Active return system

A particle accelerator comprising:

• a voltage source to provide a radio frequency (RF) voltage to a cavity to accelerate particles to produce a particle beam, the cavity having a first magnetic field for causing particles accelerated from the plasma column to move orbitally within the cavity, the RF voltage being controllable to vary in time as the particle beam increases in distance from the plasma column;
• a magnet to generate the first magnetic field in the cavity, the magnet comprising first superconducting coils to pass current in a first direction to thereby generate the first magnetic field;
• an active return system comprising second superconducting coils, each of the second superconducting coils surrounding, and being concentric with, a corresponding first superconducting coil, the second superconducting coils for passing current in a second direction that is opposite to the first direction to thereby generate a second magnetic field having a magnetic field of at least 2.5 Tesla (T), the second magnetic field having a polarity that is opposite to a polarity of the first magnetic field;
• and
• a single structure on which at least one first superconducting coil and corresponding second superconducting coil are mounted.