Abstract:
A dielectric waveguide integrated plasma lamp is disclosed for powering a small and bright bulb with a diameter of a few millimeters. The lamp is contained within a high dielectric constant material which guides the microwaves to the bulb, provides heat isolation to the drive circuit, contains the microwaves, provides structural stability and ease of manufacturing and allows efficient energy coupling to the bulb when used as a dielectric resonant oscillator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/771,788 filed on Feb. 4, 2004, now U.S. Pat. No. 7,348,732 entitled “Plasma Lamp With Dielectric Waveguide,” which is a continuation of U.S. non-provisional patent application Ser. No. 09/809,718 filed on Mar. 15, 2001, now U.S. Pat. No. 6,737,809 entitled “Plasma Lamp With Dielectric Waveguide,” which claims priority to U.S. provisional application Ser. No. 60/222,028 filed on Jul. 31, 2000, entitled “Plasma Lamp” each of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The field of the present invention relates to devices and methods for generating light, and more particularly to electrodeless plasma lamps. 
   2. Background 
   Electrodeless plasma lamps provide point-like, bright, white light sources. Because they do not use electrodes, electrodeless plasma lamps often have longer useful lifetimes than other lamps. Electrodeless plasma lamps in the prior art have certain common features. For example in U.S. Pat. Nos. 4,954,755 to Lynch et al., 4,975,625 to Lynch et al., 4,978,891 to Ury et al., 5,021,704 to Walter et al., 5,448,135 to Simpson, 5,594,303 to Simpson, 5,841,242 to Simpson et al., 5,910,710 to Simpson, and 6,031,333 to Simpson, each of which is incorporated herein by reference, the plasma lamps direct microwave energy into an air cavity, with the air cavity enclosing a bulb containing a mixture of substances that can ignite, form a plasma, and emit light. 
   The plasma lamps described in these patents are intended to provide brighter light sources with longer life and more stable spectrum than electrode lamps. However, for many applications, light sources that are brighter, smaller, less expensive, more reliable, and have long useful lifetimes are desired, but such light sources until now have been unavailable. Such applications include, for example, streetlights and emergency response vehicles. A need exists therefore, for a very bright, durable light source at low cost. 
   In the prior art, the air-filled cavity of the electrodeless plasma lamp is typically constructed in part by a metal mesh. Metal mesh is used because it contains the microwave energy within the cavity while at the same time permitting the maximum amount of visible light to escape. The microwave energy is typically generated by a magnetron or solid state electronics and is guided into the cavity through one or more waveguides. Once in the air-filled cavity, microwave energy of select frequencies resonates, where the actual frequencies that resonate depend upon the shape and size of the cavity. Although there is tolerance in the frequencies that may be used to power the lamps, in practice, the power sources are limited to microwave frequencies in the range of 1-10 GHz. 
   Because of the need to establish a resonance condition in the air-filled cavity, the cavity generally may not be smaller than one-half the wavelength of the microwave energy used to power the lamp. The air-filled cavity and thereby, the plasma lamp itself has a lower limit on its size. However, for many applications, such as for high-resolution monitors, bright lamps, and projection TVs, these sizes remain prohibitively large. A need exists therefore for a plasma lamp that is not constrained to the minimum cavity sizes illustrated by the prior art. 
   In the prior art, the bulbs are typically positioned at a point in the cavity where the electric field created by the microwave energy is at a maximum. The support structure for the bulb is preferably of a size and composition that does not interfere with the resonating microwaves, as any interference with the microwaves reduces the efficiency of the lamp. The bulbs, therefore, are typically made from quartz. Quartz bulbs, however, are prone to failure because the plasma temperature can be several thousand degrees centigrade, which can bring the quartz wall temperature to near 1000° C. Furthermore, quartz bulbs are unstable in terms of mechanical stability and optical and electrical properties over long periods. A need exists, therefore, for a light source that overcomes the above-described issues, but that is also stable in its spectral characteristics over long periods. 
   In prior art plasma lamps, the bulb typically contains a noble gas combined with a light emitter, a second element or compound which typically comprises sulfur, selenium, a compound containing sulfur or selenium, or any one of a number of metal halides. Exposing the contents of the bulb to microwave energy of high intensity causes the noble gas to become a plasma. The free electrons within the plasma excite the light emitter within the bulb. When the light emitter returns to a lower electron state, radiation is emitted. The spectrum of light emitted depends upon the characteristics of the light emitter within the bulb. Typically, the light emitter is chosen to cause emission of visible light. 
