Muon Acceleration in FFAG Rings

Muon acceleration from 6 or 10 GeV to 20 GeV in fixed-field alternating gradient (FFAG) rings is considered in this paper. The novel physics issues associated with non-scaling FFAG machines, namely the very acceleration process and the crossing of transverse imperfection resonances are addressed. These FFAG machines are essentially strong-focusing rings with a dispersion small enough to keep muons over the full momentum range inside the same magnet aperture. We consider two cases: (i) A ring which we call Modified FODO with straight sections, just long enough - about a metre - to house normal-conducting RF cavities operating at about 200 MHz and (ii) A ring which we call Doublet Lattice with longer straight sections, long enough - a few metres - to house super-conducting RF cavities also at about 200 MHz (and with enough space for decay of the magnetic fields in neighbouring components to a level of about 10 mGauss). Parameters are derived for the lattice and RF system of an electron model. It accelerates electrons from about 10 to 20 MeV and allows one to study the critical issues of non-scaling FFAGs. For both a full machine and a model, lattice parameters such as magnet type and dimension, spacing, and number of cells are presented. The consequences of misaligned magnets are studied by simulation. Practical RF system design issues, e.g. RF power, and the limit on the bunch population due to beam loading are estimated.