Abstract:
A system and method for fueling a fusion reactor. The system includes a reactor chamber containing a stable plasma including a fusion fuel; a heating system configured to heat the plasma and increase an ion energy of the plasma to a level sufficient for producing net power from fusion reactions in the stable plasma; a plurality of magnets coaxial to the reactor chamber, the plurality of magnets producing a magnetic field sufficient to confine the stable plasma and promote rapid loss of fusion products into a scrape off layer; and a neutral beam injection system configured to inject additional quantities of the fusion fuel to sustain the power output of the fusion reaction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/873,651, filed on Sep. 4, 2013. This application is a continuation-in-part of PCT/US2013/40520, filed May 10, 2013, which claims the benefit of U.S. Provisional Application Ser. No. 61/645,459, filed May 10, 2012, and U.S. patent application Ser. No. 14/466,425, filed Aug. 22, 2014, which claims the benefit of U.S. Provisional Application Serial No. 61/868,629, filed Aug. 22, 2013. The contents of each of these applications are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under Grant #DE-ACO2-09CH11466 awarded by the Department of Energy. The government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the field of plasma physics and, in particular, to methods and apparati for introducing fuel into a plasma to facilitate nuclear fusion for the purpose of producing power. 
       BACKGROUND OF THE INVENTION 
       [0004]    Many methods for introducing fuels into nuclear fusion reactors have been used including ion jets, pellet injection, and gas puffing. Ion jets are deflected out of the device along the magnetic field lines. In pellet injection, a fuel pellet enters a plasma at supersonic speeds, ablates, and penetrates 1-2 meters. In gas puffing, a fuel in gaseous form is puffed into the plasma. The gas puffing jets are relatively slow and have a high probability of being ionized in the scrape-off-layer. In both pellet injection and gas puffing, much of the fuel is lost and little control can be exerted on the timing and placement of the injected fuel. 
         [0005]    When the fusion fuels are deuterium and tritium and the reactor is terrestrial, using extra fuel and wasting it is not a major concern. However, when using a rare fuel such as helium-3 or in a reactor located in the air or in space, fuel management becomes a key concern. 
         [0006]    Traditionally, neutral beam injection (NBI), while it adds ions to the plasma, is used for heating large plasmas. In NBI, a large number of ions are produced and accelerated through a high-voltage field and shaped into a beam. It is then neutralized when it passes through a gas cloud, via the charge-exchange process, before being passed into a reactor&#39;s core. 
         [0007]    Moderate energy neutral beams of the required current (about 100 mA) and energy (10-30 keV) have been reliably made for over 40 years. Earlier in fusion research, such beams were used for heating mirror plasmas and, later, beams of these energies but higher currents were used for heating tokamak plasmas and driving currents in them. 
       SUMMARY OF THE INVENTION 
       [0008]    Herein a method and system are disclosed describing the use of NBI for more efficient fueling of a fusion reactor, in this case a field-reversed configuration (FRC). 
         [0009]    Recently, NBI injection has been used for heating and stabilizing large field-reversed configuration (FRC) plasmas. Current and future NBI devices, designed for tokamak-type reactors, have NBI sources with much higher energies and currents than those needed for a Princeton Field-Reversed Configuration-type reactor (PFRC). 
         [0010]    Neutral beam injection for heating a plasma in an FRC has previously been described in Hartman et al. (U.S. Pat. No. 4,314,879, 1979). The FRC described there crucially differs from the current system in many ways. First, the primary claim thereof is the confinement of “gun”-produced FRC&#39;s between magnetic mirror coils and not fueling. Secondly, deuterium and tritium are the envisioned fuels, which changes the requirements and method of fueling, as discussed above. Finally and most importantly, while Hartman et al. state that the plasma is “sustained by energetic neutral particle beams”, this is clarified in the caption of Table 3, where the plasma ring is described as being heated by the neutral beam and not fueled. This can also be seen from the fact that their neutral beam requires 7.2 MW of power for heating, whereas the neutral beam of the current system only needs 650 W for refueling, a difference on the scale of four orders of magnitude. Hartman and his co-author&#39;s disregard for fuel waste is clear from their choice of a pulse plasma device, where fuel burn-up occurs at a rate of 1% per second. 
         [0011]    A more recent application of NBI in FRCs has been described in Rostoker et al. (U.S. Pat. No. 7,613,271, 2009), however, the current system removes the explicit requirement thereof that the neutral beam be polarized. 
         [0012]    Hashimoto (U.S. Pat. No. 4,480,185, 1984), Knorr (U.S. Pat. No. 4,650,631, 1984), Dawson et al. (US H554, 1988) describe the use of NBI for various other types of fusion reactors. Each envisions the use of NBI for heating tokamaks burning deuterium and tritium and none use NBI for fueling purposes. The differences between tokamaks and FRC&#39;s are large, including size, geometry, and system of power generation. These all effect how a neutral beam could be used in such a system. 
         [0013]    Sykes et al. (US20130089171, 2011) also describe using tritium as a fuel in a tokamak, in this case a spherical one, but the goal is not to produce power. Instead, the aim is to generate as many 14 MeV neutrons as possible, which is one of the very outcomes the current system is attempting to minimize, as it can lead to harmful radiation damage to the reactor&#39;s structure. 
         [0014]    Distinct from the present invention, Sykes et al. explicitly describe the fueling system as the standard tokamak method, where “the tokamak fuelling system is designed to inject the fuelling gas or solid pellets of hydrogen, deuterium, and tritium . . . in gaseous or solid form.” Instead, Sykes et al. propose using a neutral beam for plasma heating to drive the current necessary to sustain the plasma. The current invention does not propose using the neutral beam for current drive or heating. This leads to the clear difference between Sykes et al.&#39;s required energy and power (greater than 80 keV and 5-10 MW) and that of the present invention (5 keV and less than 1 kW). Finally, the ‘compact’ reactor of Sykes et al. is significantly larger (e.g. sixteen times larger) than that of the current invention. 
         [0015]    In at least one embodiment, the present invention provides a system and a method for producing and injecting a neutral beam of ions into a plasma contained in an FRC, in order to fuel a continuous fusion reaction of deuterium and helium-3. This will result in a fueling method that is less wasteful and provides finer control of fueling than described in the prior art, allowing for use of rarer fuels, such as helium-3. It also allows for the use of less fuel, a critical requirement for reactors located on aircraft and spacecraft. 
         [0016]    The present invention provides a field-reversed configuration fusion reactor system employing neutral beam injection for fueling. In at least one embodiment, the reactor includes a chamber for containing a stable plasma of fusion fuels and a heating system for heating the plasma and increasing an ion energy of the plasma to produce fusion reactions in the stable plasma. A plurality of magnets are situated coaxial to the reactor chamber and produce a magnetic field that confines the stable plasma and promotes rapid loss of fusion products into a scrape-off-layer (SOL). A fuel injection system, employing a neutral beam injector, is included for injecting additional quantities of the fusion fuel to sustain the power output of the fusion reactions in the plasma. The neutral beam injector consists of an initial chamber to ionize the fusion fuel to form a plasma, means to continuously extract the desired ions from the plasma in a desired amount, means for focusing the extracted ions into a highly directional, narrow beam, means for accelerating the ion beam to a desired energy, means for neutralizing the ion beam using the charge-exchange process, and aiming the neutralized ion beam into the core of the plasma contained in the fusion reactor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings: 
           [0018]      FIG. 1  shows an example magnetic field configuration for an FRC fusion reactor system in accordance with an exemplary embodiment of the invention. 
           [0019]      FIG. 2  shows an example configuration for a neutral beam injection system in accordance with an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one having ordinary skill in the art, that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the disclosed scenarios. Furthermore, reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
         [0021]    During the course of this description like numbers will be used to identify like elements according to the different views, which illustrate the invention. 
         [0022]    In nuclear fusion reactions, light nuclei bind temporarily then fragment to produce fast moving reaction products comprising heavy particles, which contain vast amounts of kinetic energy. This fusion process only occurs rapidly at temperatures of 5-500 keV (58-5,800 million K) such that the Coulomb force, which repulses the positively charged nuclei, is overcome. The reactivity (i.e. rate of fusion) is a function of the ion temperature and plasma density. 
         [0023]    The most important fusion reactions for practical reactors are as follows: 
         [0000]        D+T→α (3.6 MeV)+ n (14.1 MeV)   (Equation 1)
 
         [0000]        D+   3 He→ 4 He(3.7 MeV)+ p (14.7 MeV)   (Equation 2)
 
         [0000]        D+D→   3 He(0.8 MeV)+ n (2.5 MeV)   (Equation 3)
 
         [0024]    where D is deuterium, T is tritium, α is a helium nucleus, n is a neutron, p is a proton, and  3 He and  4 He are helium-3 and helium-4, respectively. The associated kinetic energy of each product is indicated in parentheses. 
         [0025]    The D-T reaction produces most of its energy in neutrons, which means that electrical energy can only be produced by using the neutron radiation to heat a working fluid, much like in a conventional boiler or a fission reactor. Due to temperature limitations of the working fluid, that conversion can only be about 30% efficient. An advantage of the D-T fuel mixture is that it produces net power at the lowest ion temperatures, of only 5-10 keV (1 keV=11.6 million K, and is a more convenient unit of temperature). However, the energetic neutrons liberated in this reaction represent a significant threat to the reactor&#39;s structure as the neutron flux degrades the electrical, mechanical, and thermal properties of the reactor components and also leaves many of their materials radioactive. For terrestrial power reactors, most of these energetic neutrons are used to breed tritium, a scarce material. Thus, the D-T fuel mixture poses significant challenges with radiation damage, material activation, and fuel availability. Pursuing a D-T reactor would require substantial research and development of nuclear materials and tritium breeding as well as several meters worth of shielding to protect reactor components and personnel from neutron radiation. 
         [0026]    The D-D fusion reactions are very attractive because the abundance of deuterium obviates the need to breed tritium. Moreover, the neutrons generated are fewer in number and lower in energy than from D-T per unit of energy produced. By selective treatment of D-D fusion&#39;s daughter products—removing the T from the plasma before it fuses but burning the prompt and decay-formed  3 He, a technique called He-catalyzed D-D fusion—the neutron production can be reduced to 10% of the D-T level, per unit of energy produced. 
         [0027]    The D- 3 He reaction is termed aneutronic, because it directly produces relatively few neutrons and requires none for breeding. The energy from the charged reaction products may be directly converted to electrical power at a much higher efficiency than D-T. However, higher ion energies or temperatures, of 50-100 keV, are required to achieve the same reactivity as D-T. Both D-D and D- 3 He reactions admit D-D side reactions, which for a D- 3 He reactor is the only source of neutron production. A known method for decreasing this neutron generation is lowering the reactant concentration ratio of D- 3 He, for example, from 1:1 to 1:9. In a thermal plasma with 100 keV ion temperatures and T ash promptly removed, neutron production drops from 2.6% to 0.5% of D-T&#39;s per unit of energy produced. This reduces the level of neutron shielding required to well under a meter. However, the lower D concentration reduces the power density by a factor of 4.5, adversely affecting the economics. A method for suppressing this neutron generation pathway was been presented in International Patent Application Serial No. PCT/US13/33767 which is incorporated by reference as if fully disclosed herein. 
         [0028]    Among all fusion fuels, D- 3 He is preferred for the applications described in this disclosure because it produces fewer neutrons while generating net power. 
         [0029]    The plasma is formed within a reactor chamber, which may be small enough to accommodate ion gyroradii that are about 1/10th of the plasma radius and mostly have axis encircling orbits. This promotes the plasma stability against the tilt mode. A heating system, such as radio frequency generating magnetic coils, must be able to raise the plasma&#39;s ion energy such that fusion reactions occur. As one example, locating multiple discrete magnets coaxially to the reactor chamber, a corrugated axial magnetic field is formed that also mediates against the unstable tilt mode. 
         [0030]    The magnetic confinement scheme can employ either permanent magnets or magnetic coils. The magnets control the size of the plasma so that it is large enough to contain fusing plasma of sufficient power while small enough to be effectively heated and allow the fusion products to quickly move into a scrape-off-layer (SOL). The SOL is a typical component of magnetic confinement fusion devices with internal closed field lines and is crucial for this system of thrust augmentation. It is found in tokamaks, stellarators, reversed-field pinches, spheromaks, and field-reversed configurations (FRCs). 
         [0031]    In order to sustain conditions in the reactor&#39;s core, the fusion fuel should be injected at a rate such that it is replenished as it is depleted. In order for the injection scheme to be successful, most of the injected propellant should pass through the lower density, cooler SOL without being ionized. The reactants should then be ionized and contained in the hot, dense core. 
         [0032]    An exemplary FRC, depicted in  FIG. 1 , forms a quasi-toroidal, closed-field, simply connected magnetic confinement device with open field lines exiting the reactor chamber approximately paraxially. A separatrix  10  divides the closed-field region  12  from the open-field region  14 , with the latter containing the SOL. The discrete flux conserving magnetic coils  16  provide magnetic pressure while allowing the heating method, such as an odd-parity (i.e. anti-parallel) rotating magnetic field (RMF o ) from external radio frequency magnetic coils, to penetrate the plasma. A toroidal current  18 , centered on the O-point null line  20 , is formed around the axis of the closed-field region  12 . The surface-to-volume ratio of the FRC is large, reducing the neutron power-load density on the reactor chamber walls of a vacuum vessel  17  and component degradation. However, some radiation from will occur and may be absorbed by a shield  19 . 
         [0033]      FIG. 2  depicts an exemplary system configuration for neutral beam injection, which is used to feed a tight beam of energetic neutral D and  3 He into the center of the reactor chamber. The neutral beam is created by injecting a feedstock gas propellant  22 , in this case helium- 3  or deuterium, into a plasma formation box  24  and charge exchange box  26 . In the plasma formation box  24 , the feedstock gas propellant  22  is weakly ionized. The ionized ions are ejected through an ion extraction grid  30  and then accelerated through a high-voltage ion acceleration grid  32  to form an ion beam  28 . The energy of the beam is determined by the magnitude of the voltage. The ion beam  28  then enters the charge exchange box  26  where electrons are passed from a neutralizing gas  36 , such as argon, to the ion beam, thereby neutralizing it and forming the neutral beam  34 . The neutral beam  34  is then injected into the reaction chamber. 
         [0034]    Not wasting fuel, especially helium- 3  which is rare and expensive, is the primary constraint on the fueling method. There are three stages in the method of particular concern: when the ion beam is neutralized via charge exchange with another gas, when the neutral beam penetrates the SOL, and ionization of the neutral beam when it reaches the core. 
         [0035]    Calculations based on charge-exchange data show that losses are minimal for many gases when there are relatively low beam energies. If argon is used as the neutralizing gas with a 20 keV, then over 97% of the ion beam can be converted into a neutral beam. 
         [0036]    Neutral beam penetration is more successful in the SOL for both fuels as the SOL&#39;s density decreases at higher temperatures. This effect is much more dramatic for deuterium, where less than 10% of the beam penetrates the SOL when the latter is cooler than approximately 100 eV. This is in stark contrast with helium-3 which would have a 50% penetration rate. Furthermore, the deuterium neutral beam penetration through the SOL is highly dependent on beam energy, suggesting that the deuterium neutral beam should have energies above that of the helium-3 neutral beam. 
         [0037]    The neutral beam, after passing through the SOL, is ionized and thus magnetically confined in the core of the device. The core&#39;s much greater density and thickness aids in this objective. Simulations for a 20 keV deuterium neutral beam, with a SOL temperature of 600 eV and electron density of 2e13 particles per cc, show that 86% of the neutral deuterium beam will be deposited in the core, under 0.01% will be transmitted through the entire device and strike the opposite wall of the reactor chamber, and the remaining ˜20% will be ionized and lost in the SOL. Likewise, 96% of a 20 keV helium-3 neutral beam is deposited in the core. 
         [0038]    Although the scenarios herein have been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the disclosed scenarios. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the disclosed scenarios as defined by the appended claims.