   Plasma lamps of the type described above frequently require high intensity microwaves to initially ignite the noble gas into plasma. However, over half of the energy used to generate and maintain the plasma is typically lost as heat, making heat dissipation a problem. Hot spots can form on the bulb causing spotting on the bulb and thereby reducing the efficiency of the lamp. Methods have been proposed to reduce the hot spots by rotating the lamp to better distribute the plasma within the lamp and by blowing constant streams of air at the lamp. These solutions, however, add structure to the lamp, thereby increasing its size and cost. Therefore, a need exists for a plasma lamp that requires less energy to ignite and maintain the plasma, and includes a minimum amount of additional structure for efficient dissipation of heat. 
   SUMMARY OF THE INVENTION 
   This invention generally provides, in one aspect, devices and methods of producing bright, spectrally stable light. 
   In accordance with one embodiment as described herein, a device for producing light comprises an electromagnetic energy source, a waveguide having a body formed of a dielectric material, and a bulb. Preferably, the waveguide is connected to the energy source for receiving electromagnetic energy from the energy source. The waveguide builds and contains the electromagnetic energy. The bulb, which is coupled to the waveguide, receives electromagnetic energy from the waveguide. The received electromagnetic energy ignites a gas-fill that forms a plasma and emits light, preferably in the visible spectral range. 
   In one preferred embodiment, the bulb is shaped to reflect light outwards through its window. The electromagnetic energy source is preferably a microwave energy source that is efficiently coupled to and preferably thermally isolated from the waveguide. Furthermore, the outer surface of the waveguide, preferably with the exception of the bulb cavity, is coated with a material to contain the microwave energy within the waveguide. The dielectric forming the waveguide preferably has a high dielectric constant, a high dielectric strength, and a low loss tangent. This permits high power densities within the waveguide. A heat sink preferably is attached to the outer surfaces of the waveguide to dissipate heat. 
   In accordance with a first alternative embodiment, the lamp is operated in resonant cavity mode. In this mode, the microwave energy directed into the waveguide has a frequency such that it resonates within the waveguide. The microwave feed and the bulb are preferably positioned at locations with respect to the waveguide that correspond to electric field maxima of the resonant frequency. 
   In accordance with a second alternative embodiment, the lamp is operated in a dielectric oscillator mode. In this mode, an energy feedback mechanism or probe is coupled to the dielectric waveguide at a point that in one embodiment corresponds to an energy maximum. The probe senses the electric field amplitude and phase within the waveguide at the point of coupling. Using the probe signal to provide feedback, the lamp may be continuously operated in resonant cavity mode, even if the resonant frequency changes as the plasma forms in the bulb and/or if the dielectric waveguide undergoes thermal expansion due to the heat generated. The probe provides feedback to the microwave source and the microwave source adjusts its output frequency to dynamically maintain a resonance state. 
   Further embodiments, variations and enhancements, including combinations of the above-described embodiments, or features thereof, are also described herein or depicted in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a sectional view of a plasma lamp according to a preferred embodiment. 
       FIGS. 2A and 2B  illustrate sectional views of alternative embodiments of a plasma lamp. 
       FIGS. 3A and 3B  illustrate a sectional view of an alternative embodiment of a plasma lamp wherein the bulb is thermally isolated from the dielectric waveguide. 
       FIGS. 4A-D  illustrate different resonant modes within a rectangular prism-shaped waveguide. 
       FIGS. 5A-C  illustrate different resonant modes within using a cylindrical prism-shaped cylindrical waveguide. 
       FIG. 6  illustrates an embodiment of the apparatus using a feedback mechanism to provide feedback to the microwave source to maintain a resonant mode of operation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Turning now to the drawings,  FIG. 1  illustrates a preferred embodiment of a dielectric waveguide integrated plasma lamp  101  (DWIPL). The DWIPL  101  preferably comprises a source  115  of electromagnetic radiation, preferably microwave radiation, a waveguide  103  having a body formed of a dielectric material, and a feed  117  coupling the radiation source  115  to the waveguide  103 . As used herein, the term “waveguide” generally refers to any device having a characteristic and purpose of at least partially confining electromagnetic energy. The DWIPL  101  further includes a bulb  107 , that is preferably disposed on an opposing side of the waveguide  103 , and contains a gas-fill, preferably comprising a noble gas and a light emitter, which when receiving electromagnetic energy at a specific frequency and intensity forms a plasma and emits light. 
