Focusing a particle beam using magnetic field flutter

1. A particle accelerator comprising:

a voltage source to sweep a radio frequency (RF) voltage in a cavity to accelerate particles from a plasma column, the cavity having a magnetic field causing particles accelerated from the plasma column to move orbitally within the cavity, the magnetic field having flux that bows at edges of the cavity;

a regenerator to provide a magnetic field bump within the cavity to thereby change successive orbits of the particles accelerated from the plasma column so that, eventually, particles output to an extraction point, the regenerator being located at a radius in the cavity relative to the plasma column; and

ferromagnetic arrangements located in the cavity proximate to the radius, each ferromagnetic arrangement providing a magnetic field bump, wherein at least two ferromagnetic arrangements are on radially adjacent sides of the regenerator, are separated from the regenerator by a space, and are configured to at least partly restore vertical focusing of particles in orbit disrupted by the regenerator.

2. The particle accelerator of claim 1 , wherein a combination of the space, the regenerator and the ferromagnetic arrangements produce magnetic field fluctuations having a component at an angle to a particle orbit to counteract at least part of a reduction in vertical focusing of particles in orbit caused by the magnetic field bump.

3. The particle accelerator of claim 1 , wherein the magnetic field is at least 4 Tesla and the magnetic field bump produced by the regenerator is at most 2 Tesla.

4. The particle accelerator of claim 1 , wherein there are only two ferromagnetic arrangements, one on each side of the regenerator.

5. The particle accelerator of claim 1 , wherein there are more than two ferromagnetic arrangements in a same radius relative to the plasma column.

6. The particle accelerator of claim 1 , wherein there are multiple ferromagnetic arrangements in each one of multiple radii relative to the plasma column.

7. The particle accelerator of claim 1 , wherein the particle accelerator is a synchrocyclotron.

8. A proton therapy system comprising:

the particle accelerator of claim 1 ; and

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

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

9. The proton therapy system of claim 8 , wherein the particle accelerator comprises a synchrocyclotron.

10. The proton therapy system of claim 9 , wherein the magnetic field is 4 Tesla or greater and wherein the synchrocyclotron is a variable-energy particle accelerator.

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

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

12. The particle accelerator of claim 1 , wherein the ferromagnetic arrangements produce magnetic field flutter, which comprises an azimuthally averaged fractional normalized magnetic field variance that is between 0.25% and 20%.

13. A particle accelerator comprising:

a voltage source to sweep a radio frequency (RF) voltage in a cavity to accelerate particles from a plasma column, the cavity having a magnetic field causing particles accelerated from the plasma column to move orbitally within the cavity, the magnetic field having flux that is substantially straight throughout the cavity;

wherein the cavity comprises alternating regions of first magnetic field and second magnetic field, the first magnetic field being different from the second magnetic field to produce magnetic fluctuations having a component at an angle that is substantially perpendicular to particle orbits within the cavity.

14. The particle accelerator of claim 13 , wherein the alternating regions have different levels of ferromagnetic material.

15. The particle accelerator of claim 13 , wherein the alternating regions comprise regions of ferromagnetic material and regions absent ferromagnetic material.

16. The particle accelerator of claim 13 , wherein the alternating regions are pie-shaped, each with a point at the plasma column and an edge at an edge of the cavity.

17. The particle accelerator of claim 13 , wherein the alternating regions have pinwheel shapes, each with a point at the plasma column and an edge at an edge of the cavity.

18. The particle accelerator of claim 13 , wherein the cavity is defined by magnetic structures; and

wherein the alternating regions comprise vertically aligned alternating regions, one of which is connected to a first magnetic structure and another of which is connected to a second magnetic structure.

19. The particle accelerator of claim 13 , wherein the magnetic fluctuations vertically focus particles in orbits in the cavity.

20. The particle accelerator of claim 13 , wherein the particle accelerator is a synchrocyclotron.

21. A proton therapy system comprising:

the particle accelerator of claim 13 ; and

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

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

22. The proton therapy system of claim 21 , wherein the particle accelerator comprises a synchrocyclotron.

23. The proton therapy system of claim 22 , wherein the magnetic field is 4 Tesla or greater and wherein the synchrocyclotron is a variable-energy particle accelerator.

24. The particle accelerator of claim 14 , further comprising:

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

25. The particle accelerator of claim 13 , wherein the alternating regions of first magnetic field and second magnetic field comprise magnetic field flutter, which comprises an azimuthally averaged fractional normalized magnetic field variance that is between 0.25% and 20%.