HYBRID STANDING WAVE/TRAVELING LINEAR ACCELERATORS PROVIDING ACCELERATED CHARGED PARTICLES OR RADIATION BEAMS

Abstract

A hybrid linear accelerator is disclosed comprising a standing wave linear accelerator section (“SW section”) followed by a travelling wave linear accelerator section (“TW section”). In one example, RF power is provided to the TW section and power not used by the TW section is provided to the SW section via a waveguide. An RF switch, an RF phase adjuster, and/or an RF power adjuster is provided along the waveguide to change the energy and/or phase of the RF power provided to the SW section. In another example, RF power is provided to both the SW section and the TW section, and RF power not used by the TW section is provided to the SW section, via an RF switch, an RF phase adjuster, and/or an RF power. In another example, an RF load is matched to the output of the TW section by an RF switch.

Claims

1. A hybrid linear accelerator comprising: a source of charged particles configured to provide an input beam of charged particles; a standing wave linear accelerator section configured to receive the input beam of charged particles and accelerate the charged particles, the standing wave linear accelerator section providing an intermediate beam of accelerated electrons; a traveling wave linear accelerator section configured to receive the intermediate beam of accelerated electrons, and to further increase the momentum and energy of the intermediate beam of accelerated electrons, the traveling wave linear accelerator section providing an output beam of charged particles; a drift tube configured to provide a path to for passage of the intermediate beam from the standing wave linear accelerator section to the traveling wave linear accelerator section, the drift tube configured to RF decouple the standing wave linear accelerator section from the traveling wave linear accelerator section to further increase the momentum and energy of the intermediate beam; an RF source configured to provide RF power to the traveling wave linear accelerator section; and a waveguide having an input coupled to an output of the traveling wave linear accelerator section and an output coupled to an input of the standing wave linear accelerator section; wherein RF power remaining after attenuation in the traveling wave linear accelerator section is fed to the standing wave linear accelerator section to accelerate the charged particles.

2. The hybrid linear accelerator of claim 1, further comprising: a switch, a phase shifter, and/or a power adjuster along the waveguide, to change the power and/or phase of the RF power provided to the standing linear accelerator section.

3. The hybrid linear accelerator of claim 2, wherein phase shifter, and/or the power adjuster are configured to provide the switch, the energy regulation of the output beam of electrons of from about 0.5 MeV to a maximum linear accelerator energy.

4. The hybrid linear accelerator of claim 1, wherein the standing wave linear accelerator is configured in the form of a buncher.

5. The hybrid linear accelerator of claim 1, wherein the source of charged particles comprises an electron gun configured to provide an input beam of electrons.

6. The hybrid linear accelerator of claim 1, further comprising. a first external magnetic system cooperative with the standing wave linear accelerator; and/or a second external magnetic system cooperative with the traveling wave linear accelerator section.

7. The hybrid linear accelerator of claim 1, further comprising: a second RF waveguide between the RF source and traveling wave linear accelerator configured to provide RF power from the RF source to the traveling wave linear accelerator section; and a high power circulator along the second RF waveguide to prevent reflected RF power from propagating back to the RF source; and/or a low power circulator along the first RF waveguide to prevent reflected RF power from propagating back to the traveling wave linear accelerator section.

8. The hybrid linear accelerator of claim 1, further comprising at least one of: an charged particle beam window and a conversion target for producing Bremsstrahlung radiation.

9. A hybrid linear accelerator comprising: a source of charged particles; a standing wave linear accelerator section configured to receive the input beam of electrons and accelerate the charged particles, the standing wave linear accelerator section providing an intermediate beam of accelerated charged particles; a traveling wave linear accelerator section configured to receive the intermediate beam of accelerated charged particles, and to further increase the momentum and energy of the accelerated electrons, the traveling wave linear accelerator providing an output beam of charged particles; a drift tube configured to provide RF decoupling between the standing wave linear accelerator section and the traveling wave linear accelerator section, while also permitting transit of the intermediate beam of accelerated electrons from the standing wave linear accelerator section to the traveling wave linear accelerator section; an RF power source; and an RF splitter configured to receive RF power from the RF power source and to bifurcate the RF power into a first portion of RF power to be provided to the standing wave linear accelerator section and a second portion of RF power to be provided to the traveling wave linear accelerator section.

