Patent Application
20070035260
The present invention relates generally to the field of standing wave linear particle beam accelerators, and more particularly to an energy switch assembly for a standing wave linear particle beam accelerator having two mechanical plungers, which, when radially inserted into two side cells or cavities of the linear particle beam accelerator, increases the output of the linear accelerator at lower energies.
Medical linear particle beam accelerators that operate at two or more energies suffer from the fundamental limitation that the internal structure cannot be optimized for multiple energies. Because the length of the accelerating structure is fixed, in order to produce both high energy and low energy beams, the cavities are operated at maximum electric field gradient in the higher energy mode and lower gradient in the lower energy mode. The lower gradient causes more electrons to be lost as they travel through the structure, resulting in loss of output in the low energy mode.
To reduce this loss in output, linear particle beam accelerators were developed which employ energy switches. Energy switches typically employ one or more plungers which can be inserted into side cavities of the linear accelerator. Such energy switches concentrate the radio frequency (RF) power in the upstream section of the accelerator structure, before the switch location. In this manner, energy switches cause the gradient to be higher in the upstream portion of the accelerator structure so that electrons are accelerated and captured more efficiently, resulting in higher output in the lower energy mode.
Energy switches may employ radial or transverse plunger arrangements. In energy switches employing a radial plunger arrangement, the plunger is inserted into the side cavity along an axis which lies in a plane normal to the electron beam, while in energy switches employing a transverse plunger arrangement, the plunger is inserted into the side cavity along an axis which lies in a plane parallel to the electron beam. Known to the art are energy switches that employ multiple plungers which are positioned within a single side cavity of the linear accelerator. Such energy switches are used to provide the linear accelerator with three or more output levels.
Prior energy switches achieve high dose rates at low energy, but, because of heating, require direct water-cooling of the plunger during operation. Thus, the energy switches are vulnerable to water leaks and require multiple control system interlocks. Additionally, the side cavity in which the plungers are positioned must have a modified, more complex structure than the other side cavities of the accelerator.
Consequently, it would be desirable to provide an energy switch assembly for standing wave linear particle beam accelerators that increases the output of the accelerator at lower energies, but which does not cause excessive heating, and thus, does not require water-cooling. Additionally, it would be desirable to provide an energy switch assembly that does not require the use of side cavities having a special internal geometry.
Accordingly, the present invention is directed to a dual-plunger energy switch assembly for standing wave linear particle beam accelerators capable of operating in a higher energy mode and a lower energy mode. The dual-plunger energy switch assembly employs two mechanical plungers that can be extended different distances inside two side cells or cavities of the linear accelerator downstream of the RF input. When the linear accelerator is operated in the higher energy mode, both plungers are retracted out of the side cavities. To achieve high output while the linear accelerator is operated in the lower energy mode, the two plungers are radially inserted into the two side cavities to adjust the electromagnetic accelerating field along the length of the accelerator. For example, in a specific embodiment, one plunger is radially inserted into a side cavity so that the plunger touches the internal surface of the side cavity opposite the plunger assembly. The second plunger is radially inserted into a second side cavity so that the plunger is adjacent to, but not touching, the internal surface of the side cavity opposite the plunger assembly. In this manner, the dual-plunger energy switch assembly increases the output of the linear accelerator at lower energies without generating excessive heating, eliminating the need for water-cooling of the energy switch assembly. The energy switch assembly also does not require the use of side cavities having a special internal geometry. In specific embodiments, the dual-plunger energy switch assembly includes a common actuator assembly mechanically coupled to the plunger assemblies for positioning the plungers within the side cavities.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.
The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which:
Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.
Referring generally to
The standing wave linear particle beam accelerator 100 includes a mechanical dual-plunger energy switch assembly 134 for detuning the frequency or changing the electromagnetic field distribution in selected side cavities 110 and supplying at least two levels of output energy from the linear accelerator 100. Detuning the frequency in a selected side cavity 110 causes that side cavity to be out of resonance with the other side cavities 110, and changing the electromagnetic field distribution in a selected side cavity 110 changes the coupling factor between adjacent accelerating cavities 108 electromagnetically coupled with that side cavity. Either detuning the frequency or changing the electromagnetic field distribution in a selected side cavity 110 changes the amplitude of the electric field profile in accelerating cavities 108 following that side cavity. Thus, coupling of the microwave energy through the selected side cavity 110 is controlled by the position of the mechanical energy switch assembly 134 for varying the output of the linear accelerator 100.
