     ln 
                      
                     
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                 
                 
                   8 
                    
                   
                     
                       
                         
                           
                             π 
                             3 
                           
                            
                           
                             ( 
                             
                               b 
                               λ 
                             
                             ) 
                           
                         
                         4 
                       
                        
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 b 
                                 a 
                               
                               ) 
                             
                             2 
                           
                           - 
                           1 
                         
                         ] 
                       
                     
                     2 
                   
                 
               
             
           
         
       
     
     Even further, using the above-referenced quality factors, the total quality factor of the QWCCR structure  100  (Q QWCCR ) can be approximated by: 
     
       
         
           
             
               Q 
               QWCCR 
             
             ≈ 
             
               
                 ( 
                 
                   
                     
                       8 
                        
                       
                         
                           
                             
                               
                                 π 
                                 3 
                               
                                
                               
                                 ( 
                                 
                                   b 
                                   λ 
                                 
                                 ) 
                               
                             
                             4 
                           
                            
                           
                             [ 
                             
                               
                                 
                                   ( 
                                   
                                     b 
                                     a 
                                   
                                   ) 
                                 
                                 2 
                               
                               - 
                               1 
                             
                             ] 
                           
                         
                         2 
                       
                     
                     
                       3 
                        
                       
                         
                           ( 
                           
                             b 
                             a 
                           
                           ) 
                         
                         4 
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           
                             b 
                             a 
                           
                           ) 
                         
                       
                     
                   
                   + 
                   
                     
                       
                         R 
                         S 
                       
                       
                         2 
                          
                         π 
                          
                         
                             
                         
                          
                         η 
                       
                     
                      
                     
                       [ 
                       
                         
                           
                             ( 
                             
                               
                                 ( 
                                 
                                   b 
                                   a 
                                 
                                 ) 
                               
                               + 
                               1 
                             
                             ) 
                           
                           
                             
                               ( 
                               
                                 b 
                                 λ 
                               
                               ) 
                             
                              
                             
                               ln 
                                
                               
                                 ( 
                                 
                                   b 
                                   a 
                                 
                                 ) 
                               
                             
                           
                         
                         + 
                         8 
                       
                       ] 
                     
                   
                   + 
                   
                     tan 
                      
                     
                       ( 
                       
                         δ 
                         e 
                       
                       ) 
                     
                   
                 
                 ) 
               
               
                 - 
                 1 
               
             
           
         
       
     
     Based on the above relationships, it can be shown that one method of minimizing losses due to radiation of electromagnetic waves by the QWCCR structure  100  is to minimize the inner radius b of the outer conductor  102  with respect to the excitation wavelength (λ). Another way of minimizing losses due to radiation of electromagnetic waves is to select an inner radius b of the outer conductor  102  that is close in dimension to the radius a of the inner conductor  104 . 
     Various physical quantities and dimensions of the QWCCR structure  100  can be adjusted to modify performance of the QWCCR structure  100 . For example, physical quantities and dimensions can be modified to maximize and/or optimize the total quality factor of the QWCCR structure  100  (Q QWCCR ). In some implementations, different dielectrics can be inserted into the QWCCR structure  100 . In one implementation, the dielectric  108  can include a composite of multiple dielectric materials. For example, a half of the dielectric  108  near a proximal end of the QWCCR structure  100  can include alumina ceramic while a half of the dielectric  108  near a distal end of the QWCCR structure  100  can include air. The resonant frequency can be based on the dimensions and the fabrication materials of the QWCCR structure  100 . Hence, modification of the dielectric  108  can modify a resonant frequency of the QWCCR structure  100 . In some implementations, the resonant frequency can be 2.45 GHz based on the dimensions of the QWCCR structure  100 . In other implementations, the resonant frequency of the QWCCR structure  100  could be within an inclusive range between 1 GHz to 100 GHz. In still other implementations, the resonant frequency of the QWCCR structure  100  could be within an inclusive range of 100 MHz to 1 GHz or an inclusive range of 100 GHz to 300 GHz. However, other resonant frequencies are contemplated within the context of the present disclosure. 
     An RF power source exciting the QWCCR structure  100  can generate a standing electromagnetic wave within the QWCCR structure  100 . In some implementations, the resonant frequency of the QWCCR structure  100  can be designed to match the frequency of an RF power source that is exciting the QWCCR structure  100  (for example, to maximize power transferred to the QWCCR structure  100 ). For example, if a desired excitation frequency corresponds to a wavelength of λ 0 , dimensions of the QWCCR structure  100  can be modified such that the electrical length of the QWCCR structure  100  is an odd-integer multiple of quarter wavelengths (for example, 
     
       
         
           
             
               
                 1 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 3 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 5 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 7 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 9 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 11 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 13 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
           
         
       
     
     etc.). The electrical length is a measure of the length of a resonator in terms of the wavelength of an electromagnetic wave used to excite the resonator. The QWCCR structure  100  can be designed for a given resonant frequency based on the dimensions of the QWCCR structure  100  (for example, adjusting dimensions of the inner conductor  104 , the outer conductor  102 , or the dielectric  108 ) or the materials of the QWCCR structure  100  (for example, adjusting materials of the inner conductor  104 , the outer conductor  102 , or the dielectric  108 ). 
     In other implementations, the resonant frequency of the QWCCR structure  100  can be designed or adjusted such that its resonant frequency does not match the frequency of an RF power source that is exciting the QWCCR structure  100  (for example, to reduce power transferred to the QWCCR structure  100 ). Analogously, the frequency of an RF power source can be de-tuned relative to the resonant frequency of a QWCCR structure  100  that is being excited by the RF power source. Additionally or alternatively, the physical quantities and dimensions of the QWCCR structure  100  can be modified to enhance the amount of energy radiated (for example, from the distal end) in the form of electromagnetic waves (for example, microwaves) from the QWCCR structure  100 . As an example, one or more elements of the QWCCR structure  100  could be movable or otherwise adjustable so as to modify the resonant properties of the QWCCR structure  100 . Enhancing the amount of energy radiated might be done at the expense of maximizing the electric field at a concentrator of the electrode  106  at the distal end of the inner conductor  104 . For example, some implementations can include slots or openings in the outer conductor  102  to increase the amount of radiated energy despite possibly reducing a quality factor of the QWCCR structure  100 . 
     In still other implementations, the physical quantities and dimensions of the QWCCR structure  100  can be designed in such a way so as to enhance the intensity of an electric field at a concentrator of the electrode  106  of the QWCCR structure  100 . Enhancing the electric field at a concentrator of the electrode  106  of the QWCCR structure  100  can result in an increase in plasma corona excitation (for example, an increase in dielectric breakdown near the concentrator), when the QWCCR structure  100  is excited with sufficiently high RF power/current. To increase electric field at a concentrator of the electrode  106  of the QWCCR structure  100 , a radius of the concentrator can be minimized (for example, configured as a very sharp structure, such as a tip). Additionally or alternatively, to increase the electric field at a tip of the QWCCR structure  100  (for example, thereby increasing the intensity and/or size of an excited plasma corona), the intrinsic impedance (η) of the dielectric  108  can be increased, the power used to excite the QWCCR structure  100  can be increased, and the total quality factor of the QWCCR structure  100  (Q QWCCR ) can be increased (for example, by increasing the volume energy storage (U) of the cavity or by minimizing the surface and radiation losses). 
     Further, the shunt capacitance (C) of a circular coaxial cavity (for example, in farads/meter, and neglecting fringing fields) can be expressed as follows: 
     
       
         
           
             C 
             = 
             
               
                 2 
                  
                 
                   πɛ 
                   0 
                 
                  
                 
                   ɛ 
                   r 
                 
               
               
                 ln 
                  
                 
                   ( 
                   
                     b 
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     where ε 0  represents the permittivity of free space, ε r  represents the relative dielectric constant of the dielectric  108  between the inner conductor  104  and the outer conductor  102 , b is the inner radius of the outer conductor  102 , and a is the radius of the inner conductor  104  (as illustrated in  FIG. 1D ). 
     Similarly, the shunt inductance (L) of a circular coaxial cavity (for example, in henrys/meter) can be expressed as follows: 
     
       
         
           
             L 
             = 
             
               
                 
                   
                     μ 
                     0 
                   
                    
                   
                     μ 
                     r 
                   
                 
                 
                   2 
                    
                   π 
                 
               
                
               
                 ln 
                  
                 
                   ( 
                   
                     b 
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     where μ 0  represents the permeability of free space, μ r  represents the relative permeability of the dielectric  108  between the inner conductor  104  and the outer conductor  102 , b is the inner radius of the outer conductor  102 , and a is the radius of the inner conductor  104  (as illustrated in  FIG. 1D ). 
     Based on the above, the complex impedance (Z) of a circular coaxial cavity (for example, in ohms, Ω) can be expressed as follows: 
     
       
         
           
             Z 
             = 
             
               
                 
                   R 
                   + 
                   
                     j 
                      
                     
                         
                     
                      
                     ω 
                      
                     
                         
                     
                      
                     L 
                   
                 
                 
                   G 
                   + 
                   
                     j 
                      
                     
                         
                     
                      
                     ω 
                      
                     
                         
                     
                      
                     C 
                   
                 
               
             
           
         
       
     
     where G represents the conductance per unit length of the dielectric between the inner conductor and the outer conductor, R represents the resistance per unit length of the QWCCR structure  100 , j represents the imaginary unit (for example, √{square root over (−1)}), ω represents the frequency at which the QWCCR structure  100  is being excited, L represents the shunt inductance of the QWCCR structure  100 , and C represents the shunt capacitance of the QWCCR structure  100 . 
     At very high frequencies (for example, GHz frequencies) the complex impedance (Z) can be approximated by: 
     
       
         
           
             
               Z 
               0 
             
             = 
             
               
                 L 
                 C 
               
             
           
         
       
     
     where Z 0  represents the characteristic impedance of the QWCCR structure  100  (in other words, the complex impedance (Z) of the QWCCR structure  100  at high frequencies). 
     As described above, the shunt inductance (L) and the shunt capacitance (C) of the QWCCR structure  100  depend on the relative permeability (μ r ) and the relative dielectric constant (ε r ), respectively, of the dielectric  108  between the inner conductor  104  and the outer conductor  102 . Thus, any modification to either the relative permeability (μ r ) or the relative dielectric constant (ε r ) of the dielectric  108  between the inner conductor  104  and the outer conductor  102  can result in a modification of the characteristic impedance (Z 0 ) of the QWCCR structure  100 . Such modifications to impedance can be measured using an impedance measurement device (for example, an oscilloscope, a spectrum analyzer, and/or an AC volt meter). 
     The above characteristic impedance (Z 0 ) represents an impedance calculated by neglecting fringing fields. In some applications and implementations, the fringing fields can be non-negligible (for example, the fringing fields can significantly impact the impedance of the QWCCR structure  100 ). Further, in such implementations, the composition of the materials surrounding the QWCCR structure  100  can affect the characteristic impedance (Z 0 ) of the QWCCR structure  100 . Measurements of such changes to characteristic impedance (Z 0 ) can provide information regarding the environment (for example, a combustion chamber) surrounding the QWCCR structure  100  (for example, the temperature, pressure, or atomic composition of the environment). A change in the characteristic impedance (Z 0 ) can coincide with a change in the cutoff frequency, resonant frequency, short-circuit condition, open-circuit condition, lumped-circuit model, mode distribution, etc. of the QWCCR structure  100 . 
       FIG. 1G  illustrates a quarter-wave resonance condition of the QWCCR structure  100 . The y-axis of the plot corresponds to a power of electromagnetic waves radiated from a distal end of the QWCCR structure  100  and the x-axis corresponds to an excitation frequency (ω) (for example, from a radio-frequency power source that is electromagnetically coupled to the QWCCR structure  100 ) used to excite the QWCCR structure  100 . As illustrated, the shape of the curve can be a Lorentzian. 
     As illustrated in  FIG. 1G  the curve has a maximum power at a quarter-wave (λ/4) resonance. This resonance can correspond to excitation frequency (ω) that has an associated excitation wavelength that is four times the length of the QWCCR structure  100 . In other words, at the resonant frequency (ω 0 ) the QWCCR structure  100  is being excited by a standing wave, where one-quarter of the length of the standing wave is equal to the length of the QWCCR structure  100 . Although not illustrated, it is understood that the QWCCR structure  100  could experience additional resonances (for example, at odd-integer multiples of the resonant wavelength: 
     
       
         
           
             
               
                 3 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 5 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 7 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 9 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 11 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
             
               
                 13 
                 4 
               
                
               
                 λ 
                 0 
               
             
             , 
           
         
       
     
     etc.). Each of the additional resonances could look similar to the resonance illustrated in  FIG. 1G  (for example, could have a Lorentzian shape). 
     As illustrated, the power of the electromagnetic waves radiated from the distal end of the QWCCR structure  100  decreases exponentially the further the excitation frequency (ω) is from the resonant frequency (ω 0 ). However, the power of the electromagnetic waves is not necessarily zero as soon as you move away from resonance. Hence, it is understood that even when excited near the quarter-wave resonance condition (in other words, proximate to the quarter-wave resonance condition), rather than exactly at the resonance condition, the QWCCR structure  100  can still radiate electromagnetic waves with non-zero power and/or provide a plasma corona, depending on arrangement. 
     When the QWCCR structure  100  is being excited such that it provides a plasma corona proximate to the distal end (for example, at the electrode  106 ), a plot with a shape similar to that of  FIG. 1G  could be provided. In such a scenario, a plot of voltage at the electrode  106  versus excitation frequency (ω) could include a Gaussian shape, rather than a Lorentzian shape. In other words, the voltage at the electrode  106  may reach a peak when excited by a resonant frequency. The voltage at the electrode  106  may fall off exponentially according to a Gaussian shape as the excitation frequency moves away from the resonant frequency. It will be understood that the Gaussian and Lorentzian shapes presently described may be based on one or more characteristics of the QWCCR structure  100 , such as its shape, quality factor, bias conditions, or other factors. 
     It is understood that when the term “proximate” is used to describe a relationship between a wavelength of a signal (for example, a signal used to excite the QWCCR structure  100 ) and a resonant wavelength of a resonator (for example, the QWCCR structure  100 ), the term “proximate” can describe a difference in length. For example, if the wavelength of the signal is “proximate to an odd-integer multiple of one-quarter of the resonant wavelength,” the wavelength of the signal can be equal to, within 0.001% of, within 0.01% of, within 0.1% of, within 1.0% of, within 5.0% of, within 10.0% of, within 15.0% of, within 20.0% of, and/or within 25.0% of one-quarter of the resonant wavelength. Additionally or alternatively, if the wavelength of the signal is “proximate to an odd-integer multiple of one-quarter of the resonant wavelength,” the wavelength of the signal can be within 0.1 nm, within 1.0 nm, within 10.0 nm, within 0.1 micrometers, within 1.0 micrometers, within 10.0 micrometers, within 0.1 millimeters, within 1.0 millimeters, and/or within 1.0 centimeters of one-quarter of the resonant wavelength, depending on context (for example, depending on the resonant wavelength). Still further, if the wavelength of the signal is “proximate to an odd-integer multiple of one-quarter of the resonant wavelength,” the wavelength of the signal can be a multiple of one-quarter of the resonant wavelength that is an odd number plus or minus 0.5, an odd number plus or minus 0.1, an odd number plus or minus 0.01, an odd number plus or minus 0.001, and/or an odd number plus or minus 0.0001. 
     The quality factor of the QWCCR structure  100  (Q QWCCR ), described above, can be used to describe the width and/or the sharpness of the resonance (in other words, how quickly the power drops off as you excite the QWCCR structure  100  further and further from the resonance condition). For example, a square root of the quality factor can correspond to the voltage modification experienced at the electrode  106  of the QWCCR structure  100  when the QWCRR structure  100  is excited at the quarter-wave resonant condition. Additionally, the quality factor may be equal to the resonant frequency (ω 0 ) divided by full width at half maximum (FWHM). The FWHM is equal to the width of the curve in terms of frequency between the two points on the curve where the power is equal to 50% of the maximum power, as illustrated). The 50% power maximum point can also be referred to as the −3 decibel (dB) point, because it is the point at which the maximum voltage at the distal end of the QWCCR structure  100  decreases by 3 dB (or 29.29% for voltage) and the maximum power radiated by the QWCCR structure  100  decreases by 3 dB (or 50% for power). In various implementations, the FWHM of the QWCCR structure  100  could have various values. For example, the FWHM could be between 5 MHz and 10 MHz, between 10 MHz and 20 MHz, between 20 MHz and 40 MHz, between 40 MHz and 60 MHz, between 60 MHz and 80 MHz, or between 80 MHz and 100 MHz. Other FWHM values are also possible. 
     Further, the quality factor of the QWCCR structure  100  (Q QWCCR ) can also take various values in various implementations. For example, the quality factor could be between 25 and 50, between 50 and 75, between 75 and 100, between 100 and 125, between 125 and 150, between 150 and 175, between 175 and 200, between 200 and 300, between 300 and 400, between 400 and 500, between 500 and 600, between 600 and 700, between 700 and 800, between 800 and 900, between 900 and 1000, or between 1000 and 1100. Other quality factor values are also possible. 
     It is understood that, in alternate implementations, alternate structures (for example, alternate quarter-wave structures) can be used to emit electromagnetic radiation and/or excite plasma coronas (for example, other structures that concentrate electric field at specific locations using points or tips with sufficiently small radii). For example, other quarter-wave resonant structures, such as a coaxial-cavity resonator (sometimes referred to as a “coaxial resonator”), a dielectric resonator, a crystal resonator, a ceramic resonator, a surface-acoustic-wave resonator, a yttrium-iron-garnet resonator, a rectangular-waveguide cavity resonator, a parallel-plate resonator, a gap-coupled microstrip resonator, etc. can be used to excite a plasma corona. 
     Further, it is understood that wherever in this disclosure the terms “resonator,” “QWCCR,” “QWCCR structure,” and “coaxial resonator,” are used, any of the structures enumerated in the preceding paragraph could be used, assuming appropriate modifications are made to a corresponding system. In addition, the terms “resonator,” “QWCCR,” “QWCCR structure,” and “coaxial resonator” are not to be construed as inclusive or all-encompassing, but rather as examples of a particular structure that could be included in a particular implementation. Still further, when a “QWCCR structure” is described, the QWCCR structure can correspond to a coaxial resonator, a coaxial resonator with an additional base conductor, a coaxial resonator excited by a signal with a wavelength that corresponds to an odd-integer multiple of one-quarter (¼) of a length of the coaxial resonator, and other structures, in various implementations. 
     Additionally, whenever any “QWCCR,” “QWCCR structure,” “coaxial resonator,” “resonator,” or any of the specific resonators in this disclosure or in the claims are described as being “configured such that, when the resonator is excited by the radio-frequency power source with a signal having a wavelength proximate to an odd-integer multiple of one-quarter (¼) of the resonant wavelength, the resonator provides at least one of a plasma corona or electromagnetic waves,” some or all of the following are contemplated, depending on context. First, the corresponding resonator could be configured to provide a plasma corona when excited by the radio-frequency power source with a signal having a wavelength proximate to an odd-integer multiple of one-quarter (¼) of a resonant wavelength of the resonator. Second, the corresponding resonator could be configured to provide electromagnetic waves when excited by the radio-frequency power source with a signal having a wavelength proximate to an odd-integer multiple of one-quarter (¼) of a resonant wavelength of the resonator. Third, the corresponding resonator could be configured to provide, when excited by the radio-frequency power source with a signal having a wavelength proximate to an odd-integer multiple of one-quarter (¼) of a resonant wavelength of the resonator, both a plasma corona and electromagnetic waves. 
     V. Example Resonator Systems 
     In some implementations, the coaxial resonator  201  can be used as an antenna (for example, instead of or in addition to generating a plasma corona). As an antenna, the coaxial resonator  201  can radiate electromagnetic waves. The electromagnetic waves can consequently influence charged particles. As illustrated in the system  200  of  FIG. 2 , such electromagnetic waves can be radiated when the coaxial resonator  201  is excited by a signal generator  202 . For example, the signal generator  202  can be coupled to the coaxial resonator  201  in order to excite the coaxial resonator  201  (for example, to excite a plasma corona and to produce electromagnetic waves). Such a coupling can include inductive coupling (for example, using an induction feed loop), parallel capacitive coupling (for example, using a parallel plate capacitor), or non-parallel capacitive coupling (for example, using an electric field applied opposite a non-zero voltage conductor end). Further, the electrical distance between the signal generator  202  and the coaxial resonator  201  can be optimized (for example, minimized or adjusted based on wavelength of an RF signal) in order to minimize the amount of energy lost to heating and/or to maximize a quality factor. Further, in some implementations, the coaxial resonator  201  can radiate acoustic waves when excited (for example, at resonance). The acoustic waves produced can induce motion in nearby particles, for example. 
     The signal generator  202  can be a device that produces periodic waveforms (for example, using an oscillator circuit). In various implementations, the signal generator  202  can produce a sinusoidal waveform, a square waveform, a triangular waveform, a pulsed waveform, or a sawtooth waveform. Further, the signal generator  202  can produce waveforms with various frequencies (for example, frequencies between 1 Hz and 1 THz). The electromagnetic waves radiated from the coaxial resonator  201  can be based on the waveform produced by the signal generator  202 . For example, if the waveforms produced by the signal generator  202  are sinusoidal waves having frequencies between 300 MHz and 300 GHz (for example, between 1 GHz and 100 GHz), the electromagnetic waves radiated by coaxial resonator  201  can be microwaves. In various implementations, the signal generator  202  can, itself, be powered by an AC power source or a DC power source. 
     Depending on the signal used by the signal generator  202  to excite the coaxial resonator  201 , the coaxial resonator  201  can additionally excite one or more plasma coronas. For example, if a large enough voltage is used to excite the coaxial resonator  201 , a plasma corona can be excited at the distal end of the electrode  106  (for example, at a concentrator of the electrode  106 ). In some implementations, a voltage step-up device can be electrically coupled between the signal generator  202  and the coaxial resonator  201 . In such scenarios, the voltage step-up device can be operable to increase an amplitude of the AC voltage used to excite the coaxial resonator  201 . 
     In some implementations, the signal generator  202  can include one or more of the following: an internal power supply; an oscillator (for example, an RF oscillator, a surface acoustic wave resonator, or a yttrium-iron-garnet resonator); and an amplifier. The oscillator can generate a time-varying current and/or voltage (for example, using an oscillator circuit). The internal power supply can provide power to the oscillator. In some implementations, the internal power supply can include, for example, a DC battery (for example, a marine battery, an automotive battery, an aircraft battery, etc.), an alternator, a generator, a solar cell, and/or a fuel cell. In other implementations, the internal power supply can include a rectified AC power supply (for example, an electrical connection to a wall socket passed through a rectifier). The amplifier can magnify the power that is output by the oscillator (for example, to provide sufficient power to the coaxial resonator  201  to excite plasma coronas). For example, the amplifier can multiply the current and/or the voltage output by the oscillator. Additionally, in some implementations, the signal generator  202  can include a dedicated controller that executes instructions to control the signal generator  202 . 
     Additionally or alternatively, as illustrated in the system  300  of  FIG. 3A , the coaxial resonator  201  can be electrically coupled (for example, using a wired connection or wirelessly) to a DC power source  302 . Further, in some implementations, an RF cancellation resonator (not shown) can prevent RF power (for example, from the signal generator  202 ) from reaching, and potentially interfering with, the DC power source  302 . The RF cancellation resonator can include resistive elements, lumped-element inductors, and/or a frequency cancellation circuit. 
     In some implementations, the DC power source  302  can include a dedicated controller that executes instructions to control the DC power source  302 . The DC power source  302  can provide a bias signal (for example, corresponding to a DC bias condition) for the coaxial resonator  201 . For example, a DC voltage difference between the inner conductor  104  and the outer conductor  102  of the coaxial resonator  201  in  FIG. 3A  can be established by the DC power source  302  by increasing the DC voltage of the inner conductor  104  and/or decreasing the DC voltage of the outer conductor  102  (given the orientation of the positive terminal and negative terminal of the DC power source  302 ). In other implementations, a DC voltage difference between the inner conductor  104  and the outer conductor  102  can be established by the DC power source  302  by decreasing the DC voltage of the inner conductor  104  and/or increasing the DC voltage of the outer conductor  102  (if the orientation of the positive terminal and negative terminal of the DC power source  302  in  FIG. 3A  were reversed). The bias signal (for example, the voltage of the bias signal and/or the current of the bias signal) output by the DC power source  302  can be adjustable. 
     By providing the coaxial resonator  201  with a bias signal, an increased voltage can be presented at a concentrator of the electrode  106 , thereby yielding an increased electric field at the concentrator of the electrode  106 . The total electric field at the concentrator can thus be a sum of the electric field from the bias signal of the DC power source  302  and the electric field from the signal generator  202  exciting the coaxial resonator  201  at a resonance condition (for example, exciting the coaxial resonator  201  at a quarter-wave resonance condition so the electric field of the signal from the signal generator  202  reaches a maximum at the distal end of the coaxial resonator  201 ). Because of this increased total electric field, an excitation of a plasma corona near the concentrator can be more probable. 
     As an alternative, rather than using a bias signal, the signal generator  202  can simply excite the coaxial resonator  201  using a higher voltage. However, this might use considerably more power than providing a bias signal and augmenting that bias signal with an AC voltage oscillation. 
     In some implementations, the DC power source  302  can be switchable (for example, can generate the bias signal when switched on and not generate the bias signal when switched off). As such, the DC power source  302  can be switched on when a plasma corona output is desired from coaxial resonator  201  and can be switched off when a plasma corona output is not desired from coaxial resonator  201 . For example, the DC power source  302  can be switched on during an ignition sequence (for example, a sequence where fuel is being ignited within a combustion chamber to begin combustion), but switched off during a reforming sequence (for example, a sequence in which electromagnetic radiation is being used to chemically modify fuel). Further, in some implementations, the electric field at the concentrator of the electrode  106  used to initiate the plasma corona can be larger than the electric field at the concentrator used to sustain the plasma corona. Hence, in some implementations, the DC power source  302  can be switched on in order to excite the plasma corona, but switched off while the plasma corona is maintained by the signal from the signal generator  202 . 
     In alternate implementations, the system  200  of  FIG. 2  and/or the system  300  of  FIG. 3A  can include a plurality of coaxial resonators  201 . If the system  200  of  FIG. 2  includes a plurality of coaxial resonators  201 , the plurality of coaxial resonators  201  can each be electrically coupled to the same signal generator (for example, such that each of the plurality of coaxial resonators  201  is excited by the same signal), can each be electrically coupled to a respective signal generator (for example, such that each of the plurality of coaxial resonators  201  is independently excited, thereby allowing for unique excitation frequency, power, etc. for each of the plurality of coaxial resonators  201 ), or one set of the plurality of coaxial resonators  201  can be connected to a common signal generator and another set of the plurality of coaxial resonators  201  can be connected to one or more other signal generators, which could be similar or different from signal generator  202 . In implementations of the system  300  that include a plurality of coaxial resonators  201 , each of the coaxial resonators  201  can be attached to a respective DC power source (for example, multiple instances of DC power source  302 ) and a common signal generator (for example, such that a bias signal can be independently switchable and/or adjustable for each coaxial resonator  201 , while maintaining a common excitation waveform across all coaxial resonators  201  in the system  300 ), different signal generators and a common DC power source (for example, such that a bias signal can be jointly switchable across all coaxial resonators  201  in the system  300 , while maintaining an independent excitation waveform for each coaxial resonator  201 ), or different DC power sources and different signal generators (for example, such that the bias signal is independently switchable for each coaxial resonator  201 , while maintaining an independent excitation waveform for each coaxial resonator  201 ). 
       FIG. 3B  illustrates a circuit diagram of the system  300  of  FIG. 3A , which includes the signal generator  202 , the DC power source  302 , and the coaxial resonator  201  (illustrated in vertical cross-section). As illustrated, similar to the QWCCR structure  100 , the coaxial resonator  201  includes an outer conductor  322 , an inner conductor  324  (including an electrode  326 ), and a dielectric  328 . In addition, when the DC power source  302  is switched off, the circuit illustrated in  FIG. 3B  may not be an open-circuit. Instead, the signal generator  202  can simply be shorted to the inner conductor  324  when the DC power source  302  is switched off. As illustrated, the outer conductor  322  can be electrically coupled to ground. Further, the signal generator  202  and the DC power source  302  can be connected in series, with their negative terminals connected to ground. The positive terminals of the signal generator  202  and the DC power source  302  can be electrically coupled to the inner conductor  324 . Consequently, the electrode  326  can also be electrically coupled to the positive terminals through an electrical coupling between the inner conductor  324  and the electrode  326 . 
     In alternate implementations, the negative terminals of the signal generator  202  and the DC power source  302  can instead be connected to the inner conductor  324  and the positive terminals can be connected to the outer conductor  322 . In this way, the signal generator  202  and the DC power source  302  can instead apply a negative voltage (relative to ground) to the electrode  326  and/or inner conductor  324 , rather than a positive voltage (relative to ground). Further, in some implementations, the negative terminals of the DC power source  302  and the signal generator  202  and/or the inner conductor  324  might not be grounded. 
     As stated above, the DC power source  302  can be switchable. In this way a positive bias signal or a negative bias signal can be selectively applied to the inner conductor  324  and/or the electrode  326  relative to the outer conductor  322 . When the DC power source  302  is switched on, a bias condition can be present, and when the DC power source  302  is switched off, a bias condition might not be present. A bias signal provided by the DC power source  302  can increase the electric potential, and thus the electric field, at the electrode  326  (for example, at a concentrator of the electrode  106 , such as a tip, edge, or blade). By increasing the electric field at the electrode  326 , dielectric breakdown and potentially plasma excitation can be more prevalent. Thus, by switching on the DC power source  302 , the amount of plasma excited at a plasma corona can be enhanced. 
     In some implementations, the voltage of the DC power source  302  can range from +1 kV to +100 kV. Alternatively, the voltage of the DC power source  302  can range from −1 kV to −100 kV. Even further, the voltage of the DC power source  302  can be adjustable in some implementations. Furthermore, the voltage of the DC power source  302  can be pulsed, ramped, etc. For example, the voltage can be adjusted by a controller connected to the DC power source  302 . In such implementations, the voltage of the DC power source  302  can be adjusted by the controller according to sensor data (for example, sensor data corresponding to temperature, pressure, fuel composition, etc.). 
     As illustrated in  FIG. 4A , an example system  400  can include a controller  402 . In various implementations, the controller  402  can include a variety of components. For example, the controller  402  can include a desktop computing device, a laptop computing device, a server computing device (for example, a cloud server), a mobile computing device, a microcontroller (for example, embedded within a control system of a power-generation turbine, an automobile, or an aircraft), and/or a microprocessor. As illustrated, the controller  402  can be communicatively coupled to the signal generator  202 , the DC power source  302 , an impedance sensor  404 , and one or more other sensors  406 . Through the communicative couplings, the controller  402  can receive signals/data from various components of the system  400  and control/provide data to various components of the system  400 . For example, the controller  402  can switch the DC power source  302  in order to provide a time-modulated bias signal to the coaxial resonator  201  (for example, during an ignition sequence within a combustion chamber adjacent to, coupled to, or surrounding the coaxial resonator  201 ). 
     Further, a “communicative coupling,” as presently disclosed, is understood to cover a broad variety of connections between components, based on context. “Communicative couplings” can include direct and/or indirect couplings between components in various implementations. In some implementations, for example, a “communicative coupling” can include an electrical coupling between two (or more) components (for example, a physical connection between the two (or more) components that allows for electrical interaction, such as a direct wired connection used to read a sensor value from a sensor). Additionally or alternatively, a “communicative coupling” can include an electromagnetic coupling between two (or more) components (for example, a connection between the two (or more) components that allows for electromagnetic interaction, such as a wireless interaction based on optical coupling, inductive coupling, capacitive coupling, or coupling though evanescent electric and/or magnetic fields). In addition, a “communicative coupling” can include a connection (for example, over the public internet) in which one or more of the coupled components can transmit signals/data to and/or receive signals/data from one or more of the other coupled components. In various implementations, the “communicative coupling” can be unidirectional (in other words, one component sends signals and another component receives the signals) or bidirectional (in other words, both components send and receive signals). Other directionality combinations are also possible for communicative couplings involving more than two components. One example of a communicative coupling could be the controller  402  communicatively coupled to the coaxial resonator  201 , where the controller  402  reads a voltage and/or current value from the resonator directly. Another example of a communicative coupling could be the controller  402  communicating with a remote server over the public Internet to access a look-up table. Additional communicative couplings are also contemplated in the present disclosure. 
     In some implementations, the controller  402  can control one or more settings of the signal generator  202  (for example, waveform shape, output frequency, output power amplitude, output current amplitude, or output voltage amplitude) or the DC power source  302  (for example, switching on or off or adjusting the level of the bias signal). For example, the controller  402  can control the bias signal of the DC power source  302  (for example, a voltage of the bias signal) based on a calculated voltage used to excite a plasma corona (for example, based on conditions within a combustion chamber). The calculated voltage can account for the voltage amplitude being output by the signal generator  202 , in some implementations. The calculated voltage can ensure, for example, that the bias signal has a small effect on any standing electromagnetic wave formed within the coaxial resonator  201  based on an output of the signal generator  202 . 
     The controller  402  can be located nearby the signal generator  202 , the DC power source  302 , the impedance sensor  404 , and/or the one or more other sensors  406 . For example, the controller  402  may be connected by a wire connection to the signal generator  202 , the DC power source  302 , the impedance sensor  404 , and/or the one or more other sensors  406 . Alternatively, the controller  402  can be remotely located relative to the signal generator  202 , the DC power source  302 , the impedance sensor  404 , and/or the one or more other sensors  406 . For example, the controller  402  can communicate with the signal generator  202 , the DC power source  302 , the impedance sensor  404 , and/or the one or more other sensors  406  over BLUETOOTH®, over BLUETOOTH LOW ENERGY (BLE)®, over the public Internet, over WIFI® (IEEE 802.11 standards), over a wireless wide area network (WWAN), etc. 
     In some implementations, the controller  402  can be communicatively coupled to fewer components within the system  400  (for example, only communicatively coupled to the DC power source  302 ). Further, in implementations that include fewer components than illustrated in the system  400  (for example, in implementations, having only the coaxial resonator  201 , the signal generator  202 , and the controller  402 ), the controller  402  can interact with fewer components of the system  400 . For instance, the controller can interact only with the signal generator  202 . 
     The impedance sensor  404  can be connected to the coaxial resonator  201  (for example, one lead to the inner conductor  324  of the coaxial resonator  201  and one lead to the outer conductor  322  of the coaxial resonator  201 ) to measure an impedance of the coaxial resonator  201 . In some implementations, the impedance sensor  404  can include an oscilloscope, a spectrum analyzer, and/or an AC volt meter. The impedance measured by the impedance sensor  404  can be transmitted to the controller  402  (for example, as a digital signal or an analog signal). In some implementations, the impedance sensor  404  can be integrated with the controller  402  or connected to the controller  402  through a printed circuit board (PCB) or other mechanism. The impedance data can be used by the controller  402  to perform calculations and to adjust control of the signal generator  202  and/or the DC power source  302 . 