10. The hybrid linear accelerate of claim 9, further comprising: an RF switch, an RF phase shifter, and an RF power adjuster between the traveling wave linear accelerator section and the RF splitter, the RF switch, the RF phase shifter, and the RF power adjuster being configured to feed the standing wave standing wave linear accelerator section RF power not used by the traveling wave linear accelerator section, and/or to change a phase relationship between the standing wave linear accelerator and the traveling wave linear accelerator section.

11. The hybrid linear accelerator of claim 10, wherein the switch, the phase shifter, and/or the power adjuster are configured to provide energy regulation from about 0.5 MeV to maximum linear accelerator energy.

12. The hybrid linear accelerator of claim 9, wherein the standing wave linear accelerator is configured in the form of a buncher.

13. The hybrid linear accelerator of claim 9, wherein: the source of charged particles comprises an electron gun configured to provide an input beam of electrons.

14. The hybrid linear accelerator of claim 9, further comprising: a first external magnetic system cooperative with the standing wave linear accelerator section; and/or a second external magnetic system cooperative with the traveling wave linear accelerator section.

15. The hybrid linear accelerator of claim 9, further comprising: an RF waveguide between the RF source and RF splitter, to provide RF power to the RF splitter; and a high power circulator along the RF waveguide to prevent reflected RF power from propagating back to the RF source.

16. The hybrid linear accelerator of claim 9, further comprising: a matched RF load coupled to the traveling wave linear accelerator to absorb RF power remaining after acceleration in the traveling wave linear accelerator section.

17. The hybrid linear accelerator of claim 9, further comprising at least one of: an charged particle beam window and a conversion target for producing Bremsstrahlung radiation.

18. A hybrid linear accelerator comprising: a source of charged particles configured to provide an input beam of electrons; a standing wave linear accelerator section configured to receive the input beam of charged particles and accelerate the charged particles, the standing wave linear accelerator providing an intermediate beam of accelerated charged particles; a traveling wave linear accelerator section configured to receive the intermediate beam of accelerated charged particles, and to further increase the momentum and energy of the accelerated charged particles, the traveling wave linear accelerator section having an output; an RF coupler configured to provide RF coupling between the standing wave linear accelerator section and the traveling wave linear accelerator section and to allow transit of the intermediate beam of accelerated electrons from the standing wave linear accelerator section to the traveling wave linear accelerator section; an RF source configured to provide RF power to both the standing wave linear accelerator section and the traveling wave linear accelerator section via an RF waveguide cooperative with the RF coupler; and an RF load cooperative with the output of the traveling wave linear accelerator section; and an RF switch configured to match the RF load with the RF power output by the traveling wave linear accelerator section to absorb power remaining after attenuation in the traveling wave linear accelerator.

19. The hybrid linear accelerator of claim 18, wherein the standing wave linear accelerator is configured in the form of a buncher.

20. The hybrid linear accelerator of claim 18, wherein: the source of charged particles comprises an electron gun configured to provide an input beam of electrons.

21. The hybrid linear accelerator of claim 18, further comprising: a first external magnetic system cooperative with the standing wave linear accelerator section; and/or a second magnetic system cooperative with the traveling wave linear accelerator section.

22. The hybrid linear accelerator of claim 18, further comprising: an RF waveguide between the RF source and the RF coupler; and a high power circulator along the RF waveguide to prevent reflected RF power from propagating back to the RF source.

23. The hybrid linear accelerator of claim 18, wherein energy regulation of the output beam of electrons provides energy regulation from about 0.5 MeV to a maximum linear accelerator energy.

24. The hybrid linear accelerator of claim 18, further comprising at least one of: a charged particle beam window and a conversion target for producing Bremsstrahlung radiation.

25. A method of accelerating charged particles by a hybrid linear accelerator comprising a standing wave linear accelerator section and a traveling wave linear accelerator section following the standing wave section, the method comprising: providing charged particles to the standing wave linear accelerator section; providing RF power to the hybrid linear accelerator to cause acceleration of the charged particles by the standing wave linear accelerator section and the traveling wave linear accelerator section; and adjusting RF power and/or phase in at least a portion of the hybrid linear accelerator to regulate energy and/or dose of the intermediate electron beam output by the traveling wave linear accelerator section.