   In a preferred embodiment, the microwave radiation source  115  feeds the waveguide  103  microwave energy via the feed  117 . The waveguide contains and guides the microwave energy to a cavity  105  preferably located on an opposing side of the waveguide  103  from the feed  117 . Disposed within the cavity  105  is the bulb  107  containing the gas-fill. Microwave energy is preferably directed into the enclosed cavity  105 , and in turn the bulb  107 . This microwave energy generally frees electrons from their normal state and thereby transforms the noble gas into a plasma. The free electrons of the noble gas excite the light emitter. The de-excitation of the light emitter results in the emission of light. As will become apparent, the different embodiments of DWIPLs disclosed herein offer distinct advantages over the plasma lamps in the prior art, such as an ability to produce brighter and spectrally more stable light, greater energy efficiency, smaller overall lamp sizes, and longer useful life spans. 
   The microwave source  115  in  FIG. 1  is shown schematically as solid state electronics, however, other devices commonly known in the art that can operate in the 0.5-30 GHz range may also be used as a microwave source, including but not limited to klystrons and magnetrons. The preferred range for the microwave source is from about 500 MHz to about 10 GHz. 
   Depending upon the heat sensitivity of the microwave source  115 , the microwave source  115  may be thermally isolated from the bulb  107 , which during operation preferably reaches temperatures between about 700° C. and about 1000° C. Thermal isolation of the bulb  107  from the source  115  provides a benefit of avoiding degradation of the source  115 . Additional thermal isolation of the microwave source  115  may be accomplished by any one of a number of methods commonly known in the art, including but not limited to using an insulating material or vacuum gap occupying an optional space  16  between the source  115  and waveguide  103 . If the latter option is chosen, appropriate microwave feeds are used to couple the microwave source  115  to the waveguide  103 . 
   In  FIG. 1 , the feed  117  that transports microwaves from the source  115  to the waveguide  103  preferably comprises a coaxial probe. However, any one of several different types of microwave feeds commonly known in the art may be used, such as microstrip lines or fin line structures. 
   Due to mechanical and other considerations such as heat, vibration, aging, or shock, when feeding microwave signals into a dielectric material, contact between the feed  117  and the waveguide  103  is preferably maintained using a positive contact mechanism  121 . The contact mechanism  121  provides constant pressure between the feed  117  and the waveguide  103  to minimize the probability that microwave energy will be reflected back through the feed  117  and not transmitted into the waveguide  103 . In providing constant pressure, the contact mechanism  121  compensates for small dimensional changes in the microwave feed  117  and the waveguide  103  that may occur due to thermal heating or mechanical shock. The contact mechanism may be a spring loaded device, such as is illustrated in  FIG. 1 , a bellows type device, or any other device commonly known in the art that can sustain a constant pressure for continuously and steadily transferring microwave energy. 
   When coupling the feed  117  to the waveguide  103 , intimate contact is preferably made by depositing a metallic material  123  directly on the waveguide  103  at its point of contact with the feed  117 . The metallic material  123  eliminates gaps that may disturb the coupling and is preferably comprised of gold, silver, or platinum, although other conductive materials may also be used. The metallic material  123  may be deposited using any one of several methods commonly known in the art, such as depositing the metallic material  123  as a liquid and then firing it in an oven to provide a solid contact. 
   In  FIG. 1 , the waveguide  103  is preferably the shape of a rectangular prism, however, the waveguide  103  may also have a cylindrical prism shape, a sphere-like shape, or any other shape, including a complex, irregular shape the resonant frequencies of which are preferably determined through electromagnetic simulation tools, that can efficiently guide microwave energy from the feed  117  to the bulb  107 . The actual dimensions of the waveguide may vary depending upon the frequency of the microwave energy used and the dielectric constant of the body of waveguide  103 . 
   In one preferred embodiment, the waveguide body is approximately 12,500 mm 3  with a dielectric constant of approximately 9 and operating frequency of approximately 2.4 GHz. Waveguide bodies on this scale are significantly smaller than the waveguides in the plasma lamps of the prior art. As such, the waveguides in the preferred embodiments represent a significant advance over the prior art because the smaller size allows the waveguide to be used in many applications, where waveguide size had previously prohibited such use or made such use wholly impractical. For larger dielectric constants, even smaller sizes for the waveguides may be achieved. Besides the obvious advantages created by a reduction in size, size reduction translates into a higher power density, lower loss, and thereby, an ease in igniting the lamp. 
   Regardless of its shape and size, the waveguide  103  preferably has a body comprising a dielectric material which, for example, preferably exhibits the following properties: (1) a dielectric constant preferably greater than approximately 2; (2) a loss tangent preferably less than approximately 0.01; (3) a thermal shock resistance quantified by a failure temperature of preferably greater than approximately 200° C.; (4) a DC breakdown threshold of preferably greater than approximately 200 kilovolts/inch; (5) a coefficient of thermal expansion of preferably less than approximately 10 −5 /° C.; (6) a zero or slightly negative temperature coefficient of the dielectric constant; (7) stoichemetric stability over a preferred range of temperature, preferably from about −80° C. to about 1000° C., and (8) a thermal conductivity of preferably approximately 2 W/mK (watts per milliKelvin). 
   Certain ceramics, including alumina, zirconia, titanates, and variants or combinations of these materials, and silicone oil may satisfy many of the above preferences, and may be used because of their electrical and thermo-mechanical properties. In any event, it should be noted that the embodiments presented herein are not limited to a waveguide exhibiting all or even most of the foregoing properties. 
   In the various embodiments of the waveguide disclosed herein, such as in the example outlined above, the waveguide preferably provides a substantial thermal mass, which aids efficient distribution and dissipation of heat and provides thermal isolation between the lamp and the microwave source. 
   Alternative embodiments of DWIPLS  200 ,  220  are depicted in  FIGS. 2A-B . In  FIG. 2A , a bulb  207  and bulb cavity  205  are provided on one side of a waveguide  203 , preferably on a side opposite a feed  209 , and more preferably in the same plane as the feed  209 , where the electric field of the microwave energy is at a maximum. Where more than one maximum of the electric field is provided in the waveguide  203 , the bulb  207  and bulb cavity  205  may be positioned at one maximum and the feed  209  at another maximum. By placing the feed  209  and bulb  207  at a maximum for the electric field, a maximum amount of energy is respectively transferred and intercepted. The bulb cavity  205  is a concave form in the body of the waveguide  203 . 
   As shown in  FIG. 2B , the body of the waveguide  223  optionally protrudes outwards in a convex form, from the main part of the body of the waveguide  203  to form the bulb cavity  225 . As in  FIG. 2A , in  FIG. 2B , the bulb  227  is preferably positioned opposite to the feed  221 . However, where more than one electric field maximum is provided in the waveguide  203 , the bulb  207 ,  227  may be positioned in a plane other than the plane of the feed  209 ,  221 . 
   Returning to  FIG. 1 , the outer surfaces of the waveguide  103 , with the exception of those surfaces forming the bulb cavity  105 , are preferably coated with a thin metallic coating  119  to reflect the microwaves. The overall reflectivity of the coating  119  determines the level of energy contained within the waveguide  103 . The more energy that can be stored within the waveguide  103 , the greater the overall efficiency of the lamp  101 . The coating  119  also preferably suppresses evanescent radiation leakage. In general, the coating  119  preferably significantly eliminates any stray microwave field. 
   Microwave leakage from the bulb cavity  105  may be significantly attenuated by having a cavity  105  that is preferably significantly smaller than the microwave wavelengths used to operate the lamp  101 . For example, the length of the diagonal for the window is preferably considerably less than half of the microwave wavelength (in free space) used. 
   The bulb  107  is disposed within the bulb cavity  105 , and preferably comprises an outer wall  109  and a window  111 . In one preferred embodiment, the cavity wall of the body of the waveguide  103  acts as the outer wall of the bulb  107 . The components of the bulb  107  preferably include one or more dielectric materials, such as ceramics and sapphires. In one embodiment, the ceramics in the bulb are the same as the material used in waveguide  103 . Dielectric materials are preferred for the bulb  107  because the bulb  107  is preferably surrounded by the dielectric body of the waveguide  103  and the dielectric materials help ensure efficient coupling of the microwave energy with the gas-fill in the bulb  107 . 
   The outer wall  109  is preferably coupled to the window  111  using a seal  113 , thereby defining a bulb envelope  127  which contains the gas-fill comprising the plasma-forming gas and light emitter. The plasma-forming gas is preferably a noble gas, which enables the formation of a plasma. The light emitter is preferably a vapor formed of any one of a number of elements or compounds currently known in the art, such as sulfur, selenium, a compound containing sulfur or selenium, or any one of a number of metal halides, such as indium bromide (InBr 3 ). 
   To assist in confining the gas-fill within the bulb  107 , the seal  113  preferably comprises a hermetic seal. The outer wall  109  preferably comprises alumina because of its white color, temperature stability, low porosity, and thermal expansion coefficient. However, other materials that generally provide one or more of these properties may be used. The outer wall  109  is also preferably contoured to reflect a maximum amount of light out of the cavity  105  through the window  111 . For instance, the outer wall  109  may have a parabolic contour to reflect light generated in the bulb  107  out through the window  111 . However, other outer wall contours or configurations that facilitate directing light out through the window  111  may be used. 
   The window  111  preferably comprises sapphire for light transmittance and because its thermal expansion coefficient matches well with alumina. Other materials that have a similar light transmittance and thermal expansion coefficient may be used for the window  111 . In an alternative embodiment, the window  111  may comprise a lens to collect the emitted light. 
   As referenced above, during operation, the bulb  107  may reach temperatures of up to about 1000° C. Under such conditions, the waveguide  103  in one embodiment acts as a heat sink for the bulb  107 . By reducing the heat load and heat-induced stress upon the various components of the DWIPL  101 , the useful life span of the DWIPL  101  is generally increased beyond the life span of typical electrodeless lamps. Effective heat dissipation may be obtained by preferably placing heat-sinking fins  125  around the outer surfaces of the waveguide  103 , as depicted in  FIG. 1 . In the embodiment shown in  FIG. 2B , with the cavity  225  extending away from the main part of the body of the waveguide  223 , the DWIPL  220  may be used advantageously to remove heat more efficiently by placing fins  222  in closer proximity to the bulb  227 . 
   In another embodiment, the body of the waveguide  103  comprises a dielectric, such as a titanate, which is generally not stable at high temperatures. In this embodiment, the waveguide  103  is preferably shielded from the heat generated in the bulb  107  by placing a thermal barrier between the body of the waveguide  103  and the bulb  107 . In one alternative embodiment, the outer wall  109  acts as a thermal barrier by comprising a material with low thermal conductivity such as NZP. Other suitable material for a thermal barrier may also be used. 
     FIGS. 3A and 3B  illustrate an alternative embodiment of a DWIPL  300  wherein a vacuum gap acts as a thermal barrier. As shown in  FIG. 3A , the bulb  313  of the DWIPL  300  is disposed within a bulb cavity  315  and is separated from the waveguide  311  by a gap  317 , the thickness of which preferably varies depending upon the microwave propagation characteristics and material strength of the material used for the body of the waveguide  311  and the bulb  313 . The gap  317  is preferably a vacuum, minimizing heat transfer between the bulb  313  and the waveguide  311 . 
     FIG. 3B  illustrates a magnified view of the bulb  313 , bulb cavity  315 , and vacuum gap  317  for the DWIPL  300 . The boundaries of the vacuum gap  317  are formed by the waveguide  311 , a bulb support  319 , and the bulb  313 . The bulb support  319  may be sealed to the waveguide  311 , the support  319  extending over the edges of the bulb cavity  315  and comprising a material such as alumina that preferably has high thermal conductivity to help dissipate heat from the bulb  313 . 
   Embedded in the support  319  is an access seal  321  for establishing a vacuum within the gap  317  when the bulb  313  is in place. The bulb  313  is preferably supported by and hermetically sealed to the bulb support  319 . Once a vacuum is established in the gap  317 , heat transfers between the bulb  313  and the waveguide  311  are preferably substantially reduced. 
   Embodiments of the DWIPLs thus far described preferably operate at a microwave frequency in the range of 0.5-10 GHz. The operating frequency preferably excites one or more resonant modes supported by the size and shape of the waveguide, thereby establishing one or more electric field maxima within the waveguide. When used as a resonant cavity, at least one dimension of the waveguide is preferably an integer number of half-wavelengths long. 