In the embodiments illustrated in
In embodiments of the invention, the first plunger 138 and the first side cavity 118 are closer to the electron beam source 102 than the second plunger 142 and the second side cavity 128. That is, the first plunger 138 and the first side cavity 118 are “upstream” of the second plunger 142 and the second side cavity 128, while the second plunger 142 and the second side cavity 128 are “downstream” of the first plunger 138 and the first side cavity 118. Preferably, the second plunger 142 is inserted into the second side cavity 128 the second predetermined distance for detuning the second side cavity 128 and increasing the electric field amplitude in accelerating cavities between the first plunger assembly 136 and the second plunger assembly 140. For example, in one specific embodiment, the second plunger 142 is inserted into the second side cavity 128 so that the second plunger 142 is adjacent to, but not touching, the internal smile surface 146 of the second side cavity 128. In exemplary embodiments, insertion of the second plunger 142 to a position slightly away from the smile surface 146 of the second side cavity 128 slightly raises the field amplitude in the accelerating cavities 108 between the first plunger 138 and the second plunger 142, providing an appropriately slight increase in the microwave field between the first and second plungers 138 and 142 and acting as a fine tuning mechanism. With both the first plunger 138 fully inserted into the first side cavity 118 and the second plunger 142 partially inserted into the second side cavity 128, the advantage of wide mode separation due to degeneracy of accelerator modes remains valid.
For example, in the embodiments illustrated in
In embodiments of the invention, the energy switch assembly 134 includes an actuator assembly 148 mechanically coupled to the first plunger assembly 136 and the second plunger assembly 140, for positioning the first plunger 138 within the first side cavity 118 and the second plunger 142 within the second side cavity 128. In one specific embodiment, the actuator assembly 148 includes an actuator 150 positioned between the first plunger assembly 136 and the second plunger assembly 140. A first cross bar 152 and a second cross bar 154 are pivotally connected to the actuator 150, the first plunger assembly 136, and the second plunger assembly 140 for moving the first plunger 138 and the second plunger 142 between a first position where the first plunger 138 is retracted from the first side cavity 118 and the second plunger 142 is retracted from the second side cavity 128, and a second position where the first plunger 138 is fully extended into the first side cavity 118 and the second plunger 142 is partially extended a predetermined distance into the second side cavity 128. For example, a first thrust bar 156 coupled to the first plunger 138 and a second thrust bar 158 coupled to the second plunger 142 are engaged by the first cross bar 152 and the second cross bar 154. The first thrust bar 156 and the second thrust bar 158 are movable between the first and second positions via the actuator 150.
The actuator assembly 148 provides synchronous actuation of the first plunger 138 and the second plunger 142. For instance, in the embodiment illustrated, the actuator 150 comprises a pneumatic actuator capable of moving the first plunger 138 and the second plunger 142 between the first and second positions synchronously. In other embodiments, the actuator assembly 148 may include an electric actuator, a hydraulic actuator, or another type of actuator as contemplated by one of skill in the art. Preferably, the first cross bar 152 and the second cross bar 154 include slots for engaging the first thrust bar 156 and the second thrust bar 158 and preventing rotational torque on the first plunger assembly 136 and the second plunger assembly 140. For example, in one embodiment, the ends of the first thrust bar 156 and the second thrust bar 158 are cylindrical for traveling in slots formed in the first cross bar 152 and the second cross bar 154. In this manner, rotational torque on the first plunger assembly 136 and the second plunger assembly 140 may be prevented.
In one embodiment, the first plunger 138 and the second plunger 142 extend from a first tube 160 and a second tube 162 into the first side cavity 118 and the second side cavity 128. For instance, the first plunger 138 and the second plunger 142 are coupled with bushings for guiding the plungers through the first tube 160 and the second tube 162, while the first thrust bar 156 and the second thrust bar 158 travel through slots included in the first tube 160 and the second tube 162 for moving the first plunger 138 and the second plunger 142 between the first and second positions. Moreover, the first tube 160 and the second tube 162 may protect internal components of the first plunger assembly 136 and the second plunger assembly 140, including welded bellows, or the like, as well as limiting the travel of the first and second plungers. For further limiting the travel of the first plunger 138 and the second plunger 142, a first micrometer nut 164 and a second micrometer nut 166 may be included with the first plunger assembly 136 and the second plunger assembly 140. For instance, the first micrometer nut 164 and the second micrometer nut 166 may be theadably attached to the first tube 160 and the second tube 162, respectively, acting as hard stops for the first thrust bar 156 and the second thrust bar 158 for limiting the travel of the first plunger 138 and the second plunger 142.
Preferably, the energy switch assembly 134 includes a first flange 168 for securing the first plunger assembly 136 to the first side cavity 118 and a second flange 170 for securing the second plunger assembly 140 to the second side cavity 128. Moreover, a clamp plate may be included for connecting the actuator assembly 148 to the linear accelerator 100. Thus, the first flange 168, the second flange 170, and the clamp plate secure the energy switch assembly 134 to the linear accelerator 100. In exemplary embodiments, the first flange 168 and the second flange 170 are configured for allowing removal of non-vacuum components prior to bakeout. For example, in one specific embodiment, the first plunger assembly 136 and the second plunger assembly 140 include bolted conflat vacuum flanges for providing seals compatible with bakeout.
In exemplary embodiments of the present invention, microswitches 172 and 174 are included for indicating the positions of the first plunger 138 and the second plunger 142. For instance, microswitch 172 may be actuated when the energy switch assembly 134 is in the first position, while microswitch 174 may be activated when the energy switch assembly 134 is in the second position. It is contemplated that other techniques may be utilized for determining the position of the energy switch assembly 134 as contemplated by one of skill in the art. Further, microswitches 172 and 174 may be used for limiting the travel of the energy switch assembly 134. For example, the energy switch assembly 134 may be moved in a first direction until microswitch 172 is actuated, indicating that the first plunger assembly 136 and the second plunger assembly 140 are in the first position, while the energy switch assembly 134 may be moved in a second direction until microswitch 174 is actuated, indicating that the first plunger assembly 136 and the second plunger assembly 140 are in the second position.
Preferably, spring washers such as Belleville washers, or the like, are included with the energy switch assembly 134 for allowing the first plunger 138 to contact the smile surface 144 without causing excessive wear of the smile surface 144. It is contemplated that other techniques may be utilized for preventing undue force from the first plunger 138 to the smile surface 144 as well, without departing from the scope and intent of the present invention.
In exemplary embodiments of the present invention, the energy switch assembly 134 is primarily air cooled, i.e. the energy switch assembly 134, including the first plunger assembly 136 and the second plunger assembly 140, is passively cooled by its surroundings and requires no specific water cooling. In one embodiment, the energy switch assembly 134 is primarily cooled by ambient air found in the environment the linear accelerator 100 is operated in, while in another embodiment, air is forced over the energy switch assembly 134 for cooling, such as by utilizing a fan or the like. Those of skill in the art will appreciate that some conductive cooling of the energy switch assembly 134 to the outside structure of the linear accelerator 100 may occur as well, without departing from the scope and intent of the present invention. For instance, in specific embodiments, some conductive cooling of the energy switch assembly 134 to a built-in water cooling system thermally coupled with the outside structure of the linear accelerator 100 may occur.
Referring now to
Referring to
Then, a second plunger is radially inserted into a second side cavity for detuning the second side cavity and increasing the electric field amplitude in accelerating cavities between the first plunger assembly and the second plunger assembly. For instance, in one specific embodiment, a second plunger is inserted into a second side cavity so that the second plunger is adjacent to, but not touching, an internal surface of the second side cavity opposite the second plunger assembly, 204. The second side cavity electromagnetically couples a third accelerating cavity downstream of the second accelerating cavity and a fourth accelerating cavity adjacent to the third accelerating cavity, the fourth accelerating cavity having at least approximately the same resonant frequency as the third accelerating cavity.
In the exemplary embodiments, method 200 may be implemented as sets of instructions or software readable by the radiographic imaging device. Further, it is understood that the specific order or hierarchy of steps in the methods disclosed are examples of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the method can be rearranged while remaining within the scope and spirit of the present invention. The accompanying method claims present elements of the various steps in a sample order, and are not necessarily meant to be limited to the specific order or hierarchy presented.
It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.