26. The method of claim 25, further comprising: providing RF power to the traveling wave linear accelerator section by a source of RF power; providing the RF power remaining after attenuation in the traveling wave section to the standing wave section; and accelerating the charged particles in the standing wave linear accelerator section by the RF power provided to the standing wave section.

27. The method of claim 25, further comprising: adjusting the RF power and/or phase of the RF power provided to the standing wave linear accelerator section by an RF switch, an RF phase shifter, and/or an RF power adjuster to regulate energy and/or dose of the intermediate electron beam output by the traveling wave linear accelerator section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic diagram of an example of a traditional standing wave linear accelerator;

[0035] FIG. 2 is a graph of Electron Beam Energy vs. Peak Electron Beam Current showing changes to the linear accelerator load line in comparison with a corrected version based on Parmela simulations of beam dynamic and corresponding dose rate plots in a non-adapted standard single section linear accelerator;

[0036] FIG. 3 is a schematic diagram of an example of a hybrid linear accelerator of a first embodiment of the invention, where RF power remaining after attenuation in a traveling wave linear accelerator section is provided to a standing wave section of the hybrid linear accelerator;

[0037] FIG. 4 is a schematic diagram of a hybrid linear accelerator with a parallel RF feed, in accordance with a second embodiment of the invention; and

[0038] FIG. 5 is a schematic diagram of a hybrid linear accelerator with a single RF feed, in accordance with a third embodiment of the invention.

DETAILED DESCRIPTION

[0039] FIG. 3 is a schematic diagram of an example of a hybrid linear accelerator system 100 in accordance with an embodiment of the invention. The hybrid linear accelerator system 100 comprises a linear accelerator 105 having a standing wave linear accelerator section 110 and a traveling wave linear acceleration section 120. As discussed above with respect to FIG. 1 and as is known in the art, the linear accelerator 105 includes cavities or cells (not shown) through which RF power propagates to accelerate charged particles, such as electrons. The standing wave linear accelerator section 110 in this example is configured to be a buncher, but that is not required. In this example, the standing wave linear accelerator section 110 is also referred to herein as a “buncher section 110,” and the traveling wave linear acceleration section 120 is also referred to herein as a “traveling wave section 120.”

[0040] A charged particle source 140 is provided to inject a beam of charged particles 145 into the standing wave linear accelerator section 110. The charged particles may be electrons and the charged particle source 130 may be an election gun, for example, as discussed above with respect to FIG. 1. The electron gun 140 may be a triode, diode, or any other type of electron gun. The following discussion will refer to the electron gun 130 but it is understood that other types of charged particles may be injected into the standing wave buncher section 110 by other types of charged particle sources, and accelerated by the hybrid linear accelerator 100 system.

[0041] The buncher section 110 and the traveling wave section 120 are connected to each other by a drift tube 125, which provides a path for the passage of accelerated charged particles from the buncher section 110 to the traveling wave section 120. An output of the buncher section 110 is coupled to an input of the drift tube 115 though a first RF coupler 130. The output of the drift tube 115 is coupled to the input of the traveling wave section 120 via a second RF coupler 135. The drift tube 125 is configured to RF decouple the buncher section 110 from the traveling wave linear accelerator section 120, in a manner known in the art.

[0042] In accordance with this embodiment of the invention, an RF source 150 provides RF power to the cavities of the traveling wave section 120, via a waveguide 160. In this example, RF power is not provided by the RF source 150 to the standing wave linear accelerator section 110, although that is an option. A second RF coupler 135 couples the waveguide 160 to the interior of the traveling wave section 120 for propagation of the RF power through the interior of the cavities of the traveling wave section. The RF source 150 and the electron gun 140 are powered by one or more power sources (not shown), as is known in the art.

[0043] While the RF power source 150 can run RF power into the traveling wave input RF coupler 135 without an isolating device in steady state mode, a high power circulator 160 may be provided between the RF power source 150 and the second RF coupler 135, along the waveguide 160. The high power circulator 160 may be provided at or close to the RF power source, where the propagating RF power is at its highest value.

