Design study of compact medical fixed-field alternating-gradient accelerators

We have studied the various conditions and limitations for achieving compact fixed-field alternating-gradient (FFAG) accelerators to be widely used in heavy-ion cancer therapy. For the case of a normal-conducting FFAG accelerator, our linear calculation indicates 12-cell radial sectors with a field index of 10.5 as a suitable configuration. We found that its ring circumference can be as small as 70 m and that triple-cascade rings are needed to accelerate a carbon beam from 40 keV/u to 400 MeV/u. In this paper, we report a systematic analysis based on a linear optical model, a comparison of various types of FFAG, and a design example with some technical concerns. An important result is that viable radial-sector designs are possible with circumference factor C significantly lower than the value 4.45 previously quoted.