     Similarly, the other sensors  406  can also transmit data to the controller  402 . Analogous to the impedance sensor  404 , in some implementations, the other sensors  406  can be integrated with the controller  402  or connected to the controller  402  through a PCB or other mechanism. The other sensors  406  can include a variety of sensors, such as one or more of: a fuel gauge, a tachometer (for example, to measure revolutions per minute (RPM)), an altimeter, a barometer, a thermometer, a sensor that measures fuel composition, a gas chromatograph, a sensor measuring fuel-to-air ratio in a given fuel/air mixture, an anemometer, a torque sensor, a vibrometer, an accelerometer, or a load cell. 
     In some implementations, the controller  402  can be powered by the DC power source  302 . In other implementations, the controller  402  can be independently powered by a separate DC power source or an AC power source (for example, rectified within the controller  402 ). 
     As an example, a possible implementation of the controller  402  is illustrated in  FIG. 4B . As illustrated, the controller  402  can include a processor  452 , a memory  454 , and a network interface  456 . The processor  452 , the memory  454 , and the network interface  456  can be communicatively coupled over a system bus  450 . The system bus  450 , in some implementations, can be defined within a PCB. 
     The processor  452  can include one or more central processing units (CPUs), such as one or more general purpose processors and/or one or more dedicated processors (for example, application-specific integrated circuits (ASICs), digital signal processors (DSPs), or network processors). The processor  452  can be configured to execute instructions (for example, instructions stored within the memory  454 ) to perform various actions. Rather than a processor  452 , some implementations can include hardware logic (for example, one or more resistor-inductor-capacitor (RLC) circuits, flip-flops, latches, etc.) that performs actions (for example, based on the inputs from the impedance sensor  404  or the other sensors  406 ). 
     The memory  454  can store instructions that are executable by the processor  452  to carry out the various methods, processes, or operations presently disclosed. Alternatively, the method, processes, or operations can be defined by hardware, firmware, or any combination of hardware, firmware, or software. Further, the memory  454  can store data related to the signal generator  202  (for example, control signals), the DC power source  302  (for example, switching signals), the impedance sensor  404  (for example, look-up tables related to changes in impedance and/or a characteristic impedance of the coaxial resonator  201  based on certain environmental factors), and/or the other sensors  406  (for example, a look-up table of typical wind speeds based on elevation). 
     The memory  454  can include non-volatile memory. For example, the memory  454  can include a read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), a hard drive (for example, hard disk), and/or a solid-state drive (SSD). Additionally or alternatively, the memory  454  can include volatile memory. For example, the memory  454  can include a random-access memory (RAM), flash memory, dynamic random-access memory (DRAM), and/or static random-access memory (SRAM). In some implementations, the memory  454  can be partially or wholly integrated with the processor  452 . 
     The network interface  456  can enable the controller  402  to communicate with the other components of the system  400  and/or with outside computing device(s). The network interface  456  can include one or more ports (for example, serial ports) and/or an independent network interface controller (for example, an Ethernet controller). In some implementations, the network interface  456  can be communicatively coupled to the impedance sensor  404  or one or more of the other sensors  406 . Additionally or alternatively, the network interface  456  can be communicatively coupled to the signal generator  202 , the DC power source  302 , or an outside computing device (for example, a user device). Communicative couplings between the network interface  456  and other components can be wireless (for example, over WIFI®, BLUETOOTH®, BLUETOOTH LOW ENERGY (BLE)®, or a WWAN) or wireline (for example, over token ring, t-carrier connection, Ethernet, a trace in a PCB, or a wire connection). 
     In some implementations, the controller  402  can also include a user-input device (not shown). For example, the user-input device can include a keyboard, a mouse, a touch screen, etc. Further, in some implementations, the controller  402  can include a display or other user-feedback device (for example, one or more status lights, a speaker, a printer, etc.) (not shown). That status of the controller  402  can alternatively be provided to a user device through the network interface  456 . For example, a user device such as a personal computer or a mobile computing device can communicate with the controller  402  through the network interface  456  to retrieve the values of one or more of the other sensors  406  (for example, to be displayed on a display of the user device). 
     VI. Resonators with Fuel Injection 
     As illustrated in  FIG. 5 , in some implementations, the QWCCR structure  100  (or the coaxial resonator  201 ) can be attached to a fuel tank  502 . The fuel tank  502  can provide a fuel source for a combustion chamber or other environment, for example. The fuel tank  502  can contain or be connected to a fuel pump  504  through a fuel-supply line (for example, a hose or a pipe). The fuel pump  504  can transfer fuel from the fuel tank  502  into the fuel-supply line and propel the fuel through a fuel conduit  506  defined by or disposed within the inner conductor  104  of the QWCCR structure  100 . For example, the fuel pump  504  can include a mechanical pump (for example, gear pump, rotary vane pump, diaphragm pump, screw pump, peristaltic pump) or an electrical pump. In some implementations, the fuel tank  502  can include various sensors (for example, a pressure sensor, a temperature sensor, or a fuel-level sensor). Such sensors can be electrically connected to the controller  402  in order to provide data regarding the status of the fuel tank  502  to the controller  402 , for example. Additionally or alternatively, the fuel pump  504  can be connected to the controller  402 . Through such a connection, the controller  402  could control the fuel pump  504  (for example, to switch the fuel pump on and off, set a fuel injection rate, etc.). 
     In some implementations, the fuel conduit  506  can inject fuel (for example, into a combustion chamber) at one or more outlets  508  defined within the electrode  106  (for example, within a concentrator of the electrode  106 ). By conveying fuel through the fuel conduit  506  and out one or more outlets  508 , fuel can be introduced proximate to a source of ignition energy (for example, proximate to a plasma corona generated near a concentrator of the electrode  106 ), which can allow for efficient combustion and ignition. In alternate implementations, one or more outlets can be defined with other locations of the fuel conduit  506  (for example, so as not to interfere with the electric field at the concentrator of the electrode  106 ). 
     In some implementations, the fuel conduit  506  can act, at least in part, as a Faraday cage (for example, by encapsulating the fuel within a conductor that makes up the fuel conduit  506 ) to prevent electromagnetic radiation in the QWCCR structure  100  from interacting with the fuel while the fuel is transiting the fuel conduit  506 . In other structures, the fuel conduit  506  can allow electromagnetic radiation to interact with (for example, reform) the fuel within the fuel conduit  506 . 
     In some implementations, the QWCCR structure  100  can include multiple fuel conduits  506  (for example, multiple fuel conduits running from the proximal end of the QWCCR structure  100  to the distal end of the QWCCR structure  100 ). Additionally or alternatively, one or more fuel conduits  506  can be positioned within the dielectric  108  or within the outer conductor  102 . As described above, the outlet(s)  508  of the fuel conduit(s)  506  can be oriented in such as a way as to expel fuel toward concentrators (for example, tips, edges, or points) of one or more electrodes  106  (for example, toward regions where plasma coronas are likely to be excited). 
     VII. Additional Resonator Implementations 
       FIG. 6  illustrates a cross-sectional view of an example alternative coaxial resonator  600  connected to a DC power source through an additional resonator assembly acting as an RF attenuator, in accordance with example implementations. The coaxial resonator  600  is an assembly of two quarter-wave coaxial cavity resonators that are coupled together. More specifically, the coaxial resonator  600  includes a first resonator  602  and a second resonator  604  electrically coupled in a series arrangement along a longitudinal axis  606 . In some implementations, the coaxial resonator  600  includes a DC bias condition established at a node of the voltage standing wave (for example, between quarter-wave segments). In such implementations, there may be no impedance mismatch. Because there is no impedance mismatch, the diameters of the inner conductor and the outer conductor of the first resonator  602  can be different than the diameters of the inner conductor and the outer conductor of the second resonator  604 , respectively, without impacting the quality factor (Q). In such a way, the DC bias condition might not affect or interact with the AC signal coming from a signal generator. 
     The first resonator  602  and the second resonator  604  are defined by a common outer conductor wall structure  608 . The outer conductor wall structure  608  includes a first cylindrical wall  610  and a second cylindrical wall  612  centered on the longitudinal axis  606 . The first cylindrical wall  610  is constructed of a conducting material and surrounds a first cylindrical cavity  614  centered on the longitudinal axis  606 . The first cylindrical cavity  614  is filled with a dielectric  616  having a relative dielectric constant approximately equal to four (ε r ≈4), for example. 
     In the example implementation of  FIG. 6 , the first resonator  602  and the second resonator  604  adjoin one another in a connection plane  618  that is perpendicular to the longitudinal axis  606 . In other examples, the connection plane  618  might not be perpendicular to the longitudinal axis  606 , and can instead be designed with a different configuration that maintains constant impedance between the first resonator  602  and the second resonator  604 . 
     The second cylindrical wall  612  is constructed of a conducting material and surrounds a second cylindrical cavity  620  that is also centered on the longitudinal axis  606 . The second cylindrical cavity  620  is coaxial with the first cylindrical cavity  614 , but can have a greater physical length. The second cylindrical wall  612  provides the second cylindrical cavity  620  with a distal end  622  spaced along the longitudinal axis  606  from a proximal end  624  of the second cylindrical cavity  620 . 
     A center conductor structure  626  is supported within the conductor wall structure  608  of the coaxial resonator  600  by the dielectric  616 . The center conductor structure  626  includes a first center conductor  628 , a second center conductor  630 , and a radial conductor  632 . 
     The first center conductor  628  reaches within the first cylindrical cavity  614  along the longitudinal axis  606 . In the example implementation shown in  FIG. 6 , the first center conductor  628  has a proximal end  634  adjacent a proximal end  636  of the first cylindrical cavity  614 , and has a distal end  638  adjacent the distal end  624  of the first cylindrical cavity  614 . The radial conductor  632  projects radially from a location adjacent the distal end  638  of the first center conductor  628 , across the first cylindrical cavity  614 , and outward through an aperture  640 . 
     The second center conductor  630  has a proximal end  642  at the distal end  638  of the first center conductor  628 . The second center conductor  630  projects along the longitudinal axis  606  to a distal end  644  configured as an electrode tip located at or in close proximity to the distal end  622  of the second cylindrical cavity  620 . 
     To reduce any mismatch in impedances between the first resonator  602  and the second resonator  604 , the relative radial thicknesses between both the cylindrical walls  610 ,  612  and the respective center conductors  628 ,  630  are defined in relation to the relative dielectric constant of the dielectric  616  and the dielectric constant of the air or gas that fills the second cylindrical cavity  620 . In the example implementation of  FIG. 6 , the physical length of the second center conductor  630  along the longitudinal axis  606  is approximately twice the physical length of the first center conductor  628  along the longitudinal axis  606 . However, based at least in part on the dielectric  616  having a relative dielectric constant approximately equal to four, the electrical lengths of the two center conductors  628  and  630  are approximately equal. 
     In example implementations, any gaps between any of the center conductors  628 ,  630  and any outer conductor could be filled with a dielectric and/or the gap (for example, the second cylindrical cavity  620 ) could be large enough to reduce arcing (in other words, large enough such that the electric field is not of sufficient intensity to result in a dielectric breakdown of air or the intervening dielectric). As further shown in  FIG. 6 , the dielectric  616  fills the first cylindrical cavity  614  around the first center conductor  628  and the radial conductor  632 . 
     In the illustrated example, a DC power source  646  is connected to the center conductor structure  626  through the radial conductor  632  connected adjacent to a virtual short-circuit point of the DC power source  646 . 
     An RF control component, specifically, an RF frequency cancellation resonator assembly  648  is disposed between the radial conductor  632  and the DC power source  646  to restrict RF power from reaching the DC power source  646 . The RF frequency cancellation resonator assembly  648  is an additional resonator assembly having a center conductor  650 . The center conductor  650  has a first portion  652  and a second portion  654 , each of which has the same electrical length “X” illustrated in  FIG. 6  (and the same electrical length as the first center conductor  628  and the second center conductor  630 ). 
     In an example implementation, the electrical length “X” depicted in  FIG. 6  can be sized such that the center conductor  650  is an odd-integer multiple of half wavelengths (for example, 
     