[0044] A third RF coupler 170 is provided at the output of the traveling wave section 120. Accelerated charged particles, such as electrons, pass through a first output of the third RF coupler 170, to a charged particle beam window or conversion target 180, as discussed above with respect to FIG. 1.

[0045] During operation of this portion of the linear accelerator system 100, the electron beam 145 may be formed at n×10 KeV, for example. The electron beam 145 is injected into the RF structure of the buncher section 110, where the electron bunches are formed and accelerated to bring the electron beam energy into the MeV range, typically, around 1 MeV. This ensures that bunching is nearly complete and the electron beam 145 becomes close to being fully relativistic, typically, from about 0.85 to about 0.95 times the speed of light. Then, in this example, the electron beam 145 enters the traveling wave section 120 (or traveling wave sections if additional traveling wave sections are provided collinear with the traveling wave section 120), and is accelerated to a higher output energy such as from 4 MeV to 12 MeV, for example. The electrons in the electron beam 145 may be accelerated to lower or to higher energies. In one example, the accelerated electron beam 145 strikes a Bremsstrahlung conversion target 180 to produce X-rays. In another example, the accelerated electron beam 145 passes through an output window 180, such as a thin metal foil, exits and from the vacuum envelope of the accelerator into air or a different environment, such as a different gas or a liquid, water, as is known in the art.

[0046] Continuing the description of the linear accelerator system 100, the first, second and third RF couplers 130, 135, and 170 are configured to match the impedance of the external and internal RF circuit to minimize power reflections at the operating RF frequency while running at nominal energy and beam current values. In addition, the high power circulator 160 in this example prevents reflected power from propagating back to the RF source 150. Therefore, most or all of the RF power from the RF power source 150 enters the second RF coupler 135, propagates within the traveling wave linear accelerator section 120 to form an accelerating traveling wave field distribution, and transfer power the electron beam.

[0047] In accordance with this embodiment of the invention, the third RF coupler 170 has a second output connected to an input of a second RF waveguide 190. The output of the second RF waveguide 190 is connected to a second input of the first RF coupler 130. RF power remaining after propagation through the traveling wave linear accelerator section and electron acceleration propagates to the buncher section 110, via the third input coupler 170 and the waveguide 190. The buncher section 110 may replace or render superfluous the RF load commonly used in a linear accelerator to absorb the remaining power coming out of traveling wave linear accelerator section 120, substantially increasing the linear accelerator efficiency.

[0048] An RF switch, an RF phase shifter, and/or an RF power adjuster, indicated by block 200 in FIG. 3, may be provided along the second RF waveguide 190 to regulate the power and/or phase of the RF power propagating into the buncher section 110, to change the energy and/or dose of the accelerated electron beam 145 output by the traveling wave linear accelerator section 120 or Bremsstrahlung radiation generated by the system 100. One or more RF switches, RF phase adjusters, and/or RF power adjusters may be provided. The waveguide 190 and the RF switch, phase shifter, and/or power adjuster 200 form a reverse feeding sequence (RFS) to feed the buncher section 110 with RF power remaining after attenuation and electron beam acceleration in the traveling wave section 120, improving the efficiency the linear accelerator 100. The switch, phase shifter, and/or power adjuster is/are outside of the vacuum envelope of the linear accelerator 105.

[0049] The power/phase ratio of the RF power provided to the standing wave section 110 may be varied by the RF switch, the RF phase shifter, and/or the RF power adjuster 200 to achieve the desired energy, dose, and/or other output characteristics of the accelerated electron beam 145 or the Bremsstrahlung radiation generated by the system 100. Use of the RF switch, RF phase shifter and/or RF power adjuster 200 in this and other embodiments of the invention described below in conjunction with FIGS. 4 and 5, may be combined with regulation of beam current and/or input power in manners known in the art to further optimize the characteristics of the radiation beam or electron beam output by the accelerator. Broad electron energy regulation, which may comprise setting of the energy/dose within an operating range of the linear accelerator system 100, or switching the energy/dose between two or more energies and/or doses during a scanning procedure within the operating range, may be provided. The operating range of the linear accelerator system 100 may be from about 0.5 MeV to a maximum linear accelerator energy, such as 7 MeV, for example, with a broad range of input RF power and input electron beam current intensities. Different operating ranges, such as ranges with higher maximum energies and/or lower minimum energy levels may be provided.

