Abstract:
A particle accelerator system, including apparatuses and methods, that is configurable through repositioning of shorting devices therein to operate at different charged particle beam currents while maintaining optimum transfer of electromagnetic power from electromagnetic waves to one or more accelerating sections thereof, and reducing or eliminating reflections of electromagnetic waves. The particle accelerator system includes at least two accelerating sections and an electromagnetic drive subsystem with portions of the electromagnetic drive subsystem being interposed physically between the accelerating sections, thereby making the particle accelerator system compact. The electromagnetic drive subsystem includes, among other components, a 3 dB waveguide hybrid junction having a coupling window in a narrow wall thereof which is shared by the junction&#39;s rectangular-shaped waveguides. By virtue of the coupling window being positioned in a narrow wall rather than a wide wall, the maximal power of the 3 dB waveguide hybrid junction is increased significantly.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of priority to U.S. provisional patent application Ser. No. 60/414,300 which is entitled “Two Section Particle Accelerator with Controlled Beam Current” and was filed on Sep. 27, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates, generally, to the field of particle accelerators and, more specifically, to particle accelerators having controlled beam current.  
       BACKGROUND OF THE INVENTION  
       [0003]     Standing wave linear accelerators with controlled beam current are utilized in a wide variety of medical and industrial applications, including, radiography, radiotherapy, medical instrument sterilization, food irradiation, and dangerous substance neutralization. In such applications, available space is often limited and, hence, it is desirable that the accelerators be compact. For example, in a medical radiotherapy application, an accelerator, electron gun, and target are installed in an x-ray head of a movable gantry which may be moved around a patient lying on a table to direct x-ray radiation at an appropriate location of the patient&#39;s body. To achieve a sufficiently large area of irradiation with the required dose uniformity, the distance between the target and the patient should be as large as possible. In order to maximize the distance between the target and the patient, it is advantageous for the accelerator to have a short structure length and, hence, a high accelerating gradient to produce a beam of charged particles having an appropriate energy level in such a short structure.  
         [0004]     In typical standing wave linear accelerators often used in such applications, the standing wave linear accelerators comprise multiple accelerating sections with each accelerating section having an alternating series of connected accelerating and coupling cavities that form a biperiodic structure. An injector emits charged particles into an accelerating section and the charged particles are accelerated as they travel in a charged particle beam through the accelerating sections by electromagnetic fields present therein. The electromagnetic fields are created by electromagnetic power (i.e., in the form of radio frequency (RF) waves) that is produced by an RF generator (for example, a magnetron) and delivered to the accelerating sections by feeding waveguides which, generally, comprise hollow pipes having a rectangular cross-section.  
         [0005]     Unfortunately, reflections of the electromagnetic wave are often produced in the feeding waveguides with the extent of such reflections being dependent, at least in part, upon the coupling coefficients between the feeding waveguides and accelerating sections. To make matters worse, for an accelerator operating at a particular beam current, there is only one value of the coupling coefficient between a feeding waveguide and an accelerating section at which all of the power of the electromagnetic wave present in the feeding waveguide is delivered to the accelerating section without reflections. Because the coupling coefficient between each feeding waveguide and respective accelerating section is constant and cannot be changed in the known accelerators for operation at different beam currents, reflections are generated which may travel back to and damage the accelerator&#39;s magnetron and, hence, all of the power delivered by each feeding waveguide (i.e., in the form of an electromagnetic wave) is not maximally utilized for particle acceleration.  
         [0006]     To prevent such reflections from traveling back to the RF generator, some accelerator manufacturers have employed ferrite isolators or circulators to isolate the RF generator from the accelerating sections and feeding waveguides. However, ferrite isolators and circulators are expensive and their use results in RF power losses and, hence, decreased accelerator efficiency. As an alternative to ferrite isolators and circulators, the 3 dB waveguide hybrid junction was developed for use between the RF generator and the feeding waveguides. A 3 dB waveguide hybrid junction, generally, includes two parallel waveguides having rectangular cross-sections such that each waveguide, therefore, has two walls which are wider than the other two walls thereof (i.e., the wider walls being referred to sometimes herein as “wide walls”). One of the wide walls of each such waveguide comprises a common wide wall therebetween which is shared by both waveguides. Therefore, the parallel waveguides are oriented adjacent to one another by virtue of the shared, common wide wall. In addition, a 3 dB waveguide hybrid junction typically includes a coupling hole, or window, in the shared, common wide wall. When installed in an accelerator having two accelerating sections, a first end of the first waveguide of the 3 dB waveguide hybrid junction is connected to the magnetron output and a second end of the first waveguide is often connected to still another waveguide that, in turn, connects to one of the accelerating sections of the accelerator. A first end of the second waveguide of the 3 dB waveguide hybrid junction is connected to a waveguide load which receives electromagnetic power and a second end of the second waveguide is often connected to still another waveguide that connects to another of the accelerating sections of the accelerator.  
         [0007]     In operation, the 3 dB waveguide hybrid junction receives input electromagnetic power from the RF generator through the first end of the first waveguide. A first portion of the electromagnetic power travels through the first waveguide to its second end and then to an accelerating section via another connected waveguide. A second portion of the electromagnetic power travels through the coupling window in the junction&#39;s common wide wall and into the junction&#39;s second waveguide and then travels through the second end of the second waveguide and on to a different accelerating section via another connected waveguide. Reflections of electromagnetic waves received through the second end of the junction&#39;s first waveguide are directed through the coupling window and into the second waveguide. Reflections of electromagnetic waves received through the second end of the second waveguide and reflections received through the coupling window are directed through the first end of the second waveguide to the waveguide load, thereby protecting the RF generator from potential damage.  
         [0008]     While the 3 dB waveguide hybrid junction serves to protect the RF generator, high electrical fields are present along the junction&#39;s wide wall and at the edges of the coupling window therein. Thus, by virtue of the coupling window being positioned in the junction&#39;s wide wall, the maximal power of the 3 dB waveguide hybrid junction is limited. Also, the turns or bends in the waveguides that often connect the 3 dB waveguide hybrid junction to the accelerating sections of an accelerator results in the accelerator having larger overall dimensions, making the accelerator less desirable for the applications described above.  
         [0009]     Therefore, there exists in the industry, a need for a particle accelerator that is compact, that makes maximal use of electromagnetic power to accelerate charged particles at different beam currents, and that does not include a 3 dB waveguide hybrid junction with limited maximal power, that addresses these and other problems or difficulties which exist now or in the future.  
       SUMMARY OF THE INVENTION  
       [0010]     Broadly described, the present invention comprises a particle accelerator system with controlled charged particle beam current and methods of operating same. More particularly, the present invention comprises a particle accelerator system which is configurable to operate at different charged particle beam currents while maintaining optimum transfer of electromagnetic power from an RF generator to one or more accelerating sections thereof and reducing or eliminating reflections of electromagnetic waves. The particle accelerator system of the present invention includes at least two accelerating sections and an electromagnetic drive subsystem with portions of the electromagnetic drive subsystem being interposed physically between the accelerating sections. The electromagnetic drive subsystem includes, among other components, a 3 dB waveguide hybrid junction having a coupling window in a wide wall thereof which is shared by the junction&#39;s waveguides.  
         [0011]     Advantageously, the particle accelerator system includes movable shorting devices which are positionable in a plurality of positions relative to the accelerator system&#39;s longitudinal axis, thereby enabling the coupling coefficients between the accelerator system&#39;s feeder waveguides and accelerating sections to be changed by moving the shorting devices into different positions. Because there is only one value of the coupling coefficients between the feeder waveguides and the accelerating sections at which all of the power of the electromagnetic waves of the feeder waveguides is delivered to the accelerating sections without reflections and is maximally utilized for charged particle acceleration for each charged particle beam current at which the particle accelerator system is operated, the movability of the movable shorting devices into a plurality of positions allows optimal setting of the coupling coefficients for operation of the particle accelerator system at any charged particle beam current desired and, hence, allows the particle accelerator system to be operated at a plurality of different charged particle beam currents at peak efficiency. When the coupling coefficients are so optimized, the magnitude of the longitudinal component of the electric field produced at the accelerator system&#39;s longitudinal axis is also optimized at a maximum.  
         [0012]     Also advantageously, the particle accelerator system includes an electromagnetic drive subsystem having feeder waveguides which are physically interposed between the system&#39;s accelerating sections. A drift tube formed in a common narrow wall shared by the feeder waveguides enables charged particles to travel between the accelerating sections during the system&#39;s operation. The common narrow wall shared by the feeder waveguides is also shared by the waveguides of a 3 dB waveguide hybrid junction, thereby causing each of the feeder waveguides to be connected to a respective waveguide of the 3 dB waveguide hybrid junction in a coaxial relationship. By virtue of the feeder waveguides being interposed physically between the system&#39;s accelerating sections and by virtue of the coaxial relationship of the feeder waveguides and respective waveguides of the 3 dB waveguide hybrid junction (i.e., thereby requiring no turns, or bends, in the waveguides and, hence, less power loss in the waveguides), the particle accelerator system of the present invention is more compact and more efficient than other known particle accelerator systems.  
         [0013]     Further, the particle accelerator system&#39;s 3 dB waveguide hybrid junction includes a coupling window in the common narrow wall shared by the feeder waveguides and the junction&#39;s waveguides. Because the coupling window is located in a narrow wall of the junction&#39;s waveguides as opposed to being located in a wide wall of the junction&#39;s waveguides, the maximal power of the junction is significantly higher than that of other known 3 dB waveguide hybrid junctions having a coupling window in a wide wall thereof.  
         [0014]     Other advantages and benefits of the present invention will become apparent upon reading and understanding the present specification when taken in conjunction with the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  displays a schematic sectional view of a particle accelerator system in accordance with an exemplary embodiment of the present invention.  
         [0016]      FIG. 2  displays a schematic sectional view of the particle accelerator system of  FIG. 1  taken along lines  2 - 2 .  
         [0017]      FIG. 3  displays a schematic sectional view of the electromagnetic drive subsystem of the particle accelerator system of  FIG. 2  taken along lines  3 - 3 .  
         [0018]      FIG. 4  displays a pictorial view of the feeder and shorting waveguides of the electromagnetic drive subsystem of  FIG. 3 .  
         [0019]      FIG. 5  displays a graphical illustration of the relationship between the shorting device position and the electric field magnitude at the longitudinal axis of the particle accelerator system in accordance with the exemplary embodiment of the present invention.  
