A Fixed-Field Alternating Gradient accelerator (FFAG) is a circular particle accelerator concept on which development was started in the early 50s, and that can be characterized by its time-independent magnetic fields (fixed-field, like in a cyclotron) and the use of strong focusing (alternating gradient, like in a synchrotron).[1][2] Thus, FFAG accelerators combine the cyclotron's advantage of continuous, unpulsed operation, with the synchrotron's relatively inexpensive small magnet ring, of narrow bore.
Although the development of FFAGs had not been pursued for over a decade starting from 1967, it has gained interest since the mid-1980s for usage in neutron spallation sources, and as driver for muon colliders since the mid-1990s.[1]