[0050] If the RF switch and/or RF phase shifter are slow or fast devices, electron beams or X-rays may be switched during operation “slowly,” when the time of the variation from one energy/dose level is substantially greater than pulse length and/or pulse repetition period, or “fast,” such as within times comparable to the pulse length and/or pulse repetition period, including variation within a pulse, and from pulse-to-pulse energy and dose switching (collectively called “fast switching”), respectively. Suitable controls may be provided control the operation and configuration of the RF switch, RF phase shifter, and/or RF power adjuster of the block 200 to set the desired energy/dose or switch between the desired energy/dose during operation.

[0051] Appropriate RF switches, RF phase shifters, and RF power adjusters that may be used in the block 200 are commercially available. The RF switch may be an on/off RF switch or an RF switch that switches between energy or phase levels on its own or in conjunction with an RF phase shifter and/or power adjuster, for example. Both fast and slow devices may be provided in the block 200 to provide versatility. The switch of block 200 may be a gas-filled, ferrite or other RF switch known in the art. An example of a fast ferrite switch that may be used is described in G. S. Uebele, “High-Speed ferrite microwave switch, 1957 IRE National Connection Record, Vol. 5, pt. 7, pp. 227-234; Proceedings IRE Transaction on Microwave Theory and Techniques, January 1959, pp. 73-82. The phase shifter of the block 200 may comprise fast and/or slow phase shifters. An appropriate fast phase shifter may be obtained from Ampas GmBH, Grosserlach, Germany, for example.

[0052] A low power circulator 220 may be provided along the waveguide 190, between the buncher section 100 and the block 200, for example, to prevent RF power reflected from the buncher section 110 from propagating back to the traveling wave linear accelerator section 120. The circulator 220 is referred to as a “low power” circulator because the RF power in this location is much lower than the RF power provided by the RF source, due to some reflections, attenuation in the traveling wave lines accelerator 120, and power consumed by the electron beam.

[0053] A magnetic system 220, such as an external focusing solenoid or a permanent periodic magnet (PPM) system, is optionally provided proximate and in cooperation with the buncher section 110 and/or the traveling wave section 120 to focus the electron beam 145 as it passes through the buncher section 110 and/or the traveling wave section 120. The magnet system 200 may be omitted, because it only provides a small improvement in current transmission and increases complexity, power consumption, and consequently the cost of the hybrid linear accelerator system 100 and other examples of hybrid linear accelerator systems described herein. Simulations of several specific examples demonstrated that use of an external focusing system 200 improved current transmission by only about 20%. RF fields may be used in the buncher section 110 and/or in the traveling wave section 120 to focus and transport the electron beam to the traveling wave section 120, thereby avoiding use of the external magnetic focusing system 13.

[0054] This combination of the standing wave and traveling wave sections exploits several advantages of both. For example, the main operational frequency of the linear accelerator is largely defined by the standing wave buncher section 110, while the traveling wave linear accelerator section 120 is more broadband and is easily tuned to the required resonance frequency of the standing wave buncher section. Therefore, automatic frequency control (AFC) may be based on the buncher section 110, which is common for standing wave linear accelerators. If the AFC is only based on the traveling wave section 120, the AFC needs to be much more complex to ensure steady operation of the linear accelerator. In addition, the standing wave buncher section 110 permits effective RF focusing of the electron beam while reaching the relativistic speed, and further acceleration in the traveling wave section 120 can also be used without any external magnetic system, as discussed above.

[0055] Exploring a design example of the embodiment of FIG. 3 at 9300 MHz, using a PM-1110X X-band magnetron manufactured by L-3 Electron Devices, San Carlos, Calif., for example, the design parameters for a 60 cm long hybrid RF structure were found to be superior to the existing non-hybrid configurations with similar characteristics. The hybrid RF structure delivered a steady beam at energy in broad energy range of 1 MeV to 7 MeV, with a maximum output dose rate of 1100 R/min at 1 m, which corresponds to over 1700 R/min @ 80 cm, while delivering a substantial dose rate at low energy, estimated in tens of R/min at 1 m. Such a compact linear accelerator system with record high radiation beam characteristics can be useful in many fields, such as Non-Destructive Testing (NDT), Security Screening (SI), Radiation Therapy (RT), etc.

[0056] FIG. 4 is a schematic representation of an example of a hybrid linear accelerator in accordance with a second embodiment of the invention, including a parallel RF feed. Items common to FIG. 3 are similarly numbered. The operation and capabilities of this embodiment of the invention are the same as the embodiment of FIG. 3, except as noted herein.

[0057] In this example, the buncher section 110 and the traveling wave section 120 are decoupled by the drift tube 125, as in FIG. 3. The RF source 150 provides RF power through an RF transmitting waveguide 160, via a high power circulator 165, which is then split by an RF splitter 310. A portion of the RF power determined by the dividing ratio of the RF splitter 310 is forwarded through a first arm 315 of the RF splitter to a first RF coupler 320 at the output of the buncher section 110. The remaining power is forwarded through the second arm 330 of the RF splitter 310 to the second input RF coupler 135 through RF switch, RF phase shifter, and/or RF power adjuster 340, which may be the same or similar to the block 200 used in the embodiment of FIG. 3.

[0058] The RF switch, RF phase shifter, and/or RF power adjuster 340 redistributes RF power between the buncher section 110 and the traveling wave section 120, through the RF splitter 310. The RF energy and/or phase of the RF power redistributed to the buncher section 110 may be changed to set or change the energy and/dose of the intermediate beam of electrons output by the traveling wave linear accelerator section 120. The RF switch, RF phase shifter, and/or RF power adjuster 340 may also be configured to change the phase relationship between the buncher section and the traveling wave section, also setting or changing the energy and/dose of the intermediate beam of electrons output by the traveling wave linear accelerator section 120. Broad broad energy regulation of the output beam of electrons is thereby provided. As above, the RF switch, RF phase adjuster, and/or RF power adjuster is/are outside of the vacuum envelope of the linear accelerator 105.

[0059] In the embodiment of FIG. 4, a matched RF load 350 is provided to absorb RF power remaining after attenuation in the traveling wave accelerator section 120. The remaining RF power in the traveling wave section 120 is coupled to the matched RF load 350 through the RF coupler 170 at an output of the traveling wave section.

[0060] The embodiment of FIG. 4 may not be as efficient as the embodiment of FIG. 3, since the remaining RF power is not used. As above, broad electron energy regulation, such as from about 0.5 MeV to a maximum linear accelerator energy, may be achieved while operating in a broad range of input RF power, thereby efficiently running at a variety of input electron beam current intensities at high efficiency.

[0061] FIG. 5 is a schematic representation of an example of a hybrid linear accelerator 400 in accordance with a third embodiment of the invention. Items common to FIG. 3 are similarly numbered. The operation and capabilities of this embodiment of the invention are the same as the embodiment of FIG. 3, except as noted herein.

[0062] A input RF coupler 410 serves as a combined single RF power input for both the standing wave buncher section 110 and the traveling wave linear accelerator section 120. A drift tube is not provided between the buncher section 110 and the traveling wave section 120 in this embodiment.

[0063] An RF switch 420 may be provided at the RF output of the traveling wave section 120, after an RF coupler 430. The RF switches discussed above may be used here, for example.

[0064] A matched RF load 350, as in FIG. 4, is provided after the radiation beam parameter RF switch 420, to absorb RF power remaining after acceleration in the traveling wave section 120. As above, broad electron energy regulation, such as from about 0.5 MeV to a maximum linear accelerator energy, may be achieved while operating in a broad range of input RF power, thereby efficiently running at a variety of input electron beam current intensities at high efficiency.

[0065] While one (1) standing wave linear accelerator (buncher) section 110 and one (1) traveling wave linear accelerator section 120 are shown in the examples above, additional standing wave sections and/or traveling wave sections may can be provided. If additional standing wave sections are provided, in one example only the first standing wave section is configured to be a buncher.

[0066] Linear accelerator controls and/or a modulator (not shown) may or may not provide a supplemental method of regulating electron beam current and/or input RF power to support optimization of the linear accelerator in a broad range of its parameters, in the embodiments described above.

[0067] Other modifications and implementations will occur to those skilled in the art without departing from the spirit and the scope of the claimed invention. Accordingly, the above description is not intended to limit the invention, except as indicated in the following claims.