         [0020]      FIG. 6  displays a schematic perspective view of an alternative shorting waveguide in accordance with the exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring now to the drawings in which like numerals represent like elements or steps throughout the several views,  FIG. 1  displays a schematic sectional view of a particle accelerator system  100  in accordance with an exemplary embodiment of the present invention. The particle accelerator system  100  comprises a first accelerating section  102 , a second accelerating section  104 , an electromagnetic drive subsystem  106 , and an injector  108 . Preferably, the first and second accelerating sections  102 ,  104  comprise standing-wave accelerating sections  102 ,  104  having a biperiodic accelerating structure which are operable to accelerate charged particles through the transfer of energy from electromagnetic power provided by the electromagnetic drive subsystem  106 .  
         [0022]     The first accelerating section  102  has a first end  110  and a second end  112 , and includes a plurality of accelerating cavities  114  and a plurality of coupling cavities  116  arranged in an axial arrangement. A coupling cavity  116  is interposed between consecutive pairs of accelerating cavities  114 . Each adjacent accelerating cavity  114  and coupling cavity  116  are connected by a respective drift tube  118  which is adapted to direct charged particles between each adjacent accelerating cavity  114  and coupling cavity  116 . Each adjacent accelerating cavity  114  is RF coupled to the adjacent coupling cavity  116  via two coupling slots (not shown). The injector  108  is positioned proximate the first end  110  of the first accelerating section  102  and is connected to a first accelerating cavity  114 A of the first accelerating section  102  by a drift tube  120 . The injector  108  is operable to generate charged particles and to emit them into the first accelerating cavity  114 A via drift tube  120 . Preferably, the injector  108  is operable to generate and emit charged particles comprising electrons. The first accelerating section  102  also includes a drift tube  122  connected to the last accelerating cavity  114 Z thereof and extending between the last accelerating cavity  114 Z and an output port  124  located at the second end  112  of the first accelerating section  102 . Drift tube  122  and output port  124  are adapted to direct charged particles from the first accelerating section  102  into a drift tube  250  of the electromagnetic drive subsystem  106 , as described below, for delivery to the second accelerating section  104 . The first accelerating section  102  defines an oblong-shaped slot  126  which couples the last accelerating cavity  114 Z to a feeder waveguide  204  of the electromagnetic drive subsystem  106  to enable electromagnetic power to propagate from the feeder waveguide  204  into the last accelerating cavity  114 Z and through the other accelerating cavities  114  and coupling cavities  116  in a direction generally toward the injector  108  and the first end  110  of the first accelerating section  102 .  
         [0023]     Similar to the first accelerating section  102 , the second accelerating section  104  has a first end  150  and a second end  152 , and includes a plurality of accelerating cavities  154  and a plurality of coupling cavities  156  arranged in an axial arrangement. A coupling cavity  156  is interposed between consecutive pairs of accelerating cavities  154 . Each adjacent accelerating cavity  154  and coupling cavity  156  are connected by a respective drift tube  158  which is adapted to direct charged particles between each adjacent accelerating cavity  154  and coupling cavity  156 . Each adjacent accelerating cavity  154  is RF coupled to the adjacent coupling cavity  156  via two coupling slots (not shown). The second accelerating section  104  also includes a drift tube  160  connected to the first accelerating cavity  154 A thereof and extending between the first accelerating cavity  154 A and an input port  162  located at the first end  150  of the second accelerating section  104 . Drift tube  160  and input port  162  are adapted to receive charged particles from a drift tube  250  of the electromagnetic drive subsystem  106 , as described below, and to direct them toward the first accelerating cavity  154 A. Additionally, the second accelerating section  104  includes a drift tube  164  connected to the last accelerating cavity  154 Z thereof which extends between the last accelerating cavity  154 Z and an output port  166  located at the second end  152  of the second accelerating section  104 . Drift tube  164  and output port  166  are adapted to direct charged particles from the second accelerating section  104  (and, hence, from the particle accelerator system  100 ) toward a desired target or other object. The second accelerating section  104  defines an oblong-shaped slot  168  which couples the first accelerating cavity  154 A to a feeder waveguide  206  of the electromagnetic drive subsystem  106  to allow electromagnetic power to propagate from the feeder waveguide  206  into the first accelerating cavity  154 A and through the other accelerating cavities  154  and coupling cavities  156  in a direction generally toward the second end  152  of the second accelerating section  104 .  
         [0024]     The accelerating cavities  114 ,  154  and coupling cavities  116 ,  156  of the first and second accelerating sections  102 ,  104  are, as described briefly above, arranged in an axial arrangement. As seen in  FIG. 1 , drift tubes  118 ,  120 ,  122  and output port  124  of the first accelerating section  102  and input port  162 , drift tubes  158 ,  160 ,  164 , and output port  166  of the second accelerating section  104  define a longitudinal axis  190  of the particle accelerator system  100  along which charged particles principally travel in a charged particle beam during operation of the particle accelerator system  100 . It should be noted that while the figures and accompanying description of the present application display and describe a particle accelerator system  100  having accelerating sections  102 ,  104  having accelerating cavities  114 ,  154  and coupling cavities  116 ,  156  which are arranged in an axial arrangement, the scope of the present invention further comprises particle accelerator systems having accelerating cavities and coupling cavities arranged in a different arrangement, including, without limitation, an arrangement in which coupling cavities are side-coupled to the accelerating cavities. It should also be noted that the scope of the present invention further comprises particle accelerator systems having more than two accelerating sections and accelerating sections having different numbers of accelerating cavities and coupling cavities than those described herein.  
         [0025]      FIG. 2  displays a schematic sectional view of the particle accelerator system  100  of  FIG. 1  taken along lines  2 - 2 . As seen more clearly in  FIG. 2 , the electromagnetic drive subsystem  106  comprises an RF generator  200 , a waveguide load  202 , a first feeder waveguide  204  and a second feeder waveguide  206 . The RF generator  200  is operable to generate pulses of electromagnetic waves having an appropriate frequency and power level. Preferably, the RF generator  200  includes a klystron which generates electromagnetic waves having a frequency of 2856 MHz and 6 MW of power. Also preferably, the electromagnetic wave is a radio frequency (RF) electromagnetic wave. Alternatively, the RF generator  200  may include a magnetron or other devices for generating electromagnetic waves having an appropriate frequency and power level. The waveguide load  202  is adapted to receive reflections of electromagnetic waves during the rise and fall time of RF pulses. By receiving such reflections and dissipating the energy therein, the waveguide load  202  protects the RF generator  200  from the harmful effects of such reflections and the energy thereof.  
         [0026]     As displayed in  FIGS. 1 and 2 , each feeder waveguide  204 ,  206  includes a portion thereof which is interposed between the second end  112  of the first accelerating section  102  and the first end  150  of the second accelerating section  104 . Each feeder waveguide  204 ,  206 , respectively, has, three side walls  208 A,  208 B,  210 A,  210 B,  212 A,  212 B and a common wall  214  which are, preferably, manufactured from a material such as, for example and not limitation, copper or other materials having similarly acceptable characteristics. Wall  208 A of the first feeder waveguide  204  defines a passageway  216  extending therethrough having a slot  218  which aligns with the oblong-shaped slot  126  to enable electromagnetic waves and power in the first feeder waveguide  204  to propagate via the passageway  216 , slot  218 , and oblong-shaped slot  126  into the first accelerating section  102 . Similarly, wall  210 B defines a passageway  220  therethrough having a slot  222  which aligns with the oblong-shaped slot  168  to enable electromagnetic waves and power in the second feeder waveguide  206  to propagate via the passageway  220 , slot  222 , and oblong-shaped slot  168  into the second accelerating section  104 .  
         [0027]     In accordance with the exemplary embodiment described herein, the walls  208 ,  210 ,  212 ,  214  of the feeder waveguides  204 ,  206  define the waveguides  204 ,  206  to have, generally, rectangular cross-sections with each waveguide  204 ,  206  having, respectively, two parallel wide sides  224 A,  226 A,  224 B,  226 B and two parallel narrow sides  228 A,  230 A,  228 B,  230 B. Each wide side  224 A,  226 A,  224 B,  226 B has a length designated by dimension “A” (see  FIG. 3 ) and each narrow side  228 A,  230 A,  228 B,  230 B has a width designated by dimension “B” (see  FIG. 2 ), such that dimension “A” is greater than dimension “B”. Preferably, the first feeder waveguide  204  is oriented with a portion of wall  208 A and its first wide side  224 A adjacent to the second end  112  of the first accelerating section  102  and with a portion of wall  210 A and its second wide side  226 A adjacent to the first end  150  of the second accelerating section  104 . Similarly, the second feeder waveguide  206  is oriented with a portion of wall  208 B and its first wide side  224 B adjacent to the second end  112  of the first accelerating section  102  and with a portion of wall  210 B and its second wide side  226 B adjacent to the first end  150  of the second accelerating section  104 . Also preferably, the wide sides  224 A,  226 A,  224 B,  226 B of the first and second feeder waveguides  204 ,  206  are respectively parallel due to the rectangular cross-section of the feeder waveguides  204 ,  206 , are respectively perpendicular to the longitudinal axis  190  of the particle accelerator system  100 , and define a transverse axis  232  of the particle accelerator system  100  midway therebetween which is also perpendicular to the longitudinal axis  190  of the particle accelerator system  100 . Because portions of the feeder waveguides  204 ,  206  physically reside between the accelerating sections  102 ,  104 , the particle accelerator system  100  is made to be more compact in the transverse direction (i.e., defined by the transverse axis  232 ) than other known particle accelerator systems  100 . Further, because the feeder waveguides  204 ,  206  share a common wall  214 , the particle accelerator system  100  is more compact in the longitudinal direction (i.e., defined by the longitudinal axis  190 ).  
         [0028]     It should be understood that while the figures and accompanying description of the exemplary embodiment display and describe feeder waveguides  204 ,  206  that are oriented with their wide sides  224 A,  224 B,  226 A,  226 B respectively adjacent the second end  112  of the first accelerating section  102  and the first end  150  of the second accelerating section  104 , the scope of the present invention further comprises feeder waveguides  204 ,  206  having their narrow sides  228 A,  230 A,  228 B,  230 B oriented respectively adjacent the second end  112  of the first accelerating section  102  and the first end  150  of the second accelerating section  104 . Also, it should be understood that the scope of the present invention further comprises feeder waveguides  204 ,  206  having their wide sides  224 A,  224 B,  226 A,  226 B not perpendicular to the longitudinal axis  190  of the particle accelerator system  100 , but at an angle other than ninety degrees to the longitudinal axis  190  of the particle accelerator system  100 . Additionally, it should be understood that the scope of the present invention further comprises feeder waveguides  204 ,  206  having cross-sections which are not rectangular in shape, but instead have other shapes.  
         [0029]      FIG. 3  displays a schematic sectional view of the electromagnetic drive subsystem  106  of the particle accelerator system  100  of  FIG. 2  taken along lines  3 - 3 . As illustrated in  FIG. 3 , the common wall  214  of the feeder waveguides  204 ,  206  defines a drift tube  250  therein which is, preferably, centered about the longitudinal axis  190  of the particle accelerator system  100 . The drift tube  250  has first and second ends  252 ,  254  and provides a passageway  256  for charged particles to travel between the first and second accelerating sections  102 ,  104 . The first end  252  of the drift tube  250  abuts the output port  124  of the first accelerating section  102  and the input port  162  of the second accelerating section  104 , thereby enabling the charged particles of a charged particle beam to travel, during operation of the particle accelerator system  100 , from the first accelerating section  102  through output port  124 , through passageway  256 , and through input port  162  into the second accelerating section  104 .  
         [0030]     The electromagnetic drive subsystem  106  further comprises, as seen in  FIG. 3 , a 3 dB waveguide hybrid junction  260  which is connected to the feeder waveguides  204 ,  206 , to the RF generator  200 , and to the waveguide load  202 . The 3 dB waveguide hybrid junction  260  includes a first waveguide  262  and a second waveguide  264  which are defined by respective walls  266 A,  268 A,  270 A,  266 B,  268 B,  270 B and by common wall  214  which the 3 dB waveguide hybrid junction  260 , preferably, shares with the feeder waveguides  204 ,  206 . Preferably, the first waveguide  262  has a, generally, rectangular cross-section with walls  266 A,  268 A forming wide sides  272 A,  274 A thereof and walls  270 A,  214  forming narrow sides  276 A,  278 A thereof. Each wide side  272 A,  274 A has a length designated by dimension “A” (see  FIG. 3 ) and each narrow side  276 A,  278 A has a width designated by dimension “B” (see  FIG. 2 ), such that dimension “A” is greater than dimension “B”. Walls  266 A,  268 A,  270 A,  214  also define a first output opening  280  of the 3 dB waveguide hybrid junction  260  which mates with an input opening  282  of feeder waveguide  204  so that walls  266 A,  268 A,  270 A are, respectively and preferably, coplanar with walls  208 A,  210 A,  212 A of the first feeder waveguide  204  (and, hence, sides  272 A,  274 A,  276 A,  278 A of waveguide  262  are coplanar with sides  224 A,  226 A,  228 A of the first feeder waveguide  204 ), thereby allowing electromagnetic waves and power to propagate from the first waveguide  262  of the 3 dB waveguide hybrid junction  260  into feeder waveguide  204  during operation of the particle accelerator system  100 . Additionally, walls  266 A,  268 A,  270 A,  214  also define an input opening  283  of the 3 dB waveguide hybrid junction  260  which mates with an output opening  284  of RF generator  200 , thereby enabling electromagnetic waves and power to propagate from the RF generator  200  into the first waveguide  262  of the 3 dB waveguide hybrid junction  260  during operation of the particle accelerator system  100 .  
         [0031]     Similarly and preferably, the second waveguide  264  has a, generally, rectangular cross-section with walls  266 B,  268 B forming wide sides  272 B,  274 B thereof and walls  270 B,  214  forming narrow sides  276 B,  278 B thereof. Each wide side  272 B,  274 B has a length designated by dimension “A” (see  FIG. 3 ) and each narrow side  276 B,  278 B has a width designated by dimension “B” (see  FIG. 2 ), such that dimension “A” is greater than dimension “B”. Walls  266 B,  268 B,  270 B,  214  also define a second output opening  286  of the 3 dB waveguide hybrid junction  260  which mates with an input opening  288  of feeder waveguide  206  so that walls  266 B,  268 B,  270 B are, respectively and preferably, coplanar with walls  208 B,  210 B,  212 B of the second feeder waveguide  206  (and, hence, sides  272 B,  274 B,  276 B,  278 B of waveguide  264  are coplanar with sides  224 B,  226 B,  228 B of the second feeder waveguide  206 ), thereby allowing electromagnetic waves and power to propagate from the second waveguide  264  of the 3 dB waveguide hybrid junction  260  into feeder waveguide  206  during operation of the particle accelerator system  100 . Additionally, walls  266 B,  268 B,  270 B,  214  also define a third output opening  289  of the 3 dB waveguide hybrid junction  260  which mates with an input opening  290  of waveguide load  202 , thereby enabling reflections of electromagnetic waves to propagate from the second waveguide  264  of the 3 dB waveguide hybrid junction  260  to the waveguide load  202  during operation of the particle accelerator system  100 .  
         [0032]     The portion of common wall  214  present in the 3 dB waveguide hybrid junction  260  defines a coupling window  300  which extends through the wall  214  and between first and second waveguides  262 ,  264  of the 3 dB waveguide hybrid junction  260 . The coupling window  300  is adapted to allow, during operation of the particle accelerator system  100 , electromagnetic waves and power received by the 3 dB waveguide hybrid junction  260  from the RF generator  200  to be divided to form first electromagnetic waves and second electromagnetic waves with the first electromagnetic waves having a first portion of the power of the received electromagnetic waves and the second electromagnetic waves having a second portion of the power of the received electromagnetic waves. The ratio of the first and second portions of the power of the received electromagnetic waves (and, hence, the ratio of the power of the first electromagnetic waves to the power of the second electromagnetic waves) is based, at least in part, upon the dimensions of the coupling window  300 . The coupling window  300  is further adapted to direct reflections of the first electromagnetic waves, received from the first accelerating section  102  via feeder waveguide  204  and first waveguide  262 , into second waveguide  264 . By virtue of the coupling window  300  being positioned in narrow sides  278 A,  278 B of first and second waveguides  262 ,  264  (i.e., as opposed to being positioned in wide sides  272 A,  274 A,  272 B,  274 B), the electric field at the edges of the coupling window  300  are zero and, as a consequence, the electric field of the 3 dB waveguide hybrid junction  260  is maximal (i.e., and corresponds to the maximal power of a waveguide without a coupling window  300  therein) and is not limited by the high electric fields which would, otherwise, be present at the edges of the coupling window  300  if the coupling window  300  were positioned in a wide side  272 A,  274 A,  272 B,  274 B of the first and second waveguides  262 ,  264 .  
         [0033]     The 3 dB waveguide hybrid junction  260  is configured to direct, during operation of the particle accelerator system  100 , the first electromagnetic waves and associated power through first waveguide  262  and first output opening  280  into feeder waveguide  204  and to direct the second electromagnetic waves and associated power through second waveguide  264  and second output opening  286  into feeder waveguide  206 . The 3 dB waveguide hybrid junction  260  is further configured to direct reflections of the first electromagnetic waves received by the second waveguide  264  via coupling window  300  and reflections of the second electromagnetic waves received, from the second accelerating section  104  via feeder waveguide  206  and second waveguide  264 , to the waveguide load  202  via third output opening  289  during operation of the particle accelerator system  100 . Because the 3 dB waveguide hybrid junction  260  is connected directly and linearly to the feeder waveguides  204 ,  206  that supply electromagnetic waves and associated power to the accelerating sections  102 ,  104 , there are no additional waveguides and no waveguide turns, or bends, necessary to couple the 3 dB waveguide hybrid junction  260  with the accelerating sections  102 ,  104 . As a consequence, the overall size of the particle accelerator system  100  is reduced in comparison to the size of other known particle accelerator systems which require additional waveguides and/or waveguide turns, or bends, to couple accelerating sections with an RF generator.  
         [0034]     The electromagnetic drive subsystem  106  further comprises, as seen in  FIGS. 2 and 3 , a pair of shorting waveguides  320 ,  322  which are connected, respectively, to feeder waveguides  204 ,  206 . The first and second shorting waveguides  320 ,  322  are defined by respective walls  324 A,  326 A,  328 A,  324 B,  326 B,  328 B and by common wall  214  which the shorting waveguides  320 ,  322 , preferably, share with the feeder waveguides  204 ,  206 . Preferably, the first shorting waveguide  320  has a, generally, rectangular cross-section with walls  324 A,  326 A forming wide sides  330 A,  332 A thereof and walls  328 A,  214  forming narrow sides  334 A,  336 A thereof. Each wide side  330 A,  332 A has a length designated by dimension “A” (see  FIG. 3 ) and each narrow side  334 A,  336 A has a width designated by dimension “B” (see  FIG. 2 ), such that dimension “A” is greater than dimension “B”. Walls  324 A,  326 A,  328 A,  214  also define an input opening  338  of the first shorting waveguide  320  which mates with an output opening  340  of feeder waveguide  204  (defined by walls  208 A,  210 A,  212 A,  214  of feeder waveguide  204 ) so that walls  324 A,  326 A,  328 A are, respectively and preferably, coplanar with walls  208 A,  210 A,  212 A of feeder waveguide  204  (and, hence, sides  330 A,  332 A,  334 A,  336 A of shorting waveguide  320  are coplanar with sides  224 A,  226 A,  228 A of feeder waveguide  204 ), thereby allowing the first electromagnetic waves and associated power to propagate from feeder waveguide  204  into shorting waveguide  320  during operation of the particle accelerator system  100 .  
         [0035]     Similarly and preferably, the second shorting waveguide  322  has a, generally, rectangular cross-section with walls  324 B,  326 B forming wide sides  330 B,  332 B thereof and walls  328 B,  214  forming narrow sides  334 B,  336 B thereof. Each wide side  330 B,  332 B has a length designated by dimension “A” (see  FIG. 3 ) and each narrow side  334 B,  336 B has a width designated by dimension “B” (see  FIG. 2 ), such that dimension “A” is greater than dimension “B”. Walls  324 B,  326 B,  328 B,  214  also define an input opening  342  of the first shorting waveguide  322  which mates with an output opening  344  of feeder waveguide  206  (defined by walls  208 B,  210 B,  212 B,  214  of feeder waveguide  206 ) so that walls  324 B,  326 B,  328 B are, respectively and preferably, coplanar with walls  208 B,  210 B,  212 B of feeder waveguide  204  (and, hence, sides  330 B,  332 B,  334 B,  336 B of shorting waveguide  322  are coplanar with sides  224 B,  226 B,  228 B of feeder waveguide  206 ), thereby allowing the second electromagnetic waves and associated power to propagate from feeder waveguide  206  into shorting waveguide  322  during operation of the particle accelerator system  100 .  