       
         
           
             
               
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     etc.) out of phase (in other words, 180° out of phase) with the outer conducting wall  656  and the outer conducting wall  658 , simultaneously, where λ 0  is the resonant wavelength, and where the resonant wavelength λ 0  is inversely related to the frequency of the RF power. In alternative implementations, a similar “folded” structure to the electrical length “X” could be located within the cylindrical cavity  614  to achieve a similar phase shift between the inner conductor and the outer conductor. 
     The RF frequency cancellation resonator assembly  648  also has a short outer conducting wall  656  and a long outer conducting wall  658 . The short outer conducting wall  656  has first and second ends on opposite ends of the RF frequency cancellation resonator assembly  648 . The long outer conducting wall  658  also has first and second ends on opposite ends of the RF frequency cancellation resonator assembly  648 . The first and second ends of the short outer conducting wall  656  are each on the opposite side of the RF frequency cancellation resonator assembly  648  from the corresponding first and second ends of the long outer conducting wall  658 . 
     In an example implementation, the difference in electrical length between the short outer conducting wall  656  and the long outer conducting wall  658  is substantially equal to the combined electrical length of the first portion  652  and the second portion  654 . In this example, the combined electrical length of the first portion  652  and the second portion  654  is substantially equal to twice the electrical length of the first center conductor  628 . 
     In an example implementation, the short outer conducting wall  656  and the long outer conducting wall  658  surround a cavity  660  filled with a dielectric. In operation, with this example implementation, electric current running along the outer conductor of the RF frequency cancellation resonator assembly  648  primarily follows the shortest path and run along the short outer conducting wall  656 . Accordingly, electric current on the outer conductor of the RF frequency cancellation resonator assembly  648  travels two fewer quarter-wavelengths than current running along the center conductor  650  of the RF frequency cancellation resonator assembly  648 . 
     In examples, the RF frequency cancellation resonator assembly  648  can also have an internal conducting ground plane  662  disposed within the cavity  660  and between the first portion  652  and the second portion  654  of the center conductor  650 . Based on the geometry of the cancellation resonator assembly  648 , this configuration provides a frequency cancellation circuit connected between the DC power source  646  and the radial conductor  632 . 
     Further, in examples, the RF frequency cancellation resonator assembly  648  is configured to shift a voltage supply of RF energy 180 degrees out of phase relative to the ground plane  662  of the coaxial resonator  600  due to the difference in electrical length between the short outer conducting wall  656  and the center conductor  650  of the RF frequency cancellation resonator assembly  648 . 
       FIG. 7  illustrates a cross-sectional view of another example alternative coaxial resonator  700  connected to a DC power source through an additional resonator assembly acting as an RF attenuator, in accordance with an example implementation. The coaxial resonator  700  includes a first resonator portion  702  and a second resonator portion  704  electrically coupled in a series arrangement along a longitudinal axis  706 . 
     As depicted in  FIG. 7 , the first resonator portion  702  and the second resonator portion  704  are defined by a common outer conductor wall structure  708 . The wall structure  708  includes a first cylindrical wall portion  710  and a second cylindrical wall portion  712  centered on the longitudinal axis  706 . The first cylindrical wall portion  710  is constructed of a conducting material and surrounds a first cylindrical cavity  714  centered on the longitudinal axis  706 . In this example implementation, the first cylindrical cavity  714  is filled with a dielectric  716 . 
     An annular edge  718  of the first cylindrical wall portion  710  defines a proximal end  720  of the first cylindrical cavity  714 . A proximal end of the second cylindrical wall portion  712  adjoins a distal end  722  of the first cylindrical cavity  714 . 
     The coaxial resonator  700  further includes a first center conductor portion  724  and a second center conductor portion  726  (the center conductor portions  724 ,  726  represented by the densest cross-hatching in  FIG. 7 ). For illustration, the first center conductor portion  724  and the second center conductor portion  726  are separated by the vertical dashed line in  FIG. 7 . In some implementations, both the first center conductor portion  724  and the second center conductor portion  726  can correspond to an odd-integer multiple of quarter wavelengths based on the frequency of an RF power source used to excite the coaxial resonator  700 . The second center conductor portion  726  has a proximal end  728  adjoining a distal end  730  of the first center conductor portion  724 . The second center conductor portion  726  projects along the longitudinal axis  706  to a distal end configured as a concentrator  732  (for example, a tip) of an electrode located at or in close proximity to a distal end  734  of a second cylindrical cavity  736 . 
     The coaxial resonator  700  has an aperture  738  that reaches radially outward through the first cylindrical wall portion  710 . A radial conductor  740  extends out through the aperture  738  from the longitudinal axis  706  to be connected to an RF power source (for example, the signal generator  202 ) by an RF power input line. The end of the radial conductor  740  that is closer to the longitudinal axis  706  connects to a parallel plate capacitor  742  that is in a coupling arrangement to a center conductor structure  744 . The parallel plate capacitor  742  is also in a coupling arrangement to an inline folded RF attenuator  746 . The spacing between the parallel plate capacitor  742  and the center conductor structure  744  can depend on the materials used for fabrication (for example, the materials used to fabricate the parallel plate capacitor  742 , the center conductor structure  744 , and/or the dielectric  716 ). 
     In an example, the DC power source  646  described above is connected to the center conductor structure  744  at a proximal end  748  of the center conductor structure  744  with a DC power input line. The inline folded RF attenuator  746  is disposed between the second resonator portion  704  and the DC power source  646  to restrict RF power from reaching the DC power source  646 . 
     The inline folded RF attenuator  746  includes an interior center conductor portion  750  having a proximal end  752  and a distal end  754 . The inline folded RF attenuator  746  also includes an exterior center conductor portion  756  and a transition center conductor portion  758  that connects or couples the interior center conductor portion  750  and the exterior center conductor portion  756 . 
     The exterior center conductor portion  756  has a proximal end largely in the same plane as the proximal end  752 , and a distal end largely in the same plane as the distal end  754 . For example, in the cross-sectional illustration of  FIG. 7 , the plane of the proximal end  752  and the plane of the proximal end of the exterior center conductor portion  756  can be the plane of the cross-section that is illustrated. In this example implementation, the transition center conductor portion  758  is located proximal to the distal end  754 . The exterior center conductor portion  756  surrounds the interior center conductor portion  750 . 
     In this example, the exterior center conductor portion  756  resembles a cylindrical portion of conducting material surrounding the rest of the interior center conductor portion  750 . The longitudinal lengths of the interior center conductor portion  750  and the exterior center conductor portion  756  are substantially equal to the longitudinal length of the parallel plate capacitor  742  with which they are in a coupling arrangement. The electrical length between the proximal end  752  to the distal end  754 , for both the interior center conductor portion  750  and the exterior center conductor portion  756 , is substantially equal to one quarter-wavelength. The second center conductor portion  726  and the second cylindrical wall portion  712  are both configured to have an electrical length of one quarter-wavelength. 
     The wall structure  708  includes a short outer conducting portion  760  which has a proximal end largely in the same plane as the proximal end  752 , and a distal end largely in the same plane as the distal end  754 . An outer conducting path runs from the distal end of the wall structure  708  (that is substantially coplanar with the distal end  734  of the second cylindrical cavity  736 ), along the short outer conducting portion  760 , and stops at the proximal end  720  of the first cylindrical wall portion  710 . In this example, the outer conducting path has an electrical length of two quarter-wavelengths. 
     An inner conducting path runs from the concentrator  732  to the proximal end  728  of the second center conductor portion  726 , along the outside of the transition center conductor portion  758 , then along the outside from the distal end to the proximal end of the exterior center conductor portion  756 , then along an interior wall  762  of the exterior center conductor portion  756  from its proximal end to its distal end, then along the interior center conductor portion  750  from its distal end to its proximal end. In this example, the electrical length of this inner conducting path is four quarter-wavelengths, or two half wavelengths. The difference in electrical lengths between the inner conducting path and the outer conducting path is one half wavelength. 
     With this configuration, the inline folded RF attenuator  746  operates as a radio-frequency control component connected between the DC power source  646  and the voltage supply of RF energy. The inline folded RF attenuator  746  is configured to shift a voltage supply of RF energy 180 degrees out of phase relative to the ground plane of the coaxial resonator  700 . 
     The particular arrangement depicted in  FIG. 7  is not limiting with respect to the orientation of the inline folded RF attenuator  746 . In other examples, the entire arrangement depicted in  FIG. 7  can be “stretched,” with the inline folded RF attenuator  746  being disposed further away from the concentrator  732  and not directly coupled to the parallel plate capacitor  742 . For example, the inline folded RF attenuator  746  could be separated by one quarter-wavelength from the portion of the center conductor that would remain in direct coupling arrangement with the parallel plate capacitor  742 . The coaxial resonator  700  can achieve a maximize efficiency when (i) the inline folded RF attenuator  746  is an odd-integer multiple of quarter wavelengths from the concentrator  732 ; and (ii) the inline folded RF attenuator  746  is an odd-integer multiple of quarter wavelengths in electrical length. 
     In another example, the arrangement depicted in  FIG. 7  could be more compressed, with the exterior center conductor portions  756  of the inline folded RF attenuator  746  extending longitudinally as far as the parallel plate capacitor  742  and also surrounding the portion of center conductor exposed for plasma creation. This can be implemented by arranging the center conductor structure  744  in the middle so that the exterior center conductor portions  756  extends in either direction longitudinally. Any particular geometry of this arrangement can involve adjusting the various parameters of dielectrics to ensure impedance matching and full 180 degree phase cancellation. 
     In one example, the arrangements described with respect to  FIGS. 6 and 7  and the particular combination of components that provide the RF signal to the coaxial resonators are contained in a body dimensioned approximately the size of a gap spark igniter and adapted to mate with a combustor (for example, of an internal combustion engine). As an example for illustration, a microwave amplifier could be disposed at the resonator, and the resonator could be used as the frequency determining element in an oscillator amplifier arrangement. The amplifier/oscillator could be attached at the top or back of an igniter, and could have the high voltage supply also integrated in the module with diagnostics. This example permits the use of a single, low-voltage DC power supply for feeding the module along with a timing signal. 
     VIII. Jet Engines 
     The above coaxial resonators could be usefully employed in the context of a gas turbine such as a jet turbine configured to power an aircraft. For example, a coaxial cavity resonator similar to the coaxial resonator  201  illustrated in  FIG. 2  could be used in a gas turbine. While reference is made to “QWCCR,” “QWCCR structure,” and “coaxial resonator” elsewhere in the description, it will be understood that other types of resonators are possible and contemplated. 
     An example gas turbine includes a compressor coupled to a turbine through a shaft, and the gas turbine also includes a combustion chamber or area, called a combustor. In operation, atmospheric air flows through a compressor that brings the air to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature, high-pressure gas flow. The high-temperature, high-pressure gas enters a turbine, where it expands down to an exhaust pressure, producing a shaft work output at the shaft coupled to the turbine in the process. 
     The shaft work output is used to drive the compressor and other devices (for example, an electric generator) that can be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust gases that can include a high temperature and/or a high velocity. Gas turbines can be utilized to power aircraft, trains, ships, electrical generators, pumps, gas compressors, and tanks, among other machines. 
       FIG. 8  illustrates an aircraft  800  having a jet engine  802 , according to example implementations. To help propel the aircraft  800  through the air, the aircraft  800  includes a propulsion system operable to generate thrust. The jet engine  802  is a gas turbine engine that is part of the propulsion system of the aircraft  800 . The aircraft  800  can include several jet engines (for example, 2 or 4) similar to the jet engine  802  coupled to wings of the aircraft  800 , for example. The jet engine  802  includes several components of a gas turbine such as the compressor, the combustor, and the turbine. 
       FIG. 9  illustrates several components of the jet engine  802 , according to an example implementation. As illustrated, the jet engine  802  is configured as a gas turbine engine. Large amounts of surrounding air (free stream) are continuously brought into an inlet or intake  900 . At the rear of the intake  900 , the air enters a compressor  902  (axial, centrifugal, or both). The compressor  902  operates as many rows of airfoils, with each row producing an increase in pressure. At the exit of the compressor  902 , the air is at a much higher pressure than free stream at the intake  900 . 
