Focusing a particle beam

1. A particle accelerator comprising:

a resonant cavity in which particles are accelerated, the resonant cavity having a background magnetic field having a first shape, the background magnetic field being at least 6 Tesla; and

an extraction channel for receiving particles output from the resonant cavity, the extraction channel comprising a series of focusing regions to focus a beam of received particles, wherein at least one of the focusing regions is a focusing element configured to alter a shape of the background magnetic field to a second shape that is substantially opposite to the first shape in the presence of a magnetic field gradient resulting from reduction of the background magnetic field from the resonant cavity to the extraction channel.

2. The particle accelerator of claim 1 , wherein the focusing element comprises:

ferromagnetic quadrupoles, each ferromagnetic quadrupole having a cross-sectional shape of a substantially right-angled trapezoid having a canted surface, the ferromagnetic quadrupoles being oppositely arranged one above the other such that canted surfaces of the ferromagnetic quadrupoles partially face each other; and

a magnetic field subtractor, the magnetic field subtractor being horizontally aligned next to the ferromagnetic quadrupoles such that the canted surfaces of the ferromagnetic quadrupoles diagonally face a broad surface of the magnetic field subtractor.

3. The particle accelerator of claim 2 , wherein the magnetic field subtractor is a rectangular plate, the magnetic field subtractor being configured to draw in surrounding magnetic field flux to help the ferromagnetic quadrupoles alter the background magnetic field to the second shape.

4. The particle accelerator of claim 1 , wherein the extraction channel comprises one or more field increasing elements, the one or more field increasing element comprising two magnetic field adders; and

wherein the magnetic field adders comprise rectangular plates, the magnetic field adders being vertically aligned one above the other and parallel to each other such that a broad surface of one magnetic field adder faces a broad surface of the other magnetic field adder.

5. The particle accelerator of claim 1 , wherein the extraction channel comprises one or more field decreasing elements, the one or more field decreasing element comprising magnetic field subtractors; and

wherein the magnetic field subtractors comprise rectangular plates, the magnetic field subtractors being horizontally aligned next to and parallel to each other such that a broad surface of one magnetic field subtractor faces a broad surface of the other magnetic field subtractor.

6. The particle accelerator of claim 1 , wherein the focusing regions are arranged so that each focusing region compresses the beam of received particles by about ⅙ in an axial or radial plane.

7. The particle accelerator of claim 1 , wherein at least one of the focusing regions is a focusing space, the focusing space having a magnetic field having a shape that is substantially similar to the first shape of the background magnetic field.

8. The particle accelerator of claim 7 , wherein the series of focusing regions are arranged so that focusing elements and focusing spaces alternate.

9. 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.

10. The particle accelerator of claim 1 , wherein the particle accelerator is a variable-energy particle accelerator and the particles that are output from the resonant cavity can have an energy within a range of energies; and

wherein the focusing element is configured to provide focusing within the range of energies.

11. The particle accelerator of claim 1 , wherein the particle accelerator is a variable-energy particle accelerator and the particle that are output from the resonant cavity can have an energy within a range of energies; and

wherein the focusing element is configured to provide focusing that is specific to the range of energies.

12. The particle accelerator of claim 11 , further comprising one or more magnetic shims that are movable relative to the focusing element based on the energy of the particles received from the resonant cavity.

13. The particle accelerator of claim 11 , wherein the focusing element comprises one or more coils, the one or more coils passing current that is based on the energy of the particles received from the resonant cavity.

14. A particle accelerator comprising:

a coil to provide a magnetic field to a resonant cavity such that the resonant cavity has a background magnetic field having a first shape, the background magnetic field being at least 6 Tesla;

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

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

an enclosure containing an extraction channel for receiving the particles output from the resonant cavity, the extraction channel comprising a series of focusing regions to focus a beam of received particles, wherein at least one of the focusing regions is a focusing element configured to alter a shape of the background magnetic field to a second shape that is substantially opposite to the first shape in the presence of a magnetic field gradient resulting from reduction of the background magnetic field from the resonant cavity to the extraction channel,

wherein the focusing element comprises ferromagnetic quadrupoles, each ferromagnetic quadrupole having a cross-sectional shape of a substantially right-angled trapezoid having a canted surface, the ferromagnetic quadrupoles being oppositely arranged one above the other such that canted surfaces of the ferromagnetic quadrupoles at least partially face each other.

15. The particle accelerator of claim 10 , further comprising:

a magnetic field subtractor, the magnetic field subtractor being horizontally aligned next to the ferromagnetic quadrupoles such that canted surfaces of the ferromagnetic quadrupoles diagonally face a broad surface of the magnetic field subtractor.

16. A particle accelerator comprising:

a resonant cavity in which particles are accelerated, the resonant cavity having a background magnetic field having a first shape, the background magnetic field being at least 4 Tesla; and

an extraction channel for receiving particles output from the resonant cavity, the extraction channel comprising a series of focusing regions to focus a beam of received particles, wherein at least one of the focusing regions is a focusing element configured to alter a shape of the background magnetic field to a second shape that is substantially opposite to the first shape in the presence of a magnetic field gradient resulting from reduction of the background magnetic field from the resonant cavity to the extraction channel.