         [0036]     Each shorting waveguide  320 ,  322  includes therein a shorting device  350 ,  352  which is positioned in its respective shorting waveguide  320 ,  322  at a location (i.e., a shorting plane) at which the longitudinal axis  190  of the particle accelerator system  100  (and, hence, the longitudinal axis of accelerating sections  102 ,  104  and accelerating and coupling cavities  114 ,  116 ,  154 ,  156  thereof) is between the shorting device  350 ,  352  and the coupling window  300  of the 3 dB waveguide hybrid junction  260 . Preferably, each shorting device  350 ,  352  comprises a substantially rectangular-shaped shorting plunger having a choke groove formed therein as illustrated in  FIGS. 3 and 4 . Each shorting device  350 ,  352  is, preferably, movable, prior to startup of the particle accelerator system  100 , into one of a plurality of positions (i.e., shorting planes) which are each uniquely identified by their respective distance, “z”, from a cross-sectional plane  354  of the feeder waveguides  204 ,  206  in which the longitudinal axis  190  of the particle accelerator system  100  lies (i.e., from the longitudinal axis  190  of the particle accelerator system  100 ).  
         [0037]      FIG. 4  displays the shorting devices  350 ,  352  in two such positions with the shorting devices  350 ,  352  being identified as shorting devices  350   1    352   1  when in the first position at a distance “z 1 ” relative to cross-sectional plane  354  of the feeder waveguides  204 ,  206  and as shorting devices  350   2 ,  352   2  when in the second position at a distance “Z 2 ” relative to cross-sectional plane  354  of the feeder waveguides  204 ,  206 . When the shorting devices  350 ,  352  are positioned in the first position and in the second positions, the coupling coefficients, “k”, of feeder waveguides  204 ,  206  with accelerating sections  102 ,  104  are different. Thus, by moving the shorting devices  350 ,  352  into a plurality of positions (i.e., shorting planes) relative to cross-section plane  354  (and, hence, at a plurality of distances from the longitudinal axis  190  of the particle accelerator system  100 ), the coupling coefficients, “k”, may be changed to a corresponding plurality of values which are related to the plurality of positions on a one-to-one basis. Because there is only one value of the coupling coefficients, “k”, of feeder waveguides  204 ,  206  with accelerating sections  102 ,  104  at which all power of the first and second electromagnetic waves is delivered to accelerating sections  102 ,  104  without reflections and is maximally utilized for charged particle acceleration for each charged particle beam current at which the particle accelerator system  100  may be operated, the ability to move the shorting devices  350 ,  352  into a plurality of positions allows optimal setting of the coupling coefficients, “k”, for any charged particle beam current.  
         [0038]      FIG. 5  displays a graphical illustration of the effect of moving the shorting devices  350 ,  352  relative to cross-section plane  354  to different distances, “z”, therefrom on the magnitude of the transverse component of the electric field, “E y ”, produced at the cross-section plane  354  (i.e., at z=0) with the shorting devices  350 ,  352  at such distances. The relationship is set forth mathematically as E y =E 0  sin(k(z 0 -z)), where: E 0  corresponds to the maximum possible magnitude of the transverse component of the electric field at cross-section plane  354  of the feeder waveguides  204 ,  206 ; “k” corresponds to the coupling coefficients of feeder waveguides  204 ,  206  with accelerating sections  102 ,  104 ; z 0  corresponds to the distance of the shorting devices  350 ,  352  relative to cross-sectional plane  354  of the feeder waveguides  204 ,  206  at which the transverse component of the electric field, “E y ”, has its maximum possible magnitude; and, “z” corresponds to the actual distance of the shorting devices  350 ,  352  relative to cross-section plane  354  of the feeder waveguides  204 ,  206 . In  FIG. 5 , the solid curve is associated with the case in which the shorting devices  350 ,  352  are positioned at a distance from cross-section plane  354  with the magnitude of the transverse component of the electric field, “E y ”, produced at the cross-section plane  354  being a maximum, which corresponds to the maximal coupling coefficient, “k”. If the actual distance, “z”, is such that the transverse component of the electric field, “E y ”, equals zero (i.e., the minimum possible magnitude) in plane  354 , the coupling coefficient, “k”, equals zero (i.e., the minimal coupling coefficient). The actual position of the shorting devices  350 ,  352  is selected to be between these two extreme values so that coupling coefficient, “k”, is controllable. In this case, at the operating beam current value, all power of the first and second electromagnetic waves is delivered to accelerating sections  102 ,  104  without reflections in feeder waveguides  204 ,  206 . The dashed curve is associated with a case in which the shorting devices  350 ,  352  are positioned at some interim distance from cross-section plane  354  and, hence, the magnitude of the transverse component of the electric field, “E y ”, produced at the cross-section plane  354  is not at a maximum.  