     Fuel is mixed with the compressed air exiting the compressor  902 , and the fuel-compressed air mixture is burned in a combustor  904 , generating a flow of hot, high pressure gas. The hot, high pressure gas exiting the combustor  904  then passes through a turbine  906 , which extracts energy from the flow of gas by making turbine blades spin in the flow. The energy extracted by the turbine  906  is then used to turn the compressor  902  by coupling the compressor  902  and the turbine  906  by a central shaft  908 . 
     The turbine  906  transforms or converts some energy of the hot gas to drive the compressor  902 , but there is enough energy left over to provide thrust to the jet engine  802  by increasing velocity of the flow of gas through a nozzle  910  disposed adjacent the turbine  906 . Because the exit velocity is greater than the free stream velocity, thrust is created and the aircraft  800  is propelled. 
     Several variations could be made to the jet engine  802 . For instance, the jet engine  802  could be configured as a turbofan engine or a turboprop engine where additional components are added to the several components illustrated in  FIG. 9 . 
     The combustor  904 , which can also be referred to as a burner, combustion chamber, or flame holder, comprises the area of the jet engine  802  where combustion takes place. The combustor  904  is configured to contain and maintain stable combustion despite high air flow rates. As such, in examples, the combustor  904  is configured to mix the air and fuel, ignite the air-fuel mixture, and then mix in more air to complete the combustion process. 
       FIGS. 10A, 10B, 10C, 10D, 10E, 10F . illustrate example types of combustors, according to example implementations. In particular,  FIG. 10A  illustrates a partial perspective view of an annular combustor  1000 , and  FIG. 10B  illustrates a partial frontal view of the annular combustor  1000 .  FIG. 10C  illustrates a partial perspective view of a tubular or can combustor  1002 , and  FIG. 10D  illustrates a partial frontal view of the can combustor  1002 .  FIG. 10E  illustrates a partial perspective view of a can-annular combustor  1004 , and  FIG. 10F  illustrates a partial frontal view of the can-annular combustor  1004 . 
     The annular combustor  1000  shown in  FIGS. 10A-10B  has an annular cross section and has a liner sitting inside an outer casing, which has been peeled open in  FIG. 10A  for illustration. The annular combustor  1000  does not define separate combustion zones, but rather has a continuous liner and casing forming a ring  1006  (the annulus). 
     The can combustor  1002  shown in  FIGS. 10C and 10D  includes multiple combustion cans such as combustion cans  1008 ,  1010 , and  1012  arranged in a radial array about a central shaft. Each combustion can is a self-contained cylindrical combustion chamber that has both a liner and a casing. Each combustion can has its own fuel injector, igniter, liner, and casing. The primary air from the compressor  902  is guided into each individual combustion can, where it is decelerated, mixed with fuel, and then ignited. Secondary air also comes from the compressor  902 , where it is fed outside of the liner. The secondary air is then fed, for example, through slits in the liner, into the combustion zone to cool the liner using thin film cooling. 
     In example implementations, multiple combustion cans are arranged around the jet engine  802 , and their shared exhaust is fed to the turbine  906 . However, the can combustor  1002  can weigh more than other combustor configurations and can be characterized by higher pressure drop across the combustion cans than other combustor configurations. 
     The can-annular combustor  1004  shown in  FIGS. 10E-10F  includes an annular casing  1014  and can-shaped liners, such as liner  1016 . The can-annular combustor  1004  has discrete combustion zones contained in separate liners with their own fuel injectors. Unlike the can combustor  1002 , the combustion zones of the can-annular combustor  1004  share a common ring (annulus) casing (for example, annular casing  1014 ). Each combustion zone of the can-annular combustor  1004  does not operate as a separate pressure vessel; rather, the combustion zones “communicate” with each other through liner holes or connecting tubes that allow some air to flow circumferentially between the combustion zones. Further, rather than having separate igniters for each combustion can, once combustion takes place in one or two combustion cans of the can-annular combustor  1004  cans, combustion could spread to and ignite the other combustion cans due to communication between the combustion zones through the liner holes or connecting tubes. 
     Regardless of the type of combustor, the combustion process inside the combustor  904  can determine, at least partially, many of the operating characteristics of the jet engine  802 , such as fuel efficiency, levels of emissions, and transient response (the response to changing conditions such a fuel flow and air speed). Further, also regardless of the type of combustor, the combustor  904  has several components that can be used, and these several components are described below. 
       FIG. 11  illustrates a schematic diagram of a partial view of the combustor  904 , according to an example implementation. The combustor  904  includes a casing  1100  that is configured as an outer shell of the combustor  904 . The casing  1100  can be protected from thermal loads by the air flowing in it, and can operate as a pressure vessel that withstands the difference between the high pressures inside the combustor  904  and the lower pressure outside the combustor  904 . 
     The combustor  904  also includes a diffuser  1102  that is configured to slow the high speed, highly compressed air from the compressor  902  to a velocity optimal for the combustor  904 . Reducing the velocity results in a loss in total pressure, and the diffuser  1102  is configured to limit such loss of pressure. The diffuser  1102  is also configured to limit flow distortion by avoiding flow effects like boundary layer separation. 
     The combustor  904  further includes a liner  1104  that contains the combustion process and is configured to withstand extended high temperature cycles, and therefore can be made from superalloys. Furthermore, the liner  1104  is cooled with air flow. In some example implementations, in addition to air cooling, the combustor  904  can include thermal barrier coatings to further cool the liner  1104 . 
       FIG. 12  illustrates air flow paths through the combustor  904 , according to an example implementation. Primary air is the main combustion air and is highly compressed air from the compressor  902 . The primary air can be decelerated using the diffuser  1102  and is fed through primary air holes  1200 . This air is mixed with fuel, and then combusted in a combustion zone  1202 . 
     Intermediate air is the air injected into the combustion zone  1202  through intermediate air holes  1204 . The air injected through the intermediate air holes  1204  completes the combustion processes, cooling the air down and diluting concentrations of carbon monoxide (CO) and hydrogen (H 2 ). 
     Dilution air is air injected through dilution air holes  1206  in the liner  1104  at the end of the combustion zone  1202  to help cool the air to before it reaches the turbine  906 . The dilution air can be used to produce the uniform temperature profile desired in the combustor  904 . 
     Cooling air is air that is injected through cooling air holes  1208  in the liner  1104  to generate a layer (film) of cool air to protect the liner  1104  from the high combustion temperatures. The combustor  904  is configured such that the cooling air does not directly interact with the combustion air and combustion process. 
     Referring back to  FIG. 11 , the combustor  904  further includes a snout  1106 , which is an extension of a dome  1108 . The snout  1106  operates as an air splitter, separating the primary air from the secondary air flows (intermediate, dilution, and cooling air). 
     The dome  1108  and a swirler  1110  are the components of the combustor  904  through which the primary air flows as it enters the combustion zone  1202 . The dome  1108  and the swirler  1110  are configured to generate turbulence in the flow to rapidly mix the air with fuel. The swirler  1110  establishes a local low pressure zone that forces some of the combustion products to recirculate, creating high turbulence. However, the higher the turbulence, the higher the pressure loss is for the combustor  904 , so the dome  1108  and the swirler  1110  are configured to not generate more turbulence than is sufficient to mix the fuel and air. In some examples, with the resonators disclosed in the present disclosure, the combustor  904  can be configured without the dome  1108  and the swirler  1110 . In other examples, the dome  1108  and the swirler  1110  can be made smaller when the combustor resonators disclosed in the present disclosure are used because the flame front propagation can be faster than when a conventional igniter is used. 
     The combustor  904  further includes a fuel injector  1112  configured to introduce fuel to the combustion zone  1202  and, along with the swirler  1110 , is configured to mix the fuel and air. The fuel injector  1112  can be configured as any of several types of fuel injectors including: pressure-atomizing, air blast, vaporizing, and premix/prevaporizing injectors. 
     Pressure atomizing fuel injectors rely on high fuel pressures (as much as 1200 pounds per square inch (psi)) to atomize the fuel. When using this type of fuel injector, the fuel system is configured to be sufficiently robust to withstand such high pressures. The fuel tends to be heterogeneously atomized, resulting in incomplete or uneven combustion, which generates pollutants and smoke. 
     The air-blast injector “blasts” fuel with a stream of air, atomizing the fuel into homogeneous droplets, and can cause the combustor  904  to be smokeless. This air blast injector can operate at lower fuel pressures than the pressure atomizing fuel injector. 
     The vaporizing fuel injector is similar to the air-blast injector in that the primary air is mixed with the fuel as it is injected into the combustion zone  1202 . However, with the vaporizing fuel injector the fuel-air mixture travels through a tube within the combustion zone  1202 . Heat from the combustion zone  1202  is transferred to the fuel-air mixture, vaporizing some of the fuel to enhance the mixing before the mixture is combusted. This way, the fuel is combusted with low thermal radiation, which helps protect the liner  1104 . However, the vaporizer tube can have low durability because of the low fuel flow rate within it causing the tube to be less protected from the combustion heat. 
     The premixing/prevaporizing injector is configured to mix or vaporize the fuel before it reaches the combustion zone  1202 . This way, the fuel is uniformly mixed with the air, and emissions from the jet engine  802  can be reduced. However, fuel can auto-ignite or otherwise combust before the fuel-air mixture reaches the combustion zone  1202 , and the combustor  904  can thus be damaged. 
     In some example implementations, a resonator could be configured with fuel passages disposed within the resonator, such that the resonator integrates operations of the fuel injector  1112  with operations of an igniter described below. In these examples, the resonator could be configured to perform the atomization and vaporization of the fuel in addition to mixing and preparing the fuel for combustion. The fuel would then be passed through a formed plasma to ensure ignition. Further, the presence of electromagnetic waves radiated by the resonator could be used to energize the air-fuel mixture and stimulate combustion. 
     The combustor  904  also includes an igniter  1114  configured to ignite air-fuel mixture to cause combustion. In examples, the igniter  1114  can be configured as an electrical spark igniter, similar to an automotive spark plug. However, there are several disadvantages to such configuration as described below. The igniter  1114  is disposed proximate to the combustion zone  1202  where the fuel and air are already mixed, but is located upstream from the combustion location so that it is not damaged by the combustion itself. In example implementations, once combustion is initially started by the igniter  1114 , the combustion is self-sustaining and the igniter  1114  is no longer used. In the annular combustor  1000  and the can-annular combustor  1004 , the flame can propagate from one combustion zone to another, so igniters might not be used at each combustion zone. 
     However, in some examples, combustion can stop due to operating conditions that are not favorable to sustaining combustion. For example, the aircraft  800  can operate at high altitude with low air density, which might affect combustion. In another example, a speed of the aircraft  800  can be sufficiently low to stop the combustion process. Other operating conditions could cause the combustion to stop. In these examples, the igniter  1114  could also be used to restart combustion. 
     In some systems, ignition-assisting techniques can be used to restart combustion. One such method is oxygen injection, where oxygen is fed to the ignition area, helping the fuel to easily combust. This is particularly useful in some aircraft applications where the jet engine  802  may have to restart at high altitude. Further, described in the present disclosure are igniters and systems that could lower the probability of stopping and having to restart combustion. Particularly, the igniter  1114  could be configured as any of the resonators described in the present disclosure to enhance combustion. In some examples, if the igniter  1114  is configured as a coaxial resonator, the coaxial resonator could be used as a sensor to obtain real-time measurements of the conditions inside the combustor  904  and could be used to predict when combustion would stop (for example, when a flameout would occur). Once such a prediction is made, flameout can be precluded (or its likelihood reduced) by proactively performing operations such as adding more fuel, providing additional plasma, and/or increasing compression using the compressor  902 , among other possible operations. 
     In some example implementations of the jet engine  802 , combustion can take place in locations within the jet engine  802  other than the combustor  904 . For example, in order for an aircraft to fly faster than the speed of sound, the aircraft needs to generate a high thrust to overcome a sharp rise in drag near the speed of sound. To achieve such high thrust, an afterburner can be added to the jet engine. The afterburner can be considered another type of combustor. 
       FIG. 13  illustrates the jet engine  802  including an afterburner  1300  downstream of the turbine  906 , in accordance with an example implementation. As described above with respect to  FIG. 9 , some of the energy of the exhaust gas from the combustor  904  is used to turn the turbine  906 . The afterburner  1300  is used to add energy to generate more thrust by injecting fuel directly into the hot exhaust gas exiting the turbine  906 . 
     The nozzle  910  of the jet engine  802 , as illustrated in  FIG. 13 , is extended or moved downstream in the jet engine  802  to enable placing flame holders  1302  between the turbine  906  and the exit of the jet engine  802 . As shown in  FIG. 13 , the flame holders  1302  can include multiple hoops, such as hoops  1304 ,  1306 . In another arrangement, the flame holders  1302  can include multiple parallel gutters that extend across an afterburner channel  1308  and perpendicular to the engine axis. In yet another arrangement, the flame holders  1302  can include multiple gutters extending radially from the internal surface of the afterburner channel  1308  in a star pattern with respect to the engine axis. The gutters of the flame holders  1302  can be configured with a u- or v-shaped cross section that is open on a downstream side of the gutter. The flame holders  1302  provide a zone of low velocity air so as to retain gases during their combustion in the afterburner channel  1308 . 
     In some examples, when the afterburner  1300  is turned on, additional fuel is injected through, between, or around the flame holders  1302  and into the gas exiting the turbine  906 . In other examples, fuel is injected in the afterburner  1300  upstream of the flame holders  1302 . The fuel burns and produces additional thrust. 
     After passing the turbine  906 , the gas from the turbine  906  expands, thus losing temperature. The gas from the turbine  906  is an input gas to the afterburner  1300 . Fuel is injected into the input gas from the turbine  906  to produce a fuel-air mixture within an afterburner channel  1308 . Combustion of the fuel within the fuel-air mixture within the afterburner channel  1308  results in an exhaust gas from the afterburner  1300  having a temperature and pressure greater than a temperature and pressure, respectively, of the gas from the turbine  906 . The exhaust gas resulting from combustion within the afterburner channel  1308  passes through the nozzle  910  at a higher velocity, thereby generating additional thrust. 
     In some examples, ignition within the afterburner  1300  may be hard to achieve. In particular, because velocities and temperatures do not substantially change at the inlet of the afterburner  1300 , ignition in the afterburner  1300  may be difficult to achieve when the aircraft  800  is flying at high altitudes. The difficulty is associated with the low pressure in the afterburner  1300  that affects ignition directly. Therefore, it can be desirable to have a system that better prepares the fuel for easier ignition in the afterburner  1300  at higher altitude. 
     Further, the exhaust gas from the turbine  906  that enters the afterburner  1300  has reduced oxygen and is not highly compressed due to previous combustion at the combustor  904 . Therefore, combustion in the afterburner  1300  is generally fuel-inefficient compared with combustion in the combustor  904 . Thus, the afterburner  1300  increases thrust at the cost of increased fuel inefficiency, thereby limiting its practical use to short bursts or intermittent operation. As such, the afterburner  1300  is turned on selectively when the extra thrust is used, but is otherwise turned off. It can thus be desirable to have an afterburner that is more efficient to enable using the afterburner more often and more efficiently to enable persistent, as opposed to intermittent operation. 
     The combustion taking place at the combustor  904  and the combustion taking place in the afterburner  1300  of the jet engine  802  can affect many of the operating characteristics of the jet engine  802 . As examples, combustion determines fuel efficiency, thrust levels, and levels of emissions and transient response (the response to changing conditions such a fuel flow and air speed). It can thus be desirable to have an ignition system that prepares the fuel for efficient and thorough combustion, facilitates starting and restarting ignition when desired regardless of altitude, and enables combustion of a lean fuel mixture at high compression ratios to increase efficiency. 
     IX. Example Fuel Injection Through a Dielectric of a Resonator 
     As discussed above, in a jet engine, it may be desirable to expose fuel (or a fuel mixture) to electromagnetic waves before ignition in order to reform the fuel and/or alter an energy state of the fuel. Doing so can help conserve energy during ignition and/or provide other advantages. 
     One manner of accomplishing this can be configuring a resonator to operate as a fuel injection source—particularly, by having fuel (or fuel mixtures) flow through the dielectric of the resonator, thereby exposing the fuel to electromagnetic waves as the fuel passes through the dielectric. Configuring a resonator in this manner can provide additional benefits as well, such as enabling use of the resonator both as an ignition source and as an alternative to a separate fuel injector device. Further, while fuel injection through the dielectric may be desired to achieve leaner fuel/air mixtures, the configurations and methods for fuel injection described in the present disclosure can be applied in scenarios in which a richer fuel/air mixture is desired. In any event, the resonator can also excite a plasma corona to ignite the fuel/air mixture. 
     To facilitate fuel injection through the resonator&#39;s dielectric, the resonator can include a fuel conduit through which to inject fuel into a combustion chamber of the jet engine. In some implementations, at least a portion of the fuel conduit can be proximate to the dielectric that is between the inner and outer conductors. Without limitation, the fuel conduit being proximate to the dielectric can include the fuel conduit being defined by the dielectric, the fuel conduit being arranged within the dielectric, and/or the fuel conduit being arranged along the dielectric. Further, at least a portion of the fuel conduit can include one or more channels through which the fuel can flow, and thus the dielectric can define the one or more channels, the one or more channels can be arranged within the dielectric, and/or the one or more channels can be arranged along the dielectric. 
     In line with the discussion above, each fuel conduit channel can have a particular shape, such as a linear shape, rifled shape, curved shape, etc. As an example in which one or more channels are defined by the dielectric, the dielectric can be a porous ceramic material (or a combination of multiple different pieces of a porous ceramic material assembled together) that defines a variety of linear or non-linear channels having similar or varying dimensions. As another example, the dielectric can include an air cavity between the inner and outer conductors, and the air cavity can serve as a channel through which fuel can flow. Thus, in essence, the shape of the air cavity can define the shape of the channel, and the air cavity itself can act as a portion of the fuel conduit. For instance, fuel can be injected into a portion of the fuel conduit and expelled into the air cavity, after which the fuel can flow through the air cavity towards a distal end of the resonator. 
     Furthermore, as an example in which one or more channels are arranged within the dielectric, the dielectric can be machined so as to define a channel within the dielectric. As a more particular example of this, multiple pieces of a dielectric material such as ceramic can each be machined such that, when the pieces are combined, channels are formed within the combination. As another example, at least a portion of the fuel conduit can include a thin tubing, and that tubing can be disposed within the dielectric. For instance, a fuel conduit in the form of a polyamide plastic tubing can be disposed within a ceramic material. 
     Moreover, as an example in which one or more channels are arranged along the dielectric, a channel can be included adjacent to the dielectric. For instance, a tubing can be coupled to the inner and/or outer conductor, with at least a portion of the tubing running parallel to the longitudinal axis of the resonator. A channel arranged along the dielectric can be directly adjacent to the dielectric and run alongside the dielectric. 
     In some implementations, a first portion of the fuel conduit can be arranged within, arranged along, and/or defined by, the inner conductor and/or the outer conductor, and a second portion of the fuel conduit can be arranged within, arranged along, and/or defined by, the dielectric. As so arranged, the first portion of the fuel conduit can be configured to provide fuel into the second portion. In other implementations, the dielectric between the inner and outer conductors can include multiple dielectrics, and each of the multiple dielectrics can include a portion of a fuel conduit. In operation with this arrangement, upon receipt of fuel into the resonator, the fuel can be moved through a first portion of a fuel conduit defined by a ceramic material, and then moved into a second portion of the fuel conduit defined by a different dielectric, such as air. Alternatively, upon receipt of fuel into the resonator, the fuel can be moved through a first portion of a fuel conduit defined by a porous ceramic material, and then moved into a second portion of the fuel conduit defined by another porous ceramic material having a different porosity (a higher porosity, for instance) than the first portion. 
     Furthermore, the fuel conduit can include one or more fuel inlets located at a proximal end of the fuel conduit and into which the fuel conduit receives fuel. The fuel conduit can also include one or more fuel outlets configured to expel the fuel. The location and orientation of a given fuel outlet can be selected based on the direction where the fuel is desired. As discussed above, for example, a fuel outlet can be oriented in such as a way as to expel fuel toward a concentrator of the resonator&#39;s electrode. That way, when the resonator is exciting a plasma corona proximate to the concentrator, the plasma corona can ignite the fuel that is being expelled from the fuel outlet towards the plasma corona. As another example, the fuel outlet can be oriented to expel fuel toward another portion of the fuel conduit. For instance, the fuel outlet can be an outlet of a fuel conduit disposed within a non-porous ceramic material and can be configured to expel fuel into a porous ceramic material that defines one or more additional channels of the fuel conduit through which the fuel can then flow. 
     In some implementations, one or more fuel outlets can be arranged in an annular pattern and can be configured to expel fuel in a radial pattern towards the inner conductor and/or towards the outer conductor. In these implementations, the one or more fuel outlets can include a single, annular outlet, or multiple outlets disposed in an annular pattern about a longitudinal axis of the resonator. 
       FIGS. 14A and 14B  each illustrate cutaway side views of an example resonator  1400  that can be provided in a jet engine. The resonator  1400  has an inner conductor  1402 , an outer conductor  1404 , an electrode  1406  disposed at a distal end of the resonator  1400 , and multiple dielectric sections disposed between the inner and outer conductors. As depicted in  FIG. 14A , for instance, the resonator  1400  includes a first dielectric section  1408  and a second dielectric section  1410 . In an example arrangement, each of the two dielectric sections can be the same dielectric. Alternatively, the dielectric sections can be different dielectrics. 
     The first dielectric section  1408  includes a fuel conduit  1412  having an outlet  1414  located at a distal end of the fuel conduit and having an inlet  1415  located at a proximal end of the fuel conduit. The outlet  1414  is oriented towards both the inner conductor  1402  and the second dielectric section  1410 . In an example arrangement, the first dielectric section  1408  can be a ceramic material within which the fuel conduit  1412  is disposed and through which the fuel can flow towards the outlet  1414 . Further, the second dielectric section  1410  can be either (i) entirely air or (ii) a porous ceramic material through which fuel can flow towards the electrode  1406 . As an alternate example, the first dielectric section  1408  can be a porous ceramic material and the second dielectric section  1410  can be air. Other arrangements are possible as well. 
     Further, as depicted in  FIG. 14B , the resonator  1400  includes a first dielectric section  1416 , a second dielectric section  1418 , and a third dielectric section  1420 . Arranged within both the first dielectric section  1416  and the second dielectric section  1418  is a fuel conduit  1422  having an outlet  1424  oriented towards both the inner conductor  1402  and the third dielectric section  1420 . In an example arrangement, each of the three dielectric sections can be the same dielectric. Alternatively, at least one of the dielectric sections can be different from the other(s). 
     In some implementations, the disclosed resonator can be configured to inject a single type of fuel, such as one of the fuels noted above. In addition to injecting a single type of fuel, the disclosed resonator arrangements can also be used to mix multiple different types of fuels before, during, or after the resonator provides electromagnetic waves and exposes the fuel to the electromagnetic waves. This can be accomplished in various ways. In one example, the resonator can include multiple conduits arranged within, arranged along, or defined by the dielectric, and configured to operate together to mix fuels. Each such conduit can include a respective inlet configured to receive a distinct type of fuel from a fuel source. Further, each such conduit may be physically separate from each other conduit. Still further, a first conduit can include an outlet arranged proximate to an outlet of a second conduit, and the two outlets can be oriented such that, due to their proximity and orientations, when the first conduit expels one type of fuel out of the first conduit&#39;s respective outlet and the second conduit expels a different type of fuel out of the second conduit&#39;s respective outlet, the fuels can mix together. For instance, each of the two outlets described above can be arranged to expel the respective fuels into porous dielectric material, in which the fuels can mix together. Additionally or alternatively, the two outlets can be arranged to expel the respective fuels into a cavity of air in the resonator, in which the fuels can mix together. In some implementations, the resonator can excite a plasma corona to ignite, in a combustion chamber of the jet engine, a mixture that includes multiple fuels and air. 
       FIG. 15  illustrates a cross-sectional view of another example resonator  1500  that can be provided in a jet engine. As depicted in  FIG. 15 , the resonator  1500  includes an inner conductor  1502 , an outer conductor  1504 , and a dielectric material  1506  disposed between the inner conductor  1502  and the outer conductor  1504 . Further, the inner conductor  1502  is shown projecting along a longitudinal axis  1508  to a distal end configured as a concentrator  1510  of an electrode. 
     In addition, also depicted in  FIG. 15  are various fuel conduits including: conduit  1512  having inlet  1513  and outlet  1514 , conduit  1516  having inlet  1517  and outlet  1518 , conduit  1520  having inlet  1521  and outlet  1522 , and conduit  1524  having inlet  1525  and outlet  1526 . Each such conduit is substantially parallel to longitudinal axis  1508  and has an inlet located at a proximal end of the resonator  1500  and an outlet located at a distal end of the resonator  1500 . Inlets  1513 ,  1517 ,  1521 , and  1525 , are each configured to receive fuel into a respective conduit from a fuel source. Outlets  1514 ,  1518 ,  1522 , and  1526 , are each oriented at a slight angle towards longitudinal axis  1508  and the distal end of the inner conductor  1502 , and are each configured to expel fuel towards the concentrator  1510  of the electrode. 
     Next,  FIG. 16  illustrates various cross-sectional views of a resonator  1600  that can be provided in a jet engine. As depicted, the resonator  1600  has an inner conductor  1602 , an outer conductor  1604 . At a distal end of the resonator is an electrode  1606 . In this example, two different dielectrics are disposed between the inner conductor  1602  and the outer conductor  1604 : air  1608 , and a porous dielectric material  1610 . In operation, fuel can enter through an inlet (not shown) near a proximal end of the resonator  1600 . The fuel can then flow into and through the porous dielectric material  1610 , next flowing into and through the air  1608 , and lastly being expelled out of a distal end of the resonator  1600 . 
     Sectional view A-A shows a portion of the resonator  1600  near the distal end of the resonator  1600 . In this portion, air  1608  is disposed between the inner conductor  1602  and the outer conductor  1604 . 
     Next, sectional view B-B shows a portion of the resonator  1600  slightly below a midway point between the distal end and the proximal end of the resonator  1600 . In this portion, the porous dielectric material  1610  is disposed between the inner conductor  1602  and the outer conductor  1604 , and defines channels of a fuel conduit, such as channels  1612  and  1614 . Further, sectional view C-C shows a portion of the resonator  1600  near a proximal end of the resonator  1600 . In this portion, the porous dielectric material  1610  is less porous than the portion of the resonator  1600  shown in sectional view B-B, and defines additional channels of the fuel conduit, such as channels  1616  and  1618 . 
     In practice, due to the porous nature of a porous dielectric material, the shape of the channels defined by the material can vary along various points in the material. For example, at some point along the length of the resonator  1600  between sectional views B-B and C-C, thinner channels  1616  and  1618  can merge together to become wider channel  1614 . Alternatively, both channels  1616  and  1618  can remain physically separate from one another, but each feed into channel  1614 . Either way, in operation, fuel that flows through channels  1616  and  1618  can then flow through channel  1614 , and at some point thereafter can flow into the air  1608  portion of the resonator  1600 . 
     As another example, channel  1616  and channel  1612  can be two different portions of the same channel. Likewise, channel  1618  and channel  1614  can be two different portions of the same channel through which fuel can flow. For instance, at some point along the length of the resonator  1600  between sectional views B-B and C-C, channel  1618  can widen in a funnel-like manner and to form channel  1614 . Other examples are possible as well. 
       FIG. 17A  illustrates a cross-sectional view of an example resonator  1700  that can be provided in a jet engine. The resonator  1700  is arranged in a similar manner to the resonator depicted in  FIG. 7 . As depicted in  FIG. 17A , the resonator  1700  includes an inner conductor  1702 , an outer conductor  1704 , and a dielectric material  1706  disposed between the inner conductor  1702  and the outer conductor  1704 . In particular, above axis  1708 , both the dielectric material  1706  and a cavity  1710  (air, for instance) are disposed between the inner conductor  1702  and the outer conductor  1704 . And below axis  1708 , the dielectric material  1706  is disposed between the inner conductor  1702  and the outer conductor  1704 . Further, the inner conductor  1702  is shown to project along a longitudinal axis  1712  to a distal end configured as a concentrator  1714  of an electrode located at or in close proximity to a distal end of the cavity  1710 . 
     In addition, also depicted in  FIG. 17A  are fuel conduits  1716  and  1718  that are arranged within the dielectric material  1706  and having at least some channels that are substantially parallel to longitudinal axis  1712 . In practice, the resonator  1700  can be cylindrical, and thus, fuel conduits  1716  and  1718  can take the form of physically separate conduits arranged within the dielectric material  1706 , or can take the form of a single, annular conduit arranged within the dielectric material  1706 . 
     As shown, fuel conduit  1716  includes a fuel inlet  1720  and three outlets: outlet  1722   a , outlet  1722   b , and outlet  1722   c , where outlet  1722   a  is located at axis  1708 , and both outlet  1722   b  and outlet  1722   c  are located above axis  1708 . Likewise, fuel conduit  1718  includes a fuel inlet  1724  and three outlets: outlet  1726   a , outlet  1726   b , and outlet  1726   c , where outlet  1726   a  is located at axis  1708 , and both outlet  1726   b  and  1726   c  are located above axis  1708 . Fuel inlets  1720  and  1724  are each configured to receive fuel into conduits  1716  and  1718 , respectively, from a fuel source. 
     Slightly below axis  1708 , respective channels of fuel conduits  1716  and  1718  branch off to run within the dielectric material  1706  above axis  1708 . Outlets  1722   b ,  1722   c ,  1726   b , and  1726   c , are then each oriented at a slight angle towards longitudinal axis  1712  and the distal end of the inner conductor  1702 , and are each configured to expel fuel into the cavity  1710  in a direction towards the inner conductor  1702  and in a direction towards the distal end of the resonator  1700 . Further, outlets  1722   a  and  1726   a  are oriented so that each of their longitudinal axes are substantially parallel to longitudinal axis  1712 . Thus, outlets  1722   a  and  1726   a  are each configured to expel fuel into the cavity  1710  in a direction that is largely parallel to the longitudinal axis  1712  and in a direction toward the distal end of the resonator  1700 . 
     In some implementations, multiple conduits similar to conduits  1716  and  1718  can be arranged within the dielectric material  1706  at other locations about longitudinal axis  1712 . Each of such conduits can include more outlets, less outlets, or the same number of outlets, each of which can be at the same or different locations along the conduit as those shown in  FIG. 17A . For example, one such conduit can include an outlet disposed in a distal end of the resonator (in other words, at the top of the resonator) and configured to expel fuel out towards the distal end of the inner conductor  1702  and/or towards an area entirely outside of the resonator, depending on the orientation of the outlet. In another example, a single, annular outlet can be disposed at a location along the length of the dielectric material  1706  and configured to expel fuel in a radial pattern into the cavity  1710  toward the inner conductor  1702 . Similarly, multiple outlets with similar locations and orientations as outlets  1722   b  and  1726   b  or outlets  1722   c  and  1726   c  can be disposed in the dielectric material  1706  and can be together configured to expel fuel in a radial pattern toward the inner conductor  1702 . Other examples are possible as well. 
       FIG. 17B  illustrates a cross-sectional view of an example resonator  1700  with multiple outlets, including outlet  1722   c  and  1726   c , disposed within the dielectric material  1706  in an annular pattern. The arrows shown in  FIG. 17B  represent the direction of fuel. As shown, each such outlet can be configured to expel fuel in a radial pattern toward the inner conductor  1702 . Expelling the fuel toward the inner conductor can help to direct the fuel toward a plasma corona provided at the concentrator  1714 . 
     X. Example Methods 
       FIG. 18  is a flow chart depicting operations of a representative method for combusting fuel in a jet engine. 
     At block  1800 , the method includes providing a plasma corona in a combustion chamber of a jet engine by exciting a resonator with a signal having a wavelength proximate to an odd-integer multiple of one-quarter of a resonant wavelength of the resonator, the resonator including (i) a first conductor, (ii) a second conductor, and (iii) a dielectric between the first conductor and the second conductor. In line with the discussion above, the resonator can include a coaxial cavity resonator, a dielectric resonator, a rectangular waveguide cavity resonator, or a gap-coupled microstrip resonator, or could take still other forms. 
     In some implementations, providing the plasma corona can include providing, using a direct-current power source, a bias signal between the first conductor and the second conductor. 
     At block  1802 , the method includes moving fuel from a fuel source into the combustion chamber of the jet engine by way of a fuel conduit such that the plasma corona causes combustion of the fuel. A portion of the fuel conduit is arranged proximate to the dielectric. As discussed above, the portion of the fuel conduit can be arranged along the dielectric, arranged within the dielectric, and/or defined by a shape of the dielectric. 
     In some implementations, moving the fuel can include moving the fuel using a fuel pump of the jet engine. In addition, the dielectric can include a fuel outlet that opens out into the combustion chamber, and moving the fuel can include expelling the fuel through the fuel outlet and toward a distal end of the first conductor where the resonator provides the plasma corona. 
     In some implementations, moving the fuel can include expelling the fuel in a radial pattern into the combustion chamber using multiple fuel outlets of the fuel conduit. 
     In some implementations, moving the fuel can include expelling the fuel into an area of porous material in the dielectric such that the fuel passes through the area of the porous material and enters a combustion zone of the combustion chamber. 
     In some implementations, the resonator can assume a dual role. For instance, the method can also include exciting the resonator prior to formation of the plasma corona, such that the resonator provides electromagnetic waves for pre-treating fuel that is input to the combustion zone. Similarly, in some implementations, after combustion occurs, rather than providing a plasma corona, the resonator could instead enhance an already present combustion process by providing electromagnetic waves that can reform fuel that is being input to the combustion zone and/or already in the combustion zone. 
     The order of the blocks shown in  FIG. 18  is not meant to be limiting. In some implementations, the fuel can be moved from the fuel source into the combustion chamber prior to providing the plasma corona in the combustion chamber. 
       FIG. 19  is a flow chart depicting operations of a representative method for pre-treating fuel in a jet engine. 
     At block  1900 , the method includes providing electromagnetic waves by exciting a resonator with a signal having a wavelength proximate to an odd-integer multiple of one-quarter of a resonant wavelength of the resonator, the resonator including (i) a first conductor, (ii) a second conductor, and (iii) a dielectric between the first conductor and the second conductor. In line with the discussion above, the resonator can include a coaxial cavity resonator, a dielectric resonator, a rectangular waveguide cavity resonator, or a gap-coupled microstrip resonator, or could take still other forms. 
     At block  1902 , the method includes moving fuel from a fuel source into a combustion chamber of a jet engine by way of a fuel conduit. A portion of the fuel conduit is arranged proximate to the dielectric such that the fuel moving through the fuel conduit is exposed to the electromagnetic waves, thereby pre-treating fuel within the fuel conduit so as to provide pre-treated fuel. As discussed above, the portion of the fuel conduit can be arranged along the dielectric, arranged within the dielectric, and/or defined by a shape of the dielectric. 
     In some implementations, pre-treating the fuel can include increasing an energy state of the fuel, thereby lowering an energy barrier to combustion of the fuel. In line with the discussion above, increasing the energy state of the fuel can include increasing a valence band occupancy rate. 
     In some implementations, pre-treating the fuel can include (i) liberating hydrogen atoms from the fuel, thereby making the pre-treated fuel more amenable to combustion and/or (ii) liberating hydrogen ions from the fuel, thereby making the pre-treated fuel more amenable to combustion. 
     In some implementations, the method can also include igniting the pre-treated fuel with the combustion chamber. In line with the discussion above, igniting the pre-treated fuel within the combustion chamber can include providing a plasma corona in the combustion chamber by: (i) causing a direct-current power source to provide a bias signal between the first conductor and the second conductor, and (ii) causing a radio-frequency power source to excite the resonator with the signal having the wavelength proximate to an odd-integer multiple of one-quarter of the resonant wavelength of the resonator. 
     The particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other implementations can include more or less of each element shown in a given Figure. Further, some of the illustrated elements can be combined or omitted. Yet further, an illustrative implementation can include elements that are not illustrated in the Figures. 
     A step or block that represents a processing of information can correspond to circuitry that can be configured to perform the specific logical functions of a method or technique as presently disclosed. Alternatively or additionally, a step or block that represents a processing of information can correspond to a module, a segment, or a portion of program code (including related data). The program code can include one or more instructions executable by a processor for implementing specific logical functions or actions in the method or technique. The program code and/or related data can be stored on any type of computer-readable medium such as a storage device including a disk, hard drive, or other storage medium. 
     The computer-readable medium can also include non-transitory computer-readable media such as computer-readable media that store data for short periods of time like register memory, processor cache, and random access memory (RAM). The computer-readable media can also include non-transitory computer-readable media that store program code and/or data for longer periods of time. Thus, the computer-readable media can include secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer-readable media can also be any other volatile or non-volatile storage systems. A computer-readable medium can be considered a computer-readable storage medium, for example, or a tangible storage device. 
     While various examples and implementations have been disclosed, other examples and implementations will be apparent to those skilled in the art. The various disclosed examples and implementations are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.