Abstract:
A system for fueling a plasma includes a gyrotron for radiating microwave energy into a waveguide. Also included is a module having a deuterium-tritium (DT) fuel pellet, a diamond, quartz or sapphire window, and a pusher medium located between the pellet and window that is made of frozen deuterium (D 2 ) and metallic particles. With the module in the waveguide, the gyrotron is activated. Radiation from the gyrotron is then directed into the waveguide and through the window to cause the inducement of current in the metal particles, causing the particles to become hot. The absorbed microwave energy is then transferred to the pusher medium by conduction resulting in a gaseous expansion of the pusher medium. This ejects the pellet from the waveguide and into the plasma.

Description:
This application is a continuation-in-part of application Ser. No. 11/256,662, filed Oct. 21, 2005, now abandoned. The contents of application Ser. No. 11/256,662 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to systems and methods for accelerating a projectile from standstill to a very high velocity (e.g. 3-5 km/sec). More particularly, the present invention pertains to systems and methods for injecting fuel pellets into a plasma in a plasma confinement device while controlling the core plasma density in the plasma confinement device. The present invention is particularly, but not exclusively, useful for systems and methods that use microwave energy to achieve high velocity fuel pellet injection into the plasma. 
     BACKGROUND OF THE INVENTION 
     Several applications can be envisioned wherein a projectile needs to be quickly accelerated from standstill to a very high velocity (e.g. 3-5 km/sec). For such applications, there are, obviously, certain constraints that require special consideration. For instance, when the acceleration path of the particle is curved, as opposed to being straight, centrifugal acceleration forces are created on the projectile. These forces then present additional constraints for consideration. In particular, any limitations the centrifugal acceleration forces may impose on the acceleration of the projectile must be evaluated. Even when a straight acceleration path is available, access to the path may become a significant concern. Add to this other considerations, such as a need to accelerate a series of projectiles at a high repetition rate, and a need to achieve reliable acceleration, and it becomes clear that each application requires special consideration. 
     As implied above, for specific instances wherein a projectile must be moved along a path that necessarily includes curves, the tortuous nature of the path can severely limit velocity of the projectile. Of particular concern regarding the acceleration of projectiles is the ability to fuel a plasma using projectiles (i.e. fuel pellets). It happens, however, that for several reasons, the use of fuel pellets for this purpose may be very desirable. Indeed, it is a standard practice to fuel various types of plasma confinement devices by injecting frozen hydrogenic pellets into the plasma chamber. 
     It is also well known that toroidally shaped plasma confinement devices are more efficiently fueled, if the fuel can be delivered into the plasma from its (high field side) inner wall. To do this, however, fuel pellets typically need to travel from outside the plasma confinement device and into the plasma. This may require the pellet to travel along a path that is quite tortuous. Nevertheless, in order to ensure good plasma penetration by the fuel pellets, and to have density control flexibility, it is still necessary that the fuel pellet be injected into the plasma at very high velocities. Heretofore, the practice has been to rely on whatever velocity can be practicably attained when acceleration of the pellet is accomplished before the pellet enters the plasma confinement device. 
     In light of the above, it is an object of the present invention to provide systems and methods for accelerating projectiles (fuel pellets), wherein the pellet is moved at a relatively low velocity until the pellet is in position for rapid acceleration and injection into the plasma chamber of the plasma confinement device. Another object of the present invention is to provide systems and methods for accelerating projectiles (fuel pellets) wherein a propulsion force on the pellet is initiated using microwave energy. Still another object of the present invention is to provide systems and methods for accelerating projectiles (fuel pellets) that are easy to use, are relatively simple to operate, and are comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     A system for providing fuel to a plasma has a waveguide, and a gyrotron for directing microwave energy into the waveguide. Also, the system includes a module that is pre-positioned in the waveguide to interact with microwave energy from the gyrotron. The result of this is that a fuel pellet in the module is ejected from the waveguide and into the plasma chamber to fuel plasma in a plasma confinement device More particularly, the claimed invention is currently applicable to fueling of certain magnetic confinement devices in basic thermonuclear energy research, such as tokamak devices. 
     In accordance with the present invention, the waveguide has a substantially straight section that extends between a first end and a second end. This straight section also has a predetermined, substantially uniform cross-sectional area along its length. In combination with the waveguide, the gyrotron mentioned above is used to radiate microwave energy into the straight section. Specifically, the radiation from the gyrotron is directed by the waveguide from the first end of the straight section toward its second end. Accordingly, the second end of the waveguide&#39;s straight section is connected in communication with the plasma chamber of the plasma confinement device. 
     The module that is used for the present invention is integrated in the sense it has several distinct components. In particular, the integrated module includes a fuel pellet that will be used for fueling the plasma in the chamber. Along with the fuel pellet, the integrated module also includes a window and a pusher medium that is positioned between the pellet and the window. Additionally, the module can include a metallic reflector (e.g. a Lithium foil). If used, the metallic reflector is positioned between the fuel pellet and the pusher medium. Importantly, the assembled integrated module, with all of its constituent components, is dimensioned for insertion into the straight section of the waveguide. Stated differently, all components substantially conform to the interior dimensions of the waveguide. 
     In greater detail, the fuel pellet of the module is made of frozen deuterium-tritium (DT) or simply pure deuterium (D 2 ). The window is made from a high strength material with good microwave transparency qualities (e.g. diamond, quartz or sapphire). And, the pusher medium comprises a mixture of a suitable volatile substance, preferably frozen deuterium (D 2 ) and metallic particles. More specifically, the metallic particles in the pusher medium are preferably spherical or disc-shaped conductors that are made from a low atomic number material (e.g. lithium (Li), beryllium (Be), or carbon (C)). For optimal absorption of microwave power transmitted through the pusher medium, the metallic particles, if spherical, have a mean radius “a” between one to ten microns (1 μm&lt;a&lt;10 μm). Furthermore, it is preferable that there be a separation distance “s” between the metallic particles of approximately s˜7a, i.e., (7 μm&lt;s&lt;70 μm). With this range of particle sizes and separation distances, the concentration of metallic particles in the pusher medium will be limited to about one percent or less of the volume of the pusher medium. 
     In the operation of the system of the present invention, a module is first positioned in the straight section of the waveguide. This can be done in either of two ways. For one, a complete module is pre-assembled outside the waveguide. It is then released into the waveguide so that the module enters the straight section of the waveguide through its first end. For the other, the window is permanently affixed to the waveguide, in the straight section, at its first end. Only the pusher medium, metallic reflector and fuel pellet are then pre-assembled, outside the waveguide. This combination is then released into the waveguide so that it enters the straight section of the waveguide through the second end for subsequent contact of the pusher medium with the window. In either case, a complete module is created and positioned inside the waveguide. 
     Once a module has been positioned, and is in place in the straight section of the waveguide, the gyrotron is activated. Radiation from the gyrotron is then directed by the waveguide through the window of the module to interact with the pusher medium. The microwaves interact with the metallic particles within the pusher medium inducing an alternating electrical current flow on the outer surface of the metal particles. The currents heat the metal particles to high temperatures, which in turn heats the pusher medium in contact with the particles. This heat transfer easily vaporizes the volatile pusher medium and creates a high pressure “propellant” gas which accelerates the pellet down the waveguide/guide tube and ejects it into the plasma chamber. 
     As intended for the present invention, the gyrotron will have a high power radiation output that is in a range between approximately one and two megawatts (1-2 MW). Further, the microwave energy in the radiation will preferably be selected to have a wavelength “λ” that will effectively interact with the metallic particles for absorption of the radiation in the pusher medium. In general, wavelengths greater than about one millimeter (λ&gt;1 mm) suffice for this purpose. The import here is to vaporize and continually heat the pusher medium, thereby keeping the expanding gases under high pressure during the acceleration of the pellet. All of this happens in a so-called “one shot” operation. Consequently, as the pusher medium expands, the fuel pellet will be ejected from the waveguide and into the plasma chamber. As envisioned for the present invention, the ejection of fuel pellets can be accomplished at a velocity above three kilometers per second. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a perspective view of a module in accordance with the present invention; 
         FIG. 2  is a cross section view of a type of plasma confinement device showing an incorporation of a system of the present invention; 
         FIG. 3A  is a perspective view of a waveguide as it receives a module for subsequent module activation; 
         FIG. 3B  is a view of the waveguide shown in  FIG. 3A  with the module positioned for activation; 
         FIG. 4A  is a perspective view of an alternate embodiment of a waveguide as it receives a module for subsequent module activation; 
         FIG. 4B  is a view of the waveguide shown in  FIG. 4A  with the module positioned for activation; 
         FIG. 5A  is a perspective view of a waveguide receiving a module for activation; and 
         FIG. 5B  is a perspective view of a fuel pellet being injected into a plasma after activation of a module in a waveguide. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1  a module for use with the systems and methods of the present invention is shown and is generally designated  10 . More specifically,  FIG. 1  shows that the module  10  is integrated to include, in combination, a fuel pellet  12 , a reflector  14 , a pusher medium  16  and a window  18 . For operational reasons, the order in which components of the module  10  are assembled for the present invention is important, and is not arbitrary. Specifically, the reflector  14 , if used, is positioned between the fuel pellet  12  and the pusher medium  16 , as shown. Note: the reflector  14  may be omitted if desired. If so, then the fuel pellet  12  is juxtaposed with the pusher medium  16 . In either case, with or without the reflector  14 , the window  18  is juxtaposed with the pusher medium  16 , and is positioned in the module  10  opposite the fuel pellet  12 . 
     In addition to the order in which components of the module  10  are assembled, the materials used for the various components of the module  10  are important. For example, the fuel pellet  12  for module  10  is preferably, but not limited to, frozen deuterium-tritium (DT). Depending on the particular application, however, the fuel pellet  12  can be made of any composition of matter that can be accelerated as a projectile. When used, the reflector  14  is preferably made of a very thin Lithium foil and, as a practical matter, needs only be several microns thick. Again, other reflective materials may be used. The import here is that a reflector  14  be useful for reflecting radiation (e.g. microwave power) back into the pusher medium  16  for enhanced absorption. To interact with the radiation, the pusher medium  16  is preferably a mixture of frozen deuterium (D 2 ) and metal particles  20 . And, the window  18  needs to be made of a material that is transparent to radiation (microwave power), such as diamond, quartz or sapphire. 
     With the above in mind, the composition of the pusher medium  16  is of particular interest. In the pusher medium  16  the metal particles  20  can be made of any suitable conductor, such as Lithium (Li), Beryllium (Be) or Carbon (C). Further, the metal particles  20  can be shaped as spheres or discs. Preferably, however, the metal particles  20  are shaped as discs that have a radius “a” of about four microns. Importantly, the metal particles  20  are dispersed through the frozen deuterium (D 2 ) with inter-particle spacing “s” between particles  20  that is less than approximately five microns. Also, they are dispersed in a concentration that is about one percent of the volume of the pusher medium  16 . Of particular importance here is that the “effective” or global macroscopic conductivity of the pusher medium  16  is optimized. This is done by keeping the size of the particles  20 , and the inter-particle spacing between particles  20 , well below the mm-sized wavelength “λ” of the microwave power that will be used to heat the pusher medium  16 . 
     In combination, the components of the module  10  can be joined together in any manner well known in the pertinent art. As shown in  FIG. 1 , the module  10  is assembled as a rectangular solid having a height “d” and a width “w”. For most applications, the dimensions “d” and “w” will be in a range of about 2-20 millimeters. These dimensions, of course, can be varied according to the requirements of the particular application and, in some, the width may be equal to the height (e.g. d=w). The overall length of the module  10  will also depend on requirements of the particular application. For instance, requirements such as how much fuel is required for the fuel pellet  12 , and how much propellant is needed for the pusher medium  16  may cause the dimensions of the module  10  to be varied. In each case, however, it is always important that the cross sectional area of the module  10  (e.g. w×d) conform to, and be compatible with, the cross sectional area of the waveguide that will be used for activation of the module  10 . 
     Turning now to  FIG. 2 , a particular environment in which the module  10  (cross reference  FIG. 1 ) of the present invention may be used is shown to be a plasma confinement system, generally designated  22 . It is to be appreciated, however, that the plasma confinement system  22  as shown, is only exemplary. The import of the present invention is for a system and method that employs a microwave-powered pellet accelerator useable for fueling a plasma in a variety of different environments. With this in mind, for purposes of discussion, the plasma confinement system  22  is shown to contain a plasma  24  that is confined within a chamber  26 . As intended for the present invention, and mentioned above, the purpose here is to fuel the plasma  24 . To do this, the module  10  is pre-positioned in the plasma confinement system  22 , and it is then activated to inject the fuel pellet  12  (see  FIG. 1 ) into the plasma  24 . For example, in an embodiment of the present invention as shown in  FIG. 2 , a module  10  (see  FIG. 1 ) is advanced through a waveguide  28  and is pre-positioned at a point  30  in the waveguide  28 . Once the module  10  (see  FIG. 1 ) is at the point  30 , microwave power is radiated into the waveguide  28  from a gyrotron  32 . This microwave power then activates the module  10  by heating the pusher medium  16  and causing it to rapidly expand as a gas. The intended consequence of this is that the fuel pellet  12  (see  FIG. 1 ) is ejected at a very high velocity (e.g. 3 km/sec) from the waveguide  28 , and injected into the plasma  24 . Various embodiments for doing all of this are best seen with reference to  FIGS. 3A ,  3 B,  4 A, and  4 B. 
     Referring first to  FIG. 3A , it will be seen that the waveguide  28  includes a straight section  36 . Specifically, the straight section  36  is shown to have a length “l” that extends from a first end  38  to a second end  40 . Also, the waveguide  28  is shown to have a cross sectional area that is defined by a height “d” and a width “w”. With reference to the dimensions of module  10  discussed above, it is to be appreciated that there will necessarily be some tolerance between the respective “d” and “w” of the module  10  and “d” and “w” of the waveguide  28 . Nevertheless, this tolerance can, and should, be minimized. Again, this can be done with operational considerations in mind. On this point, again for operational reasons, the particular shape of the cross section of guidewave  28  is essentially a matter of design choice (e.g. circular, rectangular, oval etc.). 
     Referring now to  FIG. 4A , in an alternate embodiment of the present invention, the waveguide  28  is shown to include a chute  42  that is located between the ends  38  and  40  of straight section  36 . In all important respects, for both embodiments of the waveguide  28  ( FIG. 3A  and  FIG. 4A ) the respective straight sections  36  are functionally identical. Most important, the sections  36  are straight so there will be no structural limitations to the rapid linear acceleration of any fuel pellet  12  when it is ejected through the end  40  of waveguide  28  by the activation of a module  10 . 
     In the operation of the present invention, there are essentially two ways by which a module  10  can be positioned in the straight section  36  of a waveguide  28  for activation. The first is illustrated in  FIGS. 3A and 3B . There it is to be appreciated that a module  10  is pre-assembled outside the plasma confinement system  22  before it is placed in the waveguide  28 . Once in the waveguide  28 , the module  10  is allowed to travel through the waveguide  28 , at a relatively low velocity (e.g. 50 m/sec), until it reaches the point  30  (see  FIG. 2  and  FIG. 3B ). At the point  30 , the module  10  is activated. Specifically, with the module  10  at point  30 , the gyrotron  32  is energized to direct radiation  44  through the waveguide  28 . The radiation  44  then interacts with the pusher medium  16  of the module  30 , to heat the pusher medium  16  and thereby cause a gaseous expansion that will eject the fuel pellet  12  of module  10  from the waveguide  28 . As indicated in  FIG. 2 , the ejection of a fuel pellet  12  will cause it to travel along the path  34 , and into the plasma  24 . There, the fuel pellet  12  is used to fuel the plasma  24 . 
     The second way by which a module  10  can be positioned in the straight section  36  of a waveguide  28  is illustrated in  FIGS. 4A and 4B . In this case, the straight section  36  is modified in at least two aspects. For one, the window  18  is permanently affixed in the section  36  at the point  30 . For another, only the fuel pellet  12 , reflector  14  (if used), and the pusher medium  16  are pre-assembled outside the plasma confinement system  22 . As shown in  FIG. 4B , after the combination of fuel pellet  12 , reflector  14  and pusher medium  16  has been inserted through the chute  42 , a module  10  is effectively assembled at the point  30 . As with the embodiment of the invention shown in  FIGS. 3A and 3B , the module  10  is then activated. 
     To underscore the versatility of the present invention,  FIG. 5A  shows a module  10  being injected into the waveguide  28  of a device (not shown). More particularly, for purposes of the present invention the device may be of any type, well known in the art, which is usable for creating a plasma  24 . Again, references to plasma confinement systems (i.e. plasma confinement system  22 ) are only for exemplary purposes. 
     As mentioned above, once the module  10  is in position in the waveguide  28 , the gyrotron  32  is used to activate the module  10 . The result of this activation is an acceleration of the fuel pellet  12  in the direction of arrow  46  (see  FIG. 5B ). Thus, the fuel pellet  12  is ejected from the waveguide  28  and into the plasma  24 . Inside the chamber  26 , the fuel pellet  12  functions to fuel the plasma  24 . In all instances, activation of the module  10  with radiation  44 , and the consequent acceleration of the fuel pellet  12  can be engineered as disclosed herein to achieve compliance with the requirements necessary for using fuel pellets  12  as fuel for the plasma  24 . 
     While the particular Microwave-Powered Pellet Accelerator as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.