Adjusting energy of a particle beam

1. A particle accelerator comprising:

a coil to provide a magnetic field to a cavity;

a particle source to provide a plasma column to the cavity;

a voltage source to provide a radio frequency (RF) voltage to the cavity to accelerate particles from the plasma column, the magnetic field causing particles accelerated from the plasma column to move orbitally within the cavity;

an enclosure containing an extraction channel to receive the particles accelerated from the plasma column at an entry point and to output the particles from the cavity at an exit point; and

a structure arranged proximate to the entry point of the extraction channel to change an energy level of the particles.

2. The particle accelerator of claim 1 , wherein the structure has multiple thicknesses; and

wherein the structure is movable relative to the extraction channel to place one of the multiple thicknesses in a path of the particles.

3. The particle accelerator of claim 2 , wherein the structure is wheel-shaped and is rotatable within the extraction channel.

4. The particle accelerator of claim 2 , wherein the structure has variable thickness ranging from a maximum thickness to a minimum thickness.

5. The particle accelerator of claim 1 , wherein the particle accelerator is rotatable relative to a fixed position; and

wherein the particle accelerator further comprises a control system to control movement of the structure based on a rotational position of the particle accelerator.

6. The particle accelerator of claim 1 , further comprising:

a regenerator to adjust the magnetic field within the cavity to thereby change successive orbits of the particles accelerated from the plasma column so that, eventually, the particles output to the extraction channel.

7. The particle accelerator of claim 1 , wherein the structure comprises at least one of the following materials: beryllium, carbon and plastic.

8. A proton therapy system comprising:

the particle accelerator of claim 1 , wherein the particles comprise protons; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein protons are output essentially directly from the particle accelerator to the patient position.

9. A particle accelerator comprising:

a coil to provide a magnetic field to a cavity;

a particle source to provide a plasma column to the cavity;

a voltage source to provide a radio frequency (RF) voltage to the cavity to accelerate particles from the plasma column, the magnetic field causing particles accelerated from the plasma column to move orbitally within the cavity;

an enclosure containing an extraction channel to receive the particles accelerated from the plasma column and to output the particles from the cavity; and

a regenerator to adjust the magnetic field within the cavity to thereby change successive orbits of the particles accelerated from the plasma column so that, eventually, the particles output to the extraction channel;

wherein the enclosure comprises magnetic structures, at least one of the magnetic structures having a slot therein, the slot containing a magnetic shim that is ferromagnetic and movable within the slot, the magnetic shim being movable relative to the regenerator to affect an amount by which the regenerator adjusts the magnetic field.

10. The particle accelerator of claim 9 , wherein the at least one of the magnetic structures has multiple slots therein, each slot containing a magnetic shim that is ferromagnetic and movable within the slot, each magnetic shim being movable relative to the regenerator to affect an amount by which the regenerator adjusts the magnetic field.

11. The particle accelerator of claim 9 , wherein the particle accelerator is rotatable relative to a fixed position; and

wherein the particle accelerator further comprises a control system to generate a control signal to control movement of the magnetic shim based on a rotational position of the particle accelerator.

12. The particle accelerator of claim 9 , wherein the magnetic shim comprises an electromagnet.

13. A proton therapy system comprising:

the particle accelerator of claim 9 , wherein the particles comprise protons; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein protons are output essentially directly from the particle accelerator to the patient position.

14. A particle accelerator comprising:

a cryostat comprising a superconducting coil, the superconducting coil conducting a current that generates a magnetic field;

magnetic structures adjacent to the cryostat, the cryostat being attached to the magnetic structures, the magnetic structures containing a cavity;

a particle source to provide a plasma column to the cavity;

a voltage source to provide a radio frequency (RF) voltage to the cavity to accelerate particles from the plasma column, the magnetic field causing particles accelerated from the plasma column to move orbitally within the cavity;

an extraction channel to receive the particles accelerated from the plasma column and to output the particles from the cavity; and

an actuator that is controllable to move the cryostat relative to the magnetic structures.

15. The particle accelerator of claim 14 , wherein the particle accelerator is rotatable relative to a fixed position; and

wherein the particle accelerator further comprises a control system to generate a control signal to control the actuator based on a rotational position of the particle accelerator.

16. The particle accelerator of claim 15 , wherein the actuator is controlled to control movement of the cryostat so as to compensate for effects of gravity on the superconducting coil.

17. A proton therapy system comprising:

the particle accelerator of claim 14 , wherein the particles comprise protons; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein protons are output essentially directly from the particle accelerator to the patient position.

18. A variable-energy synchrocyclotron comprising:

magnetic structures defining a cavity in which particles are accelerated for output as a particle beam that has a selected energy from among a range of energies;

an extraction channel to receive the particle beam; and

a structure proximate to an entry of the extraction channel to intercept the particle beam prior to the particle beam entering the extraction channel, the structure being movable based on the selected energy, the structure to absorb at least some energy of the particle beam prior to the particle beam entering the extraction channel.

19. The variable-energy synchrocyclotron of claim 18 , wherein the structure comprises a wheel having varying thickness, where different thicknesses are capable of absorbing different amounts of energy.

20. The variable-energy synchrocyclotron of claim 18 , further comprising:

a magnetic regenerator to implement a magnetic field bump at a particle orbit that corresponds to the selected energy;

wherein the magnetic regenerator is movable based on movement of the variable- energy particle accelerator.

21. The variable-energy synchrocyclotron of claim 18 , further comprising:

a magnetic regenerator to implement a magnetic field bump at a particle orbit that corresponds to the selected energy;

wherein the magnetic regenerator is movable to intercept a particle orbit having the selected energy.