Fixed-field alternating gradient accelerator

A Fixed-Field Alternating Gradient accelerator (FFAG) is a type of circular particle accelerator being developed for potential applications in physics, medicine, national security, and energy production, that has features of cyclotrons and synchrotrons. FFAG accelerators combine the cyclotron's advantage of continuous, unpulsed operation, with the synchrotron's relatively inexpensive small magnet ring, of narrow bore.

This is achieved by using magnets with strong focusing alternating-gradient quadrupole fields to confine the beam, accompanied by a dipole bending magnetic field which bends the beam to close the orbital ring. By the use of a strong radial magnetic field gradient in the dipole component, yet with a time-constant "fixed field" as the particles are accelerated, particles with larger energies move successively to slightly larger orbits, where the bending field is larger. The beam thus remains confined to a narrow ring, as in a synchrotron, yet without the synchrotron's requirement that the machine be operated in pulsed acceleration cycles.

Such machines have potential medical applications in proton therapy for cancer, for non-invasive security inspections of closed cargo containers, for the rapid acceleration of muons to high energies before they have time to decay, and as "energy amplifiers", for Accelerator-Driven Sub-critical Reactors (ADSRs) in which a neutron beam derived from a FFAG drives a slightly sub-critical fission reactor. Such ADSRs would be inherently safe, having no danger of accidental exponential runaway, and relatively little production of transuranium waste, with its long life and potential for nuclear weapons proliferation.