Programmable radio frequency waveform generator for a synchrocyclotron

Abstract

A synchrocyclotron .[.comprises.]. .Iadd.includes .Iaddend.a resonant circuit that includes electrodes having a gap therebetween across the magnetic field. An oscillating voltage input, having a variable amplitude and frequency determined by a programmable digital waveform generator generates an oscillating electric field across the gap. The synchrocyclotron can include a variable capacitor in circuit with the electrodes to vary the resonant frequency. The synchrocyclotron can further include an injection electrode and an extraction electrode having voltages controlled by the programmable digital waveform generator. The synchrocyclotron can further include a beam monitor. The synchrocyclotron can detect resonant conditions in the resonant circuit by measuring the voltage .[.and or.]. .Iadd.and/or .Iaddend.current in the resonant circuit, driven by the input voltage, and adjust the capacitance of the variable capacitor or the frequency of the input voltage to maintain the resonant conditions. The programmable waveform generator can adjust at least one of the oscillating voltage input, the voltage on the injection electrode and the voltage on the extraction electrode according to beam intensity and in response to changes in resonant conditions.

Claims

1. A synchrocyclotron comprising: a magnetic field generator .Iadd.to produce a magnetic field.Iaddend.; a resonant circuit, comprising: electrodes, disposed .Iadd.within a cavity .Iaddend.between magnetic poles, having a gap therebetween across .[.a.]. .Iadd.the .Iaddend.magnetic field; and a variable reactive element in circuit with the electrodes to vary a resonant frequency of the resonant circuit; .[.and.]. .Iadd.an extraction channel to receive charged particles from the cavity and to output a particle beam from the synchrocyclotron; a sensor to measure a voltage in the resonant circuit; and .Iaddend. a voltage input generator to .[.provide.]. .Iadd.adjust .Iaddend.a .Iadd.frequency of a .Iaddend.voltage input to the resonant circuit .Iadd.based on an intensity of the particle beam output by the synchrocyclotron and a frequency of the voltage measured in the resonant circuit.Iaddend., the voltage input being an oscillating voltage that varies in frequency over a time of acceleration of .Iadd.the .Iaddend.charged particles.

2. The synchrocyclotron as claimed in claim 1 wherein an amplitude of the voltage input is varied.

3. The synchrocyclotron of claim 1 further including an ion source for injecting charged particles into the synchrocyclotron.

4. The synchrocyclotron of claim 1 further including an extraction electrode disposed between the magnetic poles for extracting .[.a.]. .Iadd.the .Iaddend.particle beam from the synchrocyclotron.

5. The synchrocyclotron of claim 1 .[.further including a.]. .Iadd.wherein the .Iaddend.sensor .Iadd.is .Iaddend.for detecting resonant conditions in the resonant circuit.

6. The synchrocyclotron of claim 5 further including means for adjusting at least one of .[.a.]. .Iadd.the .Iaddend.frequency of the voltage input, reactance of the variable reactive element, and resonant frequency of the resonant circuit based on the resonant conditions.

7. The synchrocyclotron of claim 1 further including a beam monitor for detecting a particle beam extracted from the synchrocyclotron.

8. The synchrocyclotron of claim 7 wherein the beam monitor is configured to measure at least one of particle beam intensity, particle beam timing, and spatial distribution of the particle beam.

9. The synchrocyclotron as claimed in claim 1 .[.further including.]. .Iadd.wherein the voltage input generator comprises .Iaddend.a programmable digital waveform generator for generating the oscillating voltage.

10. The synchrocyclotron of claim 9 wherein the programmable waveform generator is configured to compensate for at least one of variations in resonant conditions of the resonant circuit and variations in .[.a.]. .Iadd.the .Iaddend.particle beam extracted from the synchrocyclotron.

11. A method of accelerating particles in a synchrocyclotron, comprising: providing particles in .Iadd.a cavity of .Iaddend.the synchrocyclotron; providing a resonant circuit .Iadd.in the cavity.Iaddend., the resonant circuit comprising accelerating electrodes having a gap therebetween across a magnetic field; .[.and.]. .Iadd.using a sensor to measure a voltage in the resonant circuit; .Iaddend. .[.with.]. .Iadd.using .Iaddend.a voltage input generator, .[.applying an oscillating.]. .Iadd.adjusting, based on an intensity of a particle beam output by the synchrocyclotron and a frequency of a voltage measured in the resonant circuit, a frequency of a .Iaddend.voltage input .[.that varies in frequency during acceleration of the particles.]. to the resonant circuit, the .[.oscillating.]. voltage input creating an oscillating electric field across the gap that accelerates the particles in the .[.synchrocyclotron.]. .Iadd.cavity; and receiving the particles from the cavity in an extraction channel and outputting a particle beam from the extraction channel.Iaddend..

12. The method as in claim 11, further including generating the .[.oscillating.]. voltage input.

13. The method of claim 11, further including varying an amplitude of the .[.oscillating.]. voltage input.

14. The method of claim 11, wherein providing particles includes injecting particles into the synchrocyclotron.

15. The method of claim 11, further including detecting resonant conditions in the resonant circuit.

16. The method of claim 15, further including adjusting at least one of a frequency of the .[.oscillating.]. voltage input, a reactance of a variable reactive element, and a resonant frequency of the resonant circuit based on the resonant conditions.

17. The method of claim 11, further including extracting accelerated particles from the synchrocyclotron to form .[.a.]. .Iadd.the .Iaddend.particle beam.

18. The method as in claim 17, further including detecting variations in the particle beam.

19. The method as in claim 18, wherein the variations include variations in at least one of particle beam intensity, particle beam timing, and spatial distribution of the particle beam.

20. The method as in claim 18, further including compensating for detected variations in the particle beam.

.Iadd.21. A particle accelerator configured to generate a particle beam, the particle accelerator comprising: an ion source to provide pulses of charged particles that are accelerated in a cavity to form the particle beam; a beam monitor to obtain information about the particle beam, the information comprising information about an intensity of the particle beam; and a waveform generator comprising a controller to receive the information about the intensity and to write data to memory, the waveform generator comprising a driver to provide voltage to the cavity to accelerate the charged particles; and circuitry to control operation of the ion source based on the information about the intensity so that the pulses are provided at timed instances in synchronism with a sweep of a frequency of the voltage that accelerates the charged particles..Iaddend.

.Iadd.22. The particle accelerator of claim 21, further comprising: magnetic pole pieces that border the cavity, the cavity housing at least part of the ion source; wherein the voltage comprises an oscillating voltage having a frequency that varies over a time to accelerate the charged particles..Iaddend.

.Iadd.23. The particle accelerator of claim 21, wherein the voltage comprises an oscillating voltage having a frequency that varies over a time to accelerate the charged particles..Iaddend.

.Iadd.24. The particle accelerator of claim 21, further comprising: magnetic pole pieces that border the cavity, the cavity housing at least part of the ion source; wherein the voltage comprises an oscillating voltage having a frequency that varies over a time to accelerate the charged particles; and wherein the ion source is controlled to provide pulses of charged particles at a same point in each cycle of the oscillating voltage..Iaddend.

.Iadd.25. The particle accelerator of claim 21, wherein the operation of the ion source is controlled by turning the ion source on and off during a time interval in order to obtain a desired beam current..Iaddend.

.Iadd.26. The particle accelerator of claim 21, wherein the operation of the ion source is controlled by selecting a time interval and by dropping a number of pulses that occur during the time interval to obtain a desired beam current..Iaddend.

.Iadd.27. The particle accelerator of claim 21, wherein the particle accelerator comprises a synchrocyclotron..Iaddend.

.Iadd.28. The particle accelerator of claim 21, wherein the waveform generator comprises digital circuitry, the digital circuitry comprising a digital-to-analog converter (DAC) to generate an analog signal based on the data, the analog signal for controlling the ion source..Iaddend.

.Iadd.29. A particle accelerator configured to generate a particle beam, the particle accelerator comprising: an ion source to inject charged particles into a resonant cavity; magnetic pole pieces that border the resonant cavity, the resonant cavity having a radio frequency (RF) voltage to accelerate the charged particles to produce the particle beam; a feedback system to provide at least one property of the particle beam; and circuitry to receive an injection control pulse that extends over multiple cycles of the RF voltage and to control, based on the at least one property, the injection control pulse based on cycles of the RF voltage so that charged particles are injected continuously over at least some of the multiple cycles..Iaddend.

.Iadd.30. The particle accelerator of claim 29, wherein the at least one property comprises an intensity of the particle beam..Iaddend.

.Iadd.31. The particle accelerator of claim 29, wherein the at least one property comprises a spatial distribution of the particle beam..Iaddend.

.Iadd.32. The particle accelerator of claim 29, wherein the at least one property comprises a timing associated with the particle beam..Iaddend.

.Iadd.33. The particle accelerator of claim 29, wherein the injection control pulse controls operation of the ion source by turning the ion source on and off during a time interval in order to maintain a beam current..Iaddend.

.Iadd.34. The particle accelerator of claim 29, wherein the injection control pulse controls operation of the ion source by selecting a time interval and wherein the circuitry is configured to drop a number of pulses that occur during the time interval to maintain a beam current..Iaddend.

.Iadd.35. The particle accelerator of claim 29, wherein the particle accelerator comprises a synchrocyclotron..Iaddend.

.Iadd.36. The particle accelerator of claim 29, wherein the feedback system comprises a beam monitor to monitor the particle beam to obtain the at least one property..Iaddend.

.Iadd.37. The particle accelerator of claim 29, wherein the circuitry comprises a digital waveform generator..Iaddend.

.Iadd.38. A synchrocyclotron comprising: magnetic pole pieces that border a cavity; an ion source to provide, to the cavity, ions that are accelerated to form a particle beam; a voltage generator to provide voltage to the cavity, the voltage comprising an oscillating voltage having a frequency that varies over a time of acceleration of the ions; a sensor to measure the oscillating voltage in the cavity; and a control circuit to control, based on the oscillating voltage measured by the sensor, a timing at which ions are provided by the ion source, the timing being controlled so that ions are provided into the cavity for a limited time and at a same point in each of multiple frequency sweeps of the oscillating voltage, at least part of the control circuit comprising digital circuitry, the digital circuitry comprising an interface that provides a data link to a computer..Iaddend.

.Iadd.39. The synchrocyclotron of claim 38, further comprising: a beam monitor to provide information about the particle beam, the information comprising information based on an intensity of the particle beam; wherein the control circuit is configured to control the timing based also on the information based on the intensity..Iaddend.

.Iadd.40. The synchrocyclotron of claim 38, further comprising: a beam monitor to provide information about the particle beam, the information comprising information based on a spatial distribution of the particle beam; wherein the control circuit is configured to control the timing based also on the information based on the spatial distribution..Iaddend.

.Iadd.41. The synchrocyclotron of claim 38, further comprising: a beam monitor to provide information about the particle beam, the information comprising information based on a timing of the particle beam; wherein the control circuit is configured to control the timing based also on the information based on a timing of the particle beam..Iaddend.

.Iadd.42. The synchrocyclotron of claim 38, wherein the at least part of the control circuit comprises a programmable processor..Iaddend.

.Iadd.43. The synchrocyclotron of claim 38, wherein the control circuit is configured to control the timing so as to cause the ion source to provide particles into the cavity over a number of cycles of the oscillating voltage..Iaddend.

.Iadd.44. The synchrocyclotron of claim 38, wherein the control circuit is configured to control the timing by turning the ion source on and off during a time interval in order to maintain a beam current..Iaddend.

.Iadd.45. The synchrocyclotron of claim 38, wherein the digital circuitry comprises a programmable processor..Iaddend.

.Iadd.46. The synchrocyclotron of claim 38, wherein voltage generator comprises a digital waveform generator that includes the control circuit..Iaddend.

.Iadd.47. A synchrocyclotron comprising: a particle source to provide charged particles into a cavity having a magnetic field; a voltage circuit to cause acceleration of the charged particles in the cavity via an oscillating electric field, the oscillating electric field to vary over a time of acceleration of the charged particles, the voltage circuit comprising a resonant circuit that comprises accelerating electrodes having a gap therebetween across the magnetic field and a driver to drive an oscillating voltage input to produce the oscillating electric field across the gap, the oscillating voltage input to vary over the time of acceleration of the charged particles; and a control circuit to control the particle source to provide the charged particles for a limited time during each of multiple frequency sweeps of the oscillating electric field across the gap, the limited time synchronizing with an acceptance phase angle of the synchrocyclotron, at least part of the control circuit comprising digital circuitry, the digital circuitry comprising an interface that provides a data link to a computer..Iaddend.

.Iadd.48. The synchrocyclotron of claim 47, further comprising: a beam monitor to provide information about a particle beam, the information comprising information based on an intensity of the particle beam; wherein the control circuit is configured to control the particle source based also on the information based on the intensity..Iaddend.

.Iadd.49. The synchrocyclotron of claim 47, further comprising: a beam monitor to provide information about a particle beam, the information comprising information based on a spatial distribution of the particle beam; wherein the control circuit is configured to control the particle source based also on the information based on the spatial distribution..Iaddend.

.Iadd.50. The synchrocyclotron of claim 47, further comprising: a beam monitor to provide information about a particle beam, the information comprising information based on a timing of the particle beam; wherein the control circuit is configured to control the particle source based also on the information based on a timing of the particle beam..Iaddend.

.Iadd.51. The synchrocyclotron of claim 47, wherein the control circuit is configured to control the particle source based on variations of the oscillating electric field over time..Iaddend.

.Iadd.52. The synchrocyclotron of claim 47, wherein the control circuit is configured to control the particle source so as to cause the particle source to inject particles at a same point in each cycle of the oscillating electric field..Iaddend.

.Iadd.53. The synchrocyclotron of claim 47, wherein the control circuit is configured to control the particle source by turning the particle source on and off during a time interval in order to obtain a desired beam current..Iaddend.

.Iadd.54. The synchrocyclotron of claim 47, wherein the digital circuitry comprises a programmable processor..Iaddend.

.Iadd.55. The synchrocyclotron of claim 47, wherein voltage circuit and the control circuit are components of a digital waveform generator..Iaddend.

.Iadd.56. A particle accelerator comprising: a cavity having a magnetic field; an ion source to provide charged particles to the cavity; a digital waveform generator to apply an oscillating voltage input to a resonant circuit that comprises accelerating electrodes having a gap therebetween across the magnetic field to drive an oscillating electric field across the gap and accelerate the charged particles within the cavity; a beam monitor to measure a property of a particle beam comprised of the charged particles that have been accelerated; wherein the digital waveform generator is configured to control the oscillating voltage input and the ion source to compensate for variations in the property of the particle beam; and a programmable processor to control at least part of the digital waveform generator..Iaddend.

.Iadd.57. The particle accelerator of claim 56, wherein digital waveform generator is configured to vary an amplitude of the oscillating voltage input..Iaddend.

.Iadd.58. The particle accelerator of claim 56, wherein digital waveform generator is configured to vary a frequency of the oscillating voltage input..Iaddend.

.Iadd.59. The particle accelerator of claim 56, wherein digital waveform generator is configured to vary an amplitude and a frequency of oscillating voltage input..Iaddend.

.Iadd.60. The particle accelerator of claim 59, wherein digital waveform generator is configured to vary a frequency of the oscillating voltage input in order to maintain resonant conditions in the cavity..Iaddend.

.Iadd.61. The particle accelerator of claim 60, further comprising: a variable reactive element in circuit with the oscillating voltage input and the accelerating electrodes..Iaddend.

.Iadd.62. The particle accelerator of claim 56, wherein the property comprises particle beam timing..Iaddend.

.Iadd.63. The particle accelerator of claim 56, wherein the property comprises spatial distribution of the particle beam..Iaddend.

.Iadd.64. A particle accelerator comprising magnetic pole pieces that border a cavity; a voltage generator to provide voltage to the cavity, the voltage comprising an oscillating voltage having a frequency that varies over a time of acceleration of charged particles; an ion source that is controllable to output, over a period of time, pulses of charged particles that are accelerated in the cavity based on the oscillating voltage to form a particle beam; control circuitry to regulate a current of the particle beam by controlling the ion source so as not to output at least some of the pulses during the period of time; and a beam monitor to obtain at least one property of the particle beam; wherein the control circuitry is configured to control a timing at which some of the pulses are output based on the oscillating voltage and an acceptance phase angle of the synchrocyclotron and to control, based on the at least one property, the ion source so as to change a number of the pulses that are not output during the period of time across different cycles of the voltage..Iaddend.

.Iadd.65. The particle accelerator of claim 64, wherein the at least one property comprises an intensity of the particle beam..Iaddend.

.Iadd.66. The particle accelerator of claim 64, wherein the at least one property comprises a timing associated with the particle beam..Iaddend.

.Iadd.67. The particle accelerator of claim 64, wherein the at least one property comprises a spatial distribution of the particle beam..Iaddend.

.Iadd.68. The particle accelerator of claim 64, further comprising: a resonant circuit, comprising: electrodes, disposed between magnetic pole pieces, having a gap therebetween across the magnetic field; and a variable reactive element in circuit with the electrodes to vary a resonant frequency of the resonant circuit; and wherein the voltage generator is configured to provide the voltage to the resonant circuit..Iaddend.

.Iadd.69. The particle accelerator of claim 68, wherein the voltage generator is digitally controllable to change an amplitude of the voltage..Iaddend.

.Iadd.70. The particle accelerator of claim 69, wherein the control circuit is configured to regulate the current based on the at least one property..Iaddend.

.Iadd.71. A particle accelerator configured to generate a particle beam, the particle accelerator comprising: an ion source to provide charged particles into a resonant cavity; electrodes in the resonant cavity, the electrodes being separated by a gap; a voltage circuit to output a radio frequency (RF) voltage to at least one of the electrodes to produce an oscillating electric field across the gap, the RF voltage having a frequency that varies to accelerate the charged particles to form the particle beam; and a feedback system to detect information about the frequency of the oscillating electric field, and to adjust a timing of operation of the voltage circuit and the ion source based on the information so that the charged particles are provided to the resonant cavity in synchronism with a variation in the frequency of the RF voltage..Iaddend.

.Iadd.72. The particle accelerator of claim 71, wherein the feedback system comprises a voltage sensor configured to measure the oscillating electric field across the gap..Iaddend.

.Iadd.73. The particle accelerator of claim 71, wherein the feedback system comprises a digital control circuit configured to write data based on the information to memory, and to output data from memory to generate one or more waveforms upon which future outputs of the RF voltage are based..Iaddend.

.Iadd.74. The particle accelerator of claim 71, wherein the feedback system comprises: a beam monitor to detect an intensity of the particle beam; and a control circuit to control operation of the ion source based on the intensity detected..Iaddend.

.Iadd.75. The particle accelerator of claim 72, wherein the particle accelerator is a synchrocyclotron..Iaddend.

.Iadd.76. The particle accelerator of claim 71, wherein the timing of operation of the voltage circuit is varied to compensate for physical features of the particle accelerator..Iaddend.

.Iadd.77. The particle accelerator of claim 71, wherein the information comprises a peak resonant condition of the resonant cavity; and wherein the feedback system is configured to control the timing of operation of the voltage circuit based on the peak resonant condition..Iaddend.

.Iadd.78. The particle accelerator of claim 77, wherein the timing of operation of the voltage circuit is controlled so that the RF voltage output matches a voltage of the resonant circuit corresponding to the peak resonant condition..Iaddend.

.Iadd.79. The particle accelerator of claim 71, wherein the feedback system comprises a programmable processor to control the timing of operation of at least one of the voltage circuit or the ion source based on the information..Iaddend.

.Iadd.80. A synchrocyclotron configured to output a particle beam, the synchrocyclotron comprising: an ion source to provide charged particles to a resonant cavity for orbital acceleration to form the particle beam, the particle beam comprising multiple pulses; a voltage source to provide a radio frequency (RF) voltage to the resonant cavity, the voltage source comprising an amplifier to change an amplitude of the RF voltage before providing the RF voltage to the resonant cavity; a control system to control operation of the ion source in order to control intensities of individual pulses among the multiple pulses comprising the particle beam, the control system comprising a programmable processor to control the operation of the ion source based on a frequency of the RF voltage in the resonant cavity; and a beam monitor to detect an intensity of the particle beam; wherein the programmable processor is configured to control operation of the ion source based also on information about the detected intensity..Iaddend.

.Iadd.81. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled by turning the ion source on and off during a time interval..Iaddend.

.Iadd.82. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled by selectively providing charged particles from the ion source over a number of cycles of the frequency of the RF voltage..Iaddend.

.Iadd.83. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled to control a timing at which the charged particles are provided..Iaddend.

.Iadd.84. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled to inject the charged particles at a same time relative to each cycle among multiple cycles of the frequency of the RF voltage..Iaddend.

.Iadd.85. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled to inject the charged particles continuously over multiple cycles of the frequency of the RF voltage..Iaddend.

.Iadd.86. The synchrocyclotron of claim 80, further comprising: magnetic pole pieces that border the resonant cavity in which the charged particles accelerate; and a resonant circuit to control a resonant frequency within the resonant cavity; wherein the voltage source is configured to provide the RF voltage to the resonant circuit, the RF voltage comprising an oscillating voltage that varies over a time of acceleration of the charged particles; and wherein the voltage source comprises a digital waveform generator that includes the control system..Iaddend.

.Iadd.87. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled to inject the charged particles so as to compensate for detected variations of the particle beam..Iaddend.

.Iadd.88. The synchrocyclotron of claim 80, wherein the operation of the ion source is controlled to inject the charged particles periodically..Iaddend.

.Iadd.89. The synchrocyclotron of claim 80, wherein controlling intensities of individual pulses comprises controlling pulse widths of the individual pulses..Iaddend.

.Iadd.90. A synchrocyclotron comprising: magnetic pole pieces that border a cavity in which charged particles accelerate; a resonant circuit to control a resonant frequency within the cavity; a voltage circuit to provide voltage to the resonant circuit, the voltage comprising an oscillating voltage that varies over a time of acceleration of the charged particles; and a feedback system to detect parameters based on operation of the resonant circuit, and to control the voltage circuit based on a least one of the parameters so as to maintain a resonant condition in the resonant circuit; wherein the parameters comprises an amplitude of the voltage and a frequency of the voltage, and wherein the voltage circuit comprises an amplifier to change the amplitude of the voltage to maintain beam focusing..Iaddend.

.Iadd.91. The synchrocyclotron of claim 90, wherein the feedback system comprises a control circuit to control a property of the voltage based on the at least one parameter..Iaddend.

.Iadd.92. The synchrocyclotron of claim 91, wherein the control circuit is configured to control the property of the voltage by writing data about the property to memory..Iaddend.

.Iadd.93. The synchrocyclotron of claim 90, further comprising: an ion source to provide the charged particles to the cavity, the charged particles to accelerate in the cavity to form a particle beam; an extraction channel to output the particle beam from the synchrocyclotron; and a beam monitor to obtain information about a property of the particle beam; wherein the feedback system is configured to receive the information about the property of the particle beam, and to use the information to control operation of the ion source..Iaddend.

.Iadd.94. The synchrocyclotron of claim 93, wherein the property of the particle beam is an intensity of the particle beam..Iaddend.

.Iadd.95. The synchrocyclotron of claim 93, wherein the property of the particle beam is particle beam timing or spatial distribution of the particle beam..Iaddend.

.Iadd.96. The synchrocyclotron of claim 90, wherein the resonant circuit comprises: electrodes in the cavity, the electrodes having a gap therebetween across a magnetic field in the cavity produced by the magnetic pole pieces; and a variable reactive element in circuit with the electrodes to vary the resonant frequency..Iaddend.

.Iadd.97. The synchrocyclotron of claim 96, further comprising: an angular position sensor associated with the variable reactive element to output information about a position of the variable reactive element; wherein at least one parameter comprises the information about the position..Iaddend.

.Iadd.98. The synchrocyclotron of claim 90, wherein at least part of the voltage circuit comprises digital circuitry..Iaddend.

.Iadd.99. The synchrocyclotron of claim 90, wherein at least one parameter comprises a timing parameter..Iaddend.

.Iadd.100. The synchrocyclotron of claim 93, wherein the feedback system is configured to use the information to generate, based on digital data, timing waveforms to control the voltage circuit..Iaddend.

.Iadd.101. The synchrocyclotron of claim 90, wherein the feedback system is adaptive..Iaddend.

.Iadd.102. A synchrocyclotron comprising: a magnetic field generator; a resonant circuit comprising a variable reactive element to vary a resonant frequency of the resonant circuit; a source to provide a voltage input to the resonant circuit, the voltage input being an oscillating voltage that varies over a time of acceleration of charged particles; a feedback system to vary at least one property of the voltage input to the resonant circuit; and one or more sensors to detect that there has been a deviation from a peak resonant condition in the resonant circuit; wherein the voltage input is controlled by the feedback system based on the deviation in order to maintain the peak resonant condition..Iaddend.

.Iadd.103. The synchrocyclotron of claim 102, wherein the at least one property comprises amplitude of the voltage input..Iaddend.

.Iadd.104. The synchrocyclotron of claim 102, wherein the at least one property comprises frequency of the voltage input..Iaddend.

.Iadd.105. The synchrocyclotron of claim 102, wherein the at least one property comprises amplitude of the voltage input and frequency of the voltage input..Iaddend.

.Iadd.106. The synchrocyclotron of claim 102, wherein the feedback system is configured to maintain the peak resonant condition..Iaddend.

.Iadd.107. The synchrocyclotron of claim 102, further comprising: an ion source to provide the charged particles that form a particle beam following acceleration; and a beam monitor to monitor the at least one property of the particle beam; wherein the feedback system comprises control circuitry to receive the at least one property of the particle beam, and to control at least one of the voltage input or the ion source based on the at least one property to compensate for variations in the particle beam..Iaddend.

.Iadd.108. The synchrocyclotron of claim 107, wherein the at least one property comprises beam intensity, beam timing, or beam spatial distribution..Iaddend.

.Iadd.109. The synchrocyclotron of claim 107, wherein the voltage input is generated by a programmable digital waveform generator..Iaddend.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

(2) FIG. 1A is a plan cross-sectional view of a synchrocyclotron of the present invention.

(3) FIG. 1B is a side cross-sectional view of the synchrocyclotron shown in FIG. 1A.

(4) FIG. 2 is an illustration of an idealized waveform that can be used for accelerating charged particles in a synchrocyclotron shown in FIGS. 1A and 1B.

(5) FIG. 3A depicts a portion of a block diagram of a synchrocyclotron of the present invention that includes a waveform generator system.

(6) FIG. 3B depicts a portion of a block diagram of a synchrocyclotron of the present invention that includes a waveform generator system.

(7) FIG. 4 is a flow chart illustrating the principles of operation of a digital waveform generator and an adaptive feedback system (optimizer) of the present invention.

(8) FIG. 5A shows the effect of the finite propagation delay of the signal across different paths in an accelerating electrode (dee) structure.

(9) FIG. 5B shows the input waveform timing adjusted to correct for the variation in propagation delay across the dee structure.

(10) FIG. 6A shows an illustrative frequency response of the resonant system with variations due to parasitic circuit effects.

(11) FIG. 6B shows a waveform calculated to correct for the variations in frequency response due to parasitic circuit effects.

(12) FIG. 6C shows the resulting flat frequency response of the system when the waveform shown in FIG. 6B is used as input voltage.

(13) FIG. 7A shows a constant amplitude input voltage applied to the accelerating electrodes shown in FIG. 7B.

(14) FIG. 7B shows an example of the accelerating electrode geometry wherein the distance between the electrodes is reduced toward the center.

(15) FIG. 7C shows the desired and resultant electric field strength in the electrode gap as a function of radius that achieves a stable and efficient acceleration of charged particles by applying input voltage as shown in FIG. 7A to the electrode geometry shown in FIG. 7B.

(16) FIG. 7D shows input voltage input as a function of radius that directly corresponds to the electric field strength desired and can be produced using a digital waveform generator.

(17) FIG. 7E shows a parallel geometry of the accelerating electrodes which gives a direct proportionality between applied voltage and electric field strength.

(18) FIG. 7F shows the desired and resultant electric field strength in the electrode gap as a function of radius that achieves a stable and efficient acceleration of charged particles by applying input voltage as shown in FIG. 7D to the electrode geometry shown in FIG. 7E.

(19) FIG. 8A shows an example of a waveform of the accelerating voltage generated by the programmable waveform generator.

(20) FIG. 8B shows an example of a timed ion injector signal.

(21) FIG. 8C shows another example of a timed ion injector signal.

DETAILED DESCRIPTION OF THE INVENTION

(22) This invention relates to the devices and methods for generating the complex, precisely timed accelerating voltages across the dee gap in a synchrocyclotron. This invention comprises an apparatus and a method for driving the voltage across the dee gap by generating a specific waveform, where the amplitude, frequency and phase is controlled in such a manner as to create the most effective particle acceleration given the physical configuration of the individual accelerator, the magnetic field profile, and other variables that may or may not be known a priori. A synchrocyclotron needs a decreasing magnetic field in order to maintain focusing of the particles beam, thereby modifying the desired shape of the frequency sweep. There are predictable finite propagation delays of the applied electrical signal to the effective point on the dee where the accelerating particle bunch experiences the electric field that leads to continuous acceleration. The amplifier used to amplify the radio frequency (RF) signal that drives the voltage across the dee gap may also have a phase shift that varies with frequency. Some of the effects may not be known a priori, and may be only observed after integration of the entire synchrocyclotron. In addition, the timing of the particle injection and extraction on a nanosecond time scale can increase the extraction efficiency of the accelerator, thus reducing stray radiation due to particles lost in the accelerating and extraction phases of operation.

(23) Referring to FIGS. 1A and 1B, a synchrocyclotron of the present invention comprises electrical coils 2a and 2b around two spaced apart metal magnetic poles 4a and 4b configured to generate a magnetic field. Magnetic poles 4a and 4b are defined by two opposing portions of yoke 6a and 6b (shown in cross-section). The space between poles 4a and 4b defines vacuum chamber 8 or a separate vacuum chamber can be installed between the poles 4a and 4b. The magnetic field strength is generally a function of distance from the center of vacuum chamber 8 and is determined largely by the choice of geometry of coils 2a and 2b and shape and material of magnetic poles 4a and 4b.

(24) The accelerating electrodes comprise dee 10 and dee 12, having gap 13 therebetween. Dee 10 is connected to an alternating voltage potential whose frequency is changed from high to low during the accelerating cycle in order to account for the increasing relativistic mass of a charged particle and radially decreasing magnetic field (measured from the center of vacuum chamber 8) produced by coils 2a and 2b and pole portions 4a and 4b. The characteristic profile of the alternating voltage in dees 10 and 12 is show in FIG. 2 and will be discussed in details below. Dee 10 is a half-cylinder structure, hollow inside. Dee 12, also referred to as the dummy dee, does not need to be a hollow cylindrical structure as it is grounded at the vacuum chamber walls 14. Dee 12 as shown in FIGS. 1A and 1B comprises a strip of metal, e.g. copper, having a slot shaped to match a substantially similar slot in dee 10. Dee 12 can be shaped to form a mirror image of surface 16 of dee 10.

(25) Ion source 18 that includes ion source electrode 20, located at the center of vacuum chamber 8, is provided for injecting charged particles. Extraction electrodes 22 are provided to direct the charge particles into extraction channel 24, thereby forming beam 26 of the charged particles. The ion source may also be mounted externally and inject the ions substantially axially into the acceleration region.

(26) Dees 10 and 12 and other pieces of hardware that comprise a cyclotron, define a tunable resonant circuit under an oscillating voltage input that creates an oscillating electric field across gap 13. This resonant circuit can be tuned to keep the Q-factor high during the frequency sweep by using a tuning means.

(27) As used herein, Q-factor is a measure of the quality of a resonant system in its response to frequencies close to the resonant frequency. Q-factor is defined as
Q=1/R(L/C),
where R is the active resistance of a resonant circuit, L is the inductance and C is the capacitance of this circuit.

(28) Tuning means can be either a variable inductance coil or a variable capacitance. A variable capacitance device can be a vibrating reed or a rotating condenser. In the example shown in FIGS. 1A and 1B, the tuning means is rotating condenser 28. Rotating condenser 28 comprises rotating blades 30 driven by a motor 31. During each quarter cycle of motor 31, as blades 30 mesh with blades 32, the capacitance of the resonant circuit that includes dees 10 and 12 and rotating condenser 28 increases and the resonant frequency decreases. The process reverses as the blades unmesh. Thus, resonant frequency is changed by changing the capacitance of the resonant circuit. This serves the purpose of reducing by a large factor the power required to generate the high voltage applied to the dees and necessary to accelerate the beam. The shape of blades 30 and 32 can be machined so as to create the required dependence of resonant frequency on time.

(29) The blade rotation can be synchronized with the RF frequency generation so that by varying the Q-factor of the RF cavity, the resonant frequency of the resonant circuit, defined by the cyclotron, is kept close to the frequency of the alternating voltage potential applied to dees 10 and 12.

(30) The rotation of the blades can be controlled by the digital waveform generator, described below with reference to FIG. 3 and FIG. 4, in a manner that maintains the resonant frequency of the resonant circuit close to the current frequency generated by the digital waveform generator. Alternatively, the digital waveform generator can be controlled by means of an angular position sensor (not shown) on the rotating condenser shaft 33 to control the clock frequency of the waveform generator to maintain the optimum resonant condition. This method can be employed if the profile of the meshing blades of the rotating condenser is precisely related to the angular position of the shaft.

(31) A sensor that detects the peak resonant condition (not shown) can also be employed to provide feedback to the clock of the digital waveform generator to maintain the highest match to the resonant frequency. The sensors for detecting resonant conditions can measure the oscillating voltage and current in the resonant circuit. In another example, the sensor can be a capacitance sensor. This method can accommodate small irregularities in the relationship between the profile of the meshing blades of the rotating condenser and the angular position of the shaft.

(32) A vacuum pumping system 40 maintains vacuum chamber 8 at a very low pressure so as not to scatter the accelerating beam.

(33) To achieve uniform acceleration in a synchrocyclotron, the frequency and the amplitude of the electric field across the dee gap needs to be varied to account for the relativistic mass increase and radial (measured as distance from the center of the spiral trajectory of the charged particles) variation of magnetic field as well as to maintain focus of the beam of particles.

(34) FIG. 2 is an illustration of an idealized waveform that may be required for accelerating charged particles in a synchrocyclotron. It shows only a few cycles of the waveform and does not necessarily represent the ideal frequency and amplitude modulation profiles. FIG. 2 illustrates the time varying amplitude and frequency properties of the waveform used in a given synchrocyclotron. The frequency changes from high to low as the relativistic mass of the particle increases while the particle speed approaches a significant fraction of the speed of light.

(35) The instant invention uses a set of high speed digital to analog converters (DAC) that can generate, from a high speed memory, the required signals on a nanosecond time scale. Referring to FIG. 1A, both a radio frequency (RF) signal that drives the voltage across dee gap 13 and signals that drive the voltage on injector electrode 20 and extractor electrode 22 can be generated from the memory by the DACs. The accelerator signal is a variable frequency and amplitude waveform. The injector and extractor signals can be either of at least three types: continuous; discrete signals, such as pulses, that may operate over one or more periods of the accelerator waveform in synchronism with the accelerator waveform; or discrete signals, such as pulses, that may operate at precisely timed instances during the accelerator waveform frequency sweep in synchronism with the accelerator waveform. (See below with reference to FIGS. 8A-C.)

(36) FIG. 3 depicts a block diagram of a synchrocyclotron of the present invention 300 that includes particle accelerator 302, waveform generator system 319 and amplifying system 330. FIG. 3 also shows an adaptive feedback system that includes optimizer 350. The optional variable condenser 28 and drive subsystem to motor 31 are not shown.

(37) Referring to FIG. 3, particle accelerator 302 is substantially similar to the one depicted in FIGS. 1A and 1B and includes dummy dee (grounded dee) 304, dee 306 and yoke 308, injection electrode 310, connected to ion source 312, and extraction electrodes 314. Beam monitor 316 monitors the intensity of beam 318.

(38) Synchrocyclotron 300 includes digital waveform generator 319. Digital waveform generator 319 comprises one or more digital-to-analog converters (DACs) 320 that convert digital representations of waveforms stored in memory 322 into analog signals. Controller 324 controls addressing of memory 322 to output the appropriate data and controls DACs 320 to which the data is applied at any point in time. Controller 324 also writes data to memory 322. Interface 326 provides a data link to an outside computer (not shown). Interface 326 can be a fiber optic interface.

(39) The clock signal that controls the timing of the analog-to-digital conversion process can be made available as an input to the digital waveform generator. This signal can be used in conjunction with a shaft position encoder (not shown) on the rotating condenser (see FIGS. 1A and 1B) or a resonant condition detector to fine-tune the frequency generated.

(40) FIG. 3 illustrates three DACs 320a, 320b and 320c. In this example, signals from DACs 320a and 320b are amplified by amplifiers 328a and 328b, respectively. The amplified signal from DAC 320a drives ion source 312 and/or injection electrode 310, while the amplified signal from DAC 320b drives extraction electrodes 314.

(41) The signal generated by DAC 320c is passed on to amplifying system 330, operated under the control of RF amplifier control system 332. In amplifying system 330, the signal from DAC 320c is applied by RF driver 334 to RF splitter 336, which sends the RF signal to be amplified by an RF power amplifier 338. In the example shown in FIG. 3, four power amplifiers, 338a, b, c and d, are used. Any number of amplifiers 338 can be used depending on the desired extent of amplification. The amplified signal, combined by RF combiner 340 and filtered by filter 342, exits amplifying system 330 though directional coupler 344, which ensures that RF waves do not reflect back into amplifying system 330. The power for operating amplifying system 330 is supplied by power supply 346.

(42) Upon exit from amplifying system 330, the signal from DAC 320c is passed on to particle accelerator 302 through matching network 348. Matching network 348 matches impedance of a load (particle accelerator 302) and a source (amplifying system 330). Matching network 348 includes a set of variable reactive elements.

(43) Synchrocyclotron 300 can further include optimizer 350. Using measurement of the intensity of beam 318 by beam monitor 316, optimizer 350, under the control of a programmable processor can adjust the waveforms produced by DACs 320a, b and c and their timing to optimize the operation of the synchrocyclotron 300 and achieve a optimum acceleration of the charged particles.

(44) The principles of operation of digital waveform generator 319 and adaptive feedback system 350 will now be discussed with reference to FIG. 4.

(45) The initial conditions for the waveforms can be calculated from physical principles that govern the motion of charged particles in magnetic field, from relativistic mechanics that describe the behavior of a charged particle mass as well as from the theoretical description of magnetic field as a function of radius in a vacuum chamber. These calculations are performed at step 402. The theoretical waveform of the voltage at the dee gap, RF(, t), where is the frequency of the electrical field across the dee gap and t is time, is computed based on the physical principles of a cyclotron, relativistic mechanics of a charged particle motion, and theoretical radial dependency of the magnetic field.

(46) Departures of practice from theory can be measured and the waveform can be corrected as the synchrocyclotron operates under these initial conditions. For example, as will be described below with reference to FIGS. 8A-C, the timing of the ion injector with respect to the accelerating waveform can be varied to maximize the capture of the injected particles into the accelerated bunch of particles.

(47) The timing of the accelerator waveform can be adjusted and optimized, as described below, on a cycle-by-cycle basis to correct for propagation delays present in the physical arrangement of the radio frequency wiring; asymmetry in the placement or manufacture of the dees can be corrected by placing the peak positive voltage closer in time to the subsequent peak negative voltage or vice versa, in effect creating an asymmetric sine wave.

(48) In general, waveform distortion due to characteristics of the hardware can be corrected by pre-distorting the theoretical waveform RF(, t) using a device-dependent transfer function A, thus resulting in the desired waveform appearing at the specific point on the acceleration electrode where the protons are in the acceleration cycle. Accordingly, and referring again to FIG. 4, at step 404, a transfer function A(, t) is computed based on experimentally measured response of the device to the input voltage.

(49) At step 405, a waveform that corresponds to an expression RF(, t)/A(,t) is computed and stored in memory 322. At step 406, digital waveform generator 319 generates RF /A waveform from memory. The driving signal RF(, t)/A(, t) is amplified at step 408, and the amplified signal is propagated through the entire device 300 at step 410 to generate a voltage across the dee gap at step 412. A more detailed description of a representative transfer function A(,t) will be given below with reference to FIGS. 6A-C.

(50) After the beam has reached the desired energy, a precisely timed voltage can be applied to an extraction electrode or device to create the desired beam trajectory in order to extract the beam from the accelerator, where it is measured by beam monitor at step 414a. RF voltage and frequency is measured by voltage sensors at step 414b. The information about beam intensity and RF frequency is relayed back to digital waveform generator 319, which can now adjust the shape of the signal RF(, t)/A(, t) at step 406.

(51) The entire process can be controlled at step 416 by optimizer 350. Optimizer 350 can execute a semi- or fully automatic algorithm designed to optimize the waveforms and the relative timing of the waveforms. Simulated annealing is an example of a class of optimization algorithms that may be employed. On-line diagnostic instruments can probe the beam at different stages of acceleration to provide feedback for the optimization algorithm. When the optimum conditions have been found, the memory holding the optimized waveforms can be fixed and backed up for continued stable operation for some period of time. This ability to adjust the exact waveform to the properties of the individual accelerator decreases the unit-to-unit variability in operation and can compensate for manufacturing tolerances and variation in the properties of the materials used in the construction of the cyclotron.

(52) The concept of the rotating condenser (such as condenser 28 shown in FIGS. 1A and 1B) can be integrated into this digital control scheme by measuring the voltage and current of the RF waveform in order to detect the peak of the resonant condition. The deviation from the resonant condition can be fed back to the digital waveform generator 319 (see FIG. 3) to adjust the frequency of the stored waveform to maintain the peak resonant condition throughout the accelerating cycle. The amplitude can still be accurately controlled while this method is employed.

(53) The structure of rotating condenser 28 (see FIGS. 1A and 1B) can optionally be integrated with a turbomolecular vacuum pump, such as vacuum pump 40 shown in FIGS. 1A and 1B, that provides vacuum pumping to the accelerator cavity. This integration would result in a highly integrated structure and cost savings. The motor and drive for the turbo pump can be provided with a feedback element such as a rotary encoder to provide fine control over the speed and angular position of rotating blades 30, and the control of the motor drive would be integrated with the waveform generator 319 control circuitry to insure proper synchronization of the accelerating waveform.

(54) As mentioned above, the timing of the waveform of the oscillating voltage input can be adjusted to correct for propagation delays that arise in the device. FIG. 5A illustrate an example of wave propagation errors due to the difference in distances R1 and R2 from the RF input point 504 to points 506 and 508, respectively, on the accelerating surface 502 of accelerating electrode 500. The difference in distances R1 and R2 results in signal propagation delay that affects the particles as they accelerate along a spiral path (not shown) centered at point 506. If the input waveform, represented by curve 510, does not take into account the extra propagation delay caused by the increasing distance, the particles can go out of synchronization with the accelerating waveform. The input waveform 510 at point 504 on the accelerating electrode 500 experiences a variable delay as the particles accelerate outward from the center at point 506. This delay results in input voltage having waveform 512 at point 506, but a differently timed waveform 514 at point 508. Waveform 514 shows a phase shift with respect to waveform 512 and this can affect the acceleration process. As the physical size of the accelerating structure (about 0.6 meters) is a significant fraction of the wavelength of the accelerating frequency (about 2 meters), a significant phase shift is experienced between different parts of the accelerating structure.

(55) In FIG. 5B, the input voltage having waveform 516 is pre-adjusted relative to the input voltage described by waveform 510 to have the same magnitude, but opposite sign of time delay. As a result, the phase lag caused by the different path lengths across the accelerating electrode 500 is corrected. The resulting waveforms 518 and 520 are now correctly aligned so as to increase the efficiency of the particle accelerating process. This example illustrates a simple case of propagation delay caused by one easily predictable geometric effect. There may be other waveform timing effects that are generated by the more complex geometry used in the actual accelerator, and these effects, if they can be predicted or measured can be compensated for by using the same principles illustrated in this example.

(56) As described above, the digital waveform generator produces an oscillating input voltage of the form RF(, t)/A(, t), where RF(, t) is a desired voltage across the dee gap and A(, t) is a transfer function. A representative device-specific transfer function A, is illustrated by curve 600 in FIG. 6A. Curve 600 shows Q-factor as a function of frequency. Curve 600 has two unwanted deviations from an ideal transfer function, namely troughs 602 and 604. These deviation can be caused by effects due to the physical length of components of the resonant circuit, unwanted self-resonant characteristics of the components or other effects. This transfer function can be measured and a compensating input voltage can be calculated and stored in the waveform generator's memory. A representation of this compensating function 610 is shown in FIG. 6B. When the compensated input voltage 610 is applied to device 300, the resulting voltage 620 is uniform with respect to the desired voltage profile calculated to give efficient acceleration.

(57) Another example of the type of effects that can be controlled with the programmable waveform generator is shown in FIG. 7. In some synchrocyclotrons, the electric field strength used for acceleration can be selected to be somewhat reduced as the particles accelerate outward along spiral path 705. This reduction in electric field strength is accomplished by applying accelerating voltage 700, that is kept relatively constant as shown in FIG. 7A, to accelerating electrode 702. Electrode 704 is usually at ground potential. The electric field strength in the gap is the applied voltage divided by the gap length. As shown in FIG. 7B, the distance between accelerating electrodes 702 and 704 is increasing with radius R. The resulting electric field strength as a function or radius R is shown as curve 706 in FIG. 7C.

(58) With the use of the programmable waveform generator, the amplitude of accelerating voltage 708 can be modulated in the desired fashion, as shown in FIG. 7D. This modulation allows to keep the distance between accelerating electrodes 710 and 712 to remain constant, as shown in FIG. 7E. As a result, the same resulting electric field strength as a function of radius 714, shown in FIG. 7F, is produced as shown in FIG. 7C. While this is a simple example of another type of control over synchrocyclotron system effects, the actual shape of the electrodes and profile of the accelerating voltage versus radius may not follow this simple example.

(59) As mentioned above, the programmable waveform generator can be used to control the ion injector (ion source) to achieve optimal acceleration of the charged particles by precisely timing particle injections. FIG. 8A shows the RF accelerating waveform generated by the programmable waveform generator. FIG. 8B shows a precisely timed cycle-by-cycle injector signal that can drive the ion source in a precise fashion to inject a small bunch of ions into the accelerator cavity at precisely controlled intervals in order to synchronize with the acceptance phase angle of the accelerating process. The signals are shown in approximately the correct alignment, as the bunches of particles are usually traveling through the accelerator at about a 30 degree lag angle compared to the RF electric field waveform for beam stability. The actual timing of the signals at some external point such as the output of the digital-to-analog converters, may not have this exact relationship as the propagation delays of the two signals is likely to be different. With the programmable waveform generator, the timing of the injection pulses can be continuously varied with respect to the RF waveform in order to optimize the coupling of the injected pulses into the accelerating process. This signal can be enabled or disabled to turn the beam on and off. The signal can also be modulated via pulse dropping techniques to maintain a required average beam current. This beam current regulation is accomplished by choosing a macroscopic time interval that contains some relatively large number of pulses, on the order of 1000,and changing the fraction of pulses that are enabled during this interval.

(60) FIG. 8C shows a longer injection control pulse that corresponds to a multiple number of RF cycles. This pulse is generated when a bunch of protons are to be accelerated. The periodic acceleration process captures only a limited number of particles that will be accelerated to the final energy and extracted. Controlling the timing of the ion injection can result in lower gas load and consequently better vacuum conditions which reduces vacuum pumping requirements and improves high voltage and beam loss properties during the acceleration cycle. This can be used where the precise timing of the injection shown in FIG. 8B is not required for acceptable coupling of the ion source to the RF waveform phase angle. This approach injects ions for a number of RF cycles which corresponds approximately to the number of turns which are accepted by the accelerating process in the synchrocyclotron. This signal is also enabled or disabled to turn the beam on and off or modulate the average beam current.

(61) While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.