         [0039]     While the shorting devices  350 ,  352  of the exemplary embodiment described herein are movable between a plurality of positions in shorting waveguides  320 ,  322  that correspond to a plurality of different distances, “z”, relative to cross-section plane  354 ,  FIG. 6  displays a front perspective view of a shorting waveguide  370  which may be used in place of the shorting waveguides  320 ,  322 . Shorting waveguide  370  has dimensions that are substantially similar to those of shorting waveguides  320 ,  322 , thereby enabling a shorting waveguide  370  to be secured to each feeder waveguide  204 ,  206  in replacement of shorting waveguides  320 ,  322 . Preferably, shorting waveguide  370  comprises a plurality of rods  372  which are secured to an appropriate side  374  of shorting waveguide  370  at a location which results in the rods  372  being positioned at a distance, “z”, relative to cross-section plane  354  (i.e., in a shorting plane) when a shorting waveguide  370  is secured to feeder waveguide  320 ,  322  that causes the coupling coefficients, “k”, of feeder waveguides  204 ,  206  with accelerating sections  102 ,  104  to have a value at which all power of the first and second electromagnetic waves is delivered to accelerating sections  102 ,  104  without reflections and is maximally utilized for charged particle acceleration when the particle accelerator system  100  is operated at a corresponding charged particle beam current. If the particle accelerator system  100  is to be operated at a different charged particle beam current, a shorting waveguide  370  having rods  372  at different locations may be employed to optimize the coupling coefficients and to efficiently utilize power of the first and second electromagnetic waves without reflections.  
         [0040]     An exemplary particle accelerator system  100 , acceptable in accordance with the embodiment described herein, comprises a klystron RF generator  200  having a 6 MW pulse power and a 2856 MHz operating frequency. The charged particle beam current of such particle accelerator system  100  may be changed within the range of 0.1 A to 0.7 A. The coupling coefficients of the feeder waveguides  204 ,  206  and accelerating sections  102 ,  104  of such particle accelerator system  100  may be changed within the range of 1.5 to 5.0 by moving movable shorting devices  350 ,  352  thereof into appropriate positions as described above.  
         [0041]     Prior to operation of particle accelerator system  100 , shorting devices  350 ,  352  are positioned at locations appropriate to optimally set the coupling coefficients between the feeder waveguides  204 ,  206  and the accelerating sections  102 ,  104  so that all power of the first and second electromagnetic waves is delivered to accelerating sections  102 ,  104  without reflections for the charged particle beam current at which the particle accelerator system  100  is to be operated. Once the particle accelerator system  100  is in operation, injector  108  generates and emits charged particles (preferably, electrons) into the first accelerating section  102  and, concurrently, the RF generator  200  of the electromagnetic drive subsystem  106  generates electromagnetic waves which are directed into the 3 dB waveguide hybrid junction  260  thereof. After the generated electromagnetic waves and associated power are divided by the coupling window  300 , a first portion of the generated electromagnetic waves (the “first electromagnetic waves”) and associated power propagates through the first waveguide  262  of the 3 dB waveguide hybrid junction  260  and into the first feeder waveguide  204 . A second portion of the generated electromagnetic waves (the “second electromagnetic waves”) and associated power propagates through the coupling window  300 , into the second waveguide  264  of the 3 dB waveguide hybrid junction  260 , and then into the second feeder waveguide  206 . Subsequently, the first and second electromagnetic waves and associated power propagate, respectively, into and throughout the accelerating sections  102 ,  104  via the oblong-shaped slots  126 ,  168 .  
         [0042]     Any reflections of the first and second electromagnetic waves occurring during the transient startup period are directed from the first and second feeder waveguides  204 ,  206  into the second waveguide  264  of the 3 dB waveguide hybrid junction  260  (either directly from the second feeder waveguide  206  or indirectly from the first waveguide  204  via the first feeder waveguide  262  and coupling window  300  of the 3 dB waveguide hybrid junction  260 ). Once within the second waveguide  264  of the 3 dB waveguide hybrid junction  260 , the reflections are directed to the waveguide load  202  where the energy thereof is dissipated, resulting in their absorption.  
         [0043]     Contemporaneously, the charged particles emitted into the first accelerating section  102  travel through the accelerating cavities  114 , coupling cavities  116 , and drift tubes  118  thereof while being accelerated by the energy of the first electromagnetic waves and formed into a charged particle beam. Upon reaching the second end  112  of the first accelerating section  102 , the charged particles of the charged particle beam travel through output port  124  and into the drift tube  250  formed in the common wall  214  of the first and second feeder waveguides  204 ,  206  of the electromagnetic drive subsystem  106 . After traveling through the drift tube  250 , the charged particles of the charged particle beam enter the second accelerating section  104 , via input port  162 , and travel through the accelerating cavities  154 , coupling cavities  156 , and drift tubes  158  thereof while being further accelerated by the energy of the second electromagnetic waves. The charged particles of the charged particle beam exit the particle accelerator system  100  at output port  166  located at the second end  152  thereof.  
         [0044]     Whereas the present invention has been described in detail above with respect to an embodiment thereof, it is understood that variations and modifications can be effected within the spirit and scope of the invention, as described herein before and as defined in the appended claims. The corresponding structures, materials, acts, and equivalents of all means-plus-function elements, if any, in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed.