Abstract:
A device for use in a linear accelerator operable to accelerate charged particles along a beam axis is disclosed. The device includes a plurality of monolithic members connected to form a series of accelerating cavities aligned along the beam axis and coupling cavities. Each of the coupling cavities intersects with adjacent accelerating cavities at first and second coupling apertures. The first and second coupling apertures have different sizes.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a radiation emitting device, and more particularly to a linear accelerator having a monolithic cavity structure with asymmetric coupling. 
     BACKGROUND OF THE INVENTION 
     Linear accelerators are used to accelerate a variety of particles (e.g., electrons, protons, ions) for numerous applications, such as radiation therapy. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. An electron linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam may be an electron beam or photon (x-ray) beam, for example. During treatment, the radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation. 
     Linear accelerators may be used in the medical environment for a variety of applications. A beam of charged particles, e.g., electrons, from a linear accelerator may be directed at a target which is made of a material having a high atomic number, so that an X-ray beam is produced for radiation therapy. Alternatively, the beam of charged particles may be applied directly to a patient during a radiosurgical procedure. Such radio surgery has become a well-established therapy in the treatment of brain tumors. A high-energy beam may be directed at a localized region to cause a breakdown of one or both strands of the DNA molecule inside cancer cells, with the goal of at least retarding further growth and preferably providing curative cancer treatment. 
     A conventional linear accelerator includes a series of accelerating cavities that are aligned along a beam axis. A particle source, which for an electron accelerator is typically an electron gun, directs charged particles into the first accelerating cavity. As the charged particles travel through the succession of accelerating cavities, the particles are focused and accelerated by means of an electromagnetic field. For example, a radio frequency (RF) source may be coupled to the accelerator to generate the necessary field to operate the linear accelerator. The accelerated particles from a clinical linear accelerator have a high energy (e.g., up to 20 MeV). Often, the output beam is directed to a magnetic bending system that functions as an energy filter. The beam is typically bent by approximately 270 degrees. Then either the output beam of high energy particles or an X-ray beam generated by impinging a target with the output beam is employed for radiation treatment of a patient. 
     The frequency of the driving signal and the dimensions of the accelerating cavities and the beam passages between adjacent accelerating cavities determine the operating frequency of the accelerator. Optimal performance of the accelerator requires a match between the resonant frequency of the cavity structure and the frequency of the driving signal. 
     In a resonant chain of coupled cavities such as used in a standing-wave linear particle accelerator, it is often desirable to change the field strength in some cavities relative to other cavities. Adjustment of the field strength profile in an accelerator can be done by changing the coupling constants on each side of a coupling cavity. This is typically done by shifting the side cavity&#39;s longitudinal position, which makes the coupling aperture larger on one side and smaller on the other. In doing this, the side cavity&#39;s shape is generally unchanged. The side cavity remains symmetrical. This conventional method works well for accelerator designs where the side cavity is manufactured as one piece and attached to a piece which contains two main cavity halves. 
     An alternative method for manufacturing the accelerator structures is to form monolithic members such as disclosed in U.S. Pat. No. 5,734,168, by Yao, which is incorporated herein by reference in its entirety. The monolithic structure defines a portion of the main cavity and side cavity in one structure. The monolithic structure provides improvements in manufacturing such as reduced tolerances and reduced manufacturing costs, especially for higher frequency accelerators. One drawback with the monolithic structure is that the field strength adjustment as described above cannot be used. If the side cavity is shifted longitudinally, the unit cell will not contain exactly one half of a side cavity, and the frequency of this partial side cavity will be significantly shifted from the frequency of the full side cavity. This complicates the design and testing of cavities. 
     There is, therefore, a need for a monolithic cell structure that allows for adjustment of the field strength by modifying the side cavity configuration to vary the coupling constant between a side cavity and a main cavity. 
     SUMMARY OF THE INVENTION 
     A device for use in a linear accelerator operable to accelerate charged particles along a beam axis is disclosed. The device includes a plurality of monolithic members connected to form a series of accelerating cavities aligned along the beam axis and coupling cavities. Each of the coupling cavities intersects with adjacent accelerating cavities at first and second coupling apertures. The first and second coupling apertures have different sizes. 
     In another aspect of the invention, a system for delivering charged particles for medical applications generally comprises a particle accelerator having an input for connection to a source of charged particles and a plurality of accelerating cells. The particle accelerator has a beam path extending through the cells to an exit window. Each of the particle accelerating cells comprises an accelerating cavity half cell and a coupling cavity half cell. The particle accelerating cells are connected to form a series of accelerating cavities aligned along the beam axis and coupling cavities. Each of the coupling cavities intersects with adjacent accelerating cavities at first and second coupling apertures. The first and second coupling apertures have different sizes. The system further includes a signal source for energy transfer engagement with the charged particles within the particle accelerator. 
     The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a radiation treatment device having a linear accelerator according to an embodiment of the present invention and a patient positioned for treatment within the treatment device. 
     FIG. 2 is a schematic of a linear accelerator of the radiation treatment device of FIG.  1 . 
     FIG. 3 is a side sectional view of a series of monolithic members of the present invention that are connected to form a linear accelerator. 
     FIG. 4 is a front view of the monolithic member of FIG.  3 . 
     FIG. 5 is a side sectional view of the monolithic member of FIG. 4 taken along lines  5 — 5 . 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily apparent to those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail. 
     Referring now to the drawings, and first to FIG. 1, a radiation treatment device of the present invention is shown and generally indicated at  20 . The radiation treatment device  20  includes a beam shielding device within a treatment head  24 , a control unit within a housing  26  connected to a treatment processing unit (not shown). The radiation treatment device further includes a gantry  36  which can be swiveled for rotation about axis A in the course of a therapeutic treatment. The treatment head  24  is fixed to the gantry  36  for movement therewith and a linear accelerator is located within the gantry for generating high powered radiation used for therapy. The radiation emitted from the linear accelerator extends generally along axis R. Electron, photon, or any other detectable radiation may be used for the therapy. During treatment, the radiation beam is focused on a zone Z of an object P (e.g., a patient who is to be treated). The zone to be treated is located at an isocenter defined by the intersection of the rotational axis A of the gantry  36 , rotational axis T of treatment table  38 , and the radiation beam axis R. The treatment device  20  described above is provided as an example of a device for use in delivering a treatment with a linear accelerator having a monolithic structure as described below. It is to be understood that the radiation treatment device may be different than the one shown in FIG. 1 without departing from the scope of the invention. 
     FIG. 2 illustrates additional detail of the linear accelerator of the treatment device of FIG.  1 . The linear accelerator includes a particle source  42  for directing charged particles into an accelerator device  44 . In a preferred embodiment, the particle source is an electron gun which injects electrons into the input end of the accelerator device  44 . A driving source is introduced into the accelerator device by a signal source  46 . The signal source  46  introduces an electromagnetic wave having a suitable frequency. Radio frequency or high frequency sources are conventionally employed, but the selection of the frequency of the drive signal is not critical to the invention. Optionally, the frequency may be dynamically controlled by a control circuit  48  that is connected within a closed loop system (not shown). 
     Electrons introduced into the accelerator device  44  by the electron gun are accelerated along the beam axis  50  of the device. The electrons obtain a high energy by virtue of the energy-transfer relationship with the electromagnetic waves established by connection with the signal source  46 . A pulsed or steady state output beam of the electrons is emitted from an exit window  54 , which is located at the delivery end of the device  44 . The exit window  54  may include a thin metal foil. The output beam  52  of charged particles is directed to an achromatic magnetic bending system  56 , which acts as an energy filter. The output beam is bent by approximately 270 degrees and is then directed onto a target  58  such as a gold or tungsten target. Impingement of the target  58  by the output beam  52  generates an X-ray beam which is employed for radiation treatment of a patient. Alternatively, the output beam  52  may be applied directly to a patient such as during a radiosurgical procedure to treat a brain tumor. The operations of the magnetic bending system  56  and the target  58  are well known by those skilled in the art. 
     Referring now to FIG. 3, a side sectional view of a series of monolithic members  70  of the present invention is shown. The monolithic members  70  are connected together to form the linear accelerator. As shown in FIG. 3, two connected members  70  define a main accelerating cavity  72  and a side coupling cavity  74 . The accelerating cavities  72  are aligned to permit passage of beam  50  (FIGS.  2  and  3 ). The accelerating cavities  72  include projecting noses  78  which are used to improve efficiency of interaction of microwave power and electron beam. The side cavities  74  are used to electromagnetically couple the accelerating cavities  72 . The intersection region of the side cavity  74  with the acceleration cavity  72  is referred to as an iris (or coupling aperture)  80 . 
     Referring now to FIGS. 4 and 5, an individual monolithic member (half cell)  70  is shown. The member  70  includes a beam axis opening  100  which extends from a first face  102  of the monolithic member to the interior of the monolithic member. A second face is contoured to provide an abutment region  104  and a cavity-defining region  106 . The cavity-defining region  106  preferably has a generally circular cross-section. 
     As previously discussed, the member  70  is a monolithic side coupled structure. The side coupling is achieved on the member shown in FIGS.  4  and by means of an upper portion of the monolithic member. This upper portion is machined to provide the coupling cavity  74 . After pieces are assembled together, the coupling cavity  74  is off-axis of the electron beam and is connected to the accelerating cavity of the monolithic member by an opening (iris)  80 . The coupling cavity  74  is connected to each of two accelerating cavities  72 . Consequently, when a drive signal having the appropriate frequency is fed to any cavity in the structure, the electromagnetic waves are in an energy transfer relationship with an electron beam that is directed through the accelerating cavities  72 . The beam  50  of charged particles passes through each of the accelerating cavities  72  and is focused and accelerated. The exit velocity of the output beam  52  is determined by a number of factors, including the number of accelerating cavities  72  within the accelerator device  40 . 
     The members  70  are interconnected using a brazing process. Wire of brazing material is introduced into grooves and activated using conventional techniques. One example of a brazing material is the alloy made of Ag, Pd, and Ga. The contents may be 82% Ag, 9% Pd, and 9% Ga, for example. Circular grooves  114 ,  116  are formed concentrically about the beam axis opening  100 . These openings are filled with the braze material during the interconnection of the monolithic half cell members. There is also a circular groove  118  for braze material at the upper portion of the monolithic member  70 . 
     The accelerating device of FIG. 3 preferably operates in the standing wave mode that is referred to as a half-π mode (also known as π/2 mode). The frequency of excitation is such that the series of connected structures is excited in a standing wave resonance with π/2 radians phase shift between each accelerating cavity  72  and the adjacent side cavity  74 . A linear accelerator operated in half-π mode has side cavities  74  that are nominally unexcited and main accelerating cavities  72  with strong fields. When properly tuned (so that the side cavities are unexcited), the ratio of field strengths in adjoining main cavities  72  are determined by the coupling coefficients between the main cavities and the common side cavity  74  which connects them. The coupling cavities  74  are preferably resonant at roughly the same frequency as the accelerating cavities  72 . 
     More specifically, if coupling constants between two adjacent main cavities (A, B) and the connecting side cavities are k A  and k B , and the stored energy in the main cavities is U A  and U B , the ratio of stored energies is given by:            U   B       U   A       =       (       k   A       k   B       )     2                            
     where: 
     U A : stored energy in cavity A; 
     U B : stored energy in cavity B; 
     k A : coupling constant between cavity A and the connecting side cavity; and 
     k B : coupling constant between cavity B and the connecting side cavity. 
     The above equation holds for main cavities  72  of different shape or volume. If the two main cavities are identical, the field ratio is proportional to the square root of the stored energy ratio, so it is just proportional to the inverse of the coupling ratio:            E   B       E   A       =       k   A       k   B                              
     where: 
     E A : maximum longitudinal electric field strength in cavity A; 
     E B : maximum longitudinal electric field strength in cavity B; 
     k A : coupling constant between cavity A and the connecting side cavity; and 
     k B : coupling constant between cavity B and the connecting side cavity. 
     One method for adjusting field strength in conventional non-monolithic structures is to shift the side cavity&#39;s longitudinal position, which results in a larger coupling aperture (iris) on one side and a smaller iris on the other side. However, if the side cavity is shifted longitudinally, the member  70  will not contain exactly one half of a side cavity  74 , and the frequency of this partial side cavity will be significantly shifted from the frequency of the full side cavity. This complicates the design and testing of cavities. 
     The present invention resolves this problem by designing the side cavities  74  to be longitudinally asymmetric. The partial side cavity on each monolithic member  70  has its post  84  height adjusted to make each partial side cavity resonant at the identical desired frequency. This assists in the cold testing of the monolithic members, by simplifying the measurements of frequencies and coupling constants. The coupling constant may be adjusted in the design phase by changing the depth of the partial side cavity, while at the same time changing its post height to keep its frequency constant. 
     The size of the coupling aperture  80  may be determined through use of a simulation software such as Superfish, available from Los Alamos, National Laboratory, which calculates resonant frequency of a two dimensional cavity, as is well known by those skilled in the art. This can be used to calculate the initial post  84  height. Alternatively a three dimensional simulation code that accounts for the size and shape of the iris  80  may be used. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiment and these variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.