Effects of Field Distortions in IH-APF Linac for a Compact Medical Accelerator

The project on developing compact medical accelerators for the tumor therapy using carbon ions has been started at the National Institute of Radiological Sciences (NIRS). Alternating-phase-focused (APF) linac using an interdigital H-mode (IH) cavity has been proposed for the injector linac. The IH-cavity is a doubly ridged circular resonator loaded by the drift-tubes mