1.1 FFAG Accelerators and Storage Rings

Fixed Field Alternating Gradient accelerators (FFAGs) offer higher pulse repetition rates and acceptances than synchrotrons – and therefore much higher beam intensities. With a fixed magnetic field the rep rate is set by purely rf considerations, and can be several kHz, while the radially extended spiral orbit and high momentum compaction lead to large radial and momentum acceptances – at the cost of somewhat wider vacuum chambers, magnets and rf cavities. With fixed magnetic fields, modulated rf, and pulsed beams, FFAGs operate just like synchrocyclotrons, but are capable of reaching tens of GeV. The innovations were to break the magnet into radial or spiral sectors to provide edge and strong focusing, and (usually) to remove the central region – the same steps that convert a classic Lawrence cyclotron into a separated-sector isochronous ring cyclotron. The FFAG is the most general type of fixed-field accelerator (cyclotron).