     FIGS. 4A-C  illustrate three alternative embodiments of DWIPLs  410 ,  420 ,  430  operating in different resonant modes.  FIG. 4A  illustrates a DWIPL  410  operating in a first resonant mode  411  where one axis of a rectangular prism-shaped waveguide  417  has a length that is one-half the wavelength of the microwave energy used.  FIG. 4B  illustrates a DWIPL  420  operating in a resonant mode  421  where one axis of a rectangular prism-shaped waveguide  427  has a length that is equal to one wavelength of the microwave energy used.  FIG. 4C  illustrates a DWIPL  430  operating in a resonant mode  431  where one axis of a rectangular prism-shaped waveguide  437  has a length that is 1½ wavelengths of the microwave energy used. 
   In each of the DWIPLs and corresponding modes depicted in  FIGS. 4A-C , and for DWIPLs operating at any higher modes, the bulb cavity  415 ,  425 ,  435  and the feed(s)  413 ,  423 ,  433 ,  434  are preferably positioned with respect to the waveguide  417 ,  427 ,  437  at locations where the electric fields are at an operational maximum. However, the bulb cavity and the feed do not necessarily have to lie in the same plane. 
     FIG. 4C  illustrates an additional embodiment of a DWIPL  430  wherein two feeds  433 ,  434  are used to supply energy to the waveguide  437 . The two feeds  433 ,  434  may be coupled to a single microwave source or multiple sources (not shown). 
     FIG. 4D  illustrates another embodiment wherein a single energy feed  443  supplies energy into the waveguide  447  having multiple bulb cavities  415 ,  416 , each positioned with respect to the waveguide  447  at locations where the electric field is at a maximum. 
     FIGS. 5A-C  illustrate DWIPLs  510 ,  520 ,  530  having cylindrical prism-shaped waveguides  517 ,  527 ,  537 . In the embodiments depicted in  FIGS. 5A-C , the height of the cylinder is preferably less than its diameter, the diameter preferably being close to an integer multiple of the lowest order half-wavelength of energy that can resonate within the waveguide  517 ,  527 ,  537 . Placing such a dimensional restriction on the cylinder results in the lowest resonant mode being independent of the height of the cylinder. The diameter of the cylinder thereby dictates the fundamental mode of the energy within the waveguide  517 ,  527 ,  537 . The height of the cylinder can therefore be optimized for other requirements such as size and heat dissipation. In  FIG. 5A , the feed  513  is preferably positioned directly opposite the bulb cavity  515  and the zeroeth order Bessel mode  511  is preferably excited. 
   Other modes may also be excited within a cylindrical prism-shaped waveguide. For example,  FIG. 5B  illustrates a DWIPL  520  operating in a resonant mode where the cylinder  527  has a diameter that is preferably close to one wavelength of the microwave energy used. 
   As another example,  FIG. 5C  illustrates a DWIPL  520  operating in a resonant mode where the cylinder  537  has a diameter that is preferably close to ½ wavelengths of the microwave energy used.  FIG. 5C  additionally illustrates an embodiment of a DWIPL  530  whereby two feeds  533 ,  534  are used to supply energy to the cylinder-shaped waveguide  537 . As with other embodiments of the DWIPL, in a DWIPL having a cylinder-shaped waveguide, the bulb cavity  515 ,  525 ,  535  and the feed(s)  513 ,  523 ,  533 ,  534  are preferably positioned with respect to the waveguide  517 ,  527 ,  537  at locations where the electric field is at a maximum. 
   Using a dielectric waveguide has several distinct advantages. First, as discussed above, the waveguide may be used to help dissipate the heat generated in the bulb. Second, higher power densities may be achieved within a dielectric waveguide than are possible in the plasma lamps with air cavities that are currently used in the art. The energy density of a dielectric waveguide is greater, depending on the dielectric constant of the material used for the waveguide, than the energy density of an air cavity plasma lamp. 
   Referring back to the DWIPL  101  of  FIG. 1 , high resonant energy within the waveguide  103 , corresponding to a high value for Q (where Q is the ratio of the operating frequency to the frequency width of the resonance) for the waveguide results in a high evanescent leakage of microwave energy into the bulb cavity  105 . High leakage in the bulb cavity  105  leads to the quasi-static breakdown of the noble gas within the envelope  127 , thus generating the first free electrons. The oscillating energy of the free electrons scales as Iλ 2 , where I is the circulating intensity of the microwave energy and λ is the wavelength of that energy. Therefore, the higher the microwave energy, the greater is the oscillating energy of the free electrons. By making the oscillating energy greater than the ionization potential of the gas, electron-neutral collisions result in efficient build-up of plasma density. 
   Once the plasma is formed in the DWIPL and the incoming power is absorbed, the waveguide&#39;s Q value drops due to the conductivity and absorption properties of the plasma. The drop in the Q value is generally due to a change in the impedance of the waveguide. After plasma formation, the presence of the plasma in the cavity makes the bulb cavity absorptive to the resonant energy, thus changing the overall impedance of the waveguide. This change in impedance is effectively a reduction in the overall reflectivity of the waveguide. Therefore, by matching the reflectivity of the feed close to the reduced reflectivity of the waveguide, a sufficiently high Q value may be obtained even after the plasma formation to sustain the plasma. Consequently, a relatively low net reflection back into the energy source may be realized. 
   Much of the energy absorbed by the plasma eventually appears as heat, such that the temperature of the lamp may approach 1000° C. When the waveguide is also used as a heat sink, as previously described, the dimensions of the waveguide may change due to its coefficient of thermal expansion. Under such circumstances, when the waveguide expands, the microwave frequency that resonates within the waveguide changes and resonance is lost. In order for resonance to be maintained, the waveguide preferably has at least one dimension equal to an integer multiple of the half wavelength microwave frequency being generated by the microwave source. 
   One preferred embodiment of a DWIPL that compensates for this change in dimensions employs a waveguide comprising a dielectric material having a temperature coefficient for the refractive index that is approximately equal and opposite in sign to its temperature coefficient for thermal expansion. Using such a material, a change in dimensions due to thermal heating offsets the change in refractive index, minimizing the potential that the resonant mode of the cavity would be interrupted. Such materials include Titanates. A second embodiment that compensates for dimensional changes due to heat comprises physically tapering the walls of the waveguide in a predetermined manner. 
   In another preferred embodiment, schematically shown in  FIG. 6 , a DWIPL  610  may be operated in a dielectric resonant oscillator mode. In this mode, first and second microwave feeds  613 ,  615  are coupled between the dielectric waveguide  611 , which may be of any shape previously discussed, and the microwave energy source  617 . The energy source  617  is preferably broadband with a high gain and high power output and capable of driving plasma to emission. 
   The first feed  613  may generally operate as described above in other embodiments. The second feed  615  may probe the waveguide  611  to sample the field (including the amplitude and phase information contained therein) present and provide its sample as feedback to an input of the energy source  617  or amplifier. In probing the waveguide  611 , the second feed  615  also preferably acts to filter out stray frequencies, leaving only the resonant frequency within the waveguide  611 . 
   In this embodiment, the first feed  613 , second feed,  615  and bulb cavity  619  are each preferably positioned with respect to the waveguide  611  at locations where the electric field is at a maximum. Using the second feed  615 , the energy source  617  amplifies the resonant energy within the waveguide  611 . The source  617  thereby adjusts the frequency of its output to maintain one or more resonant modes in the waveguide  611 . The complete configuration thus forms a resonant oscillator. In this manner, automatic compensation may be realized for frequency shifts due to plasma formation and thermal changes in dimension and the dielectric constant. 
   The dielectric resonant oscillator mode also enables the DWIPL  610  to have an immediate re-strike capability after being turned off. As previously discussed, the resonant frequency of the waveguide  611  may change due to thermal expansion or changes in the dielectric constant caused by heat generated during operation. When the DWIPL  610  is shutdown, heat is slowly dissipated, causing instantaneous changes in the resonant frequency of the waveguide  611 . 
   However, as indicated above, in the resonant oscillator mode the energy source  617  automatically compensates for changes in the resonant frequency of the waveguide  611 . Therefore, regardless of the startup characteristics of the waveguide  611 , and providing that the energy source  617  has the requisite bandwidth, the energy source  617  will automatically compensate to achieve resonance within the waveguide  611 . The energy source immediately provides power to the DWIPL at the optimum plasma-forming frequency. 
   While embodiments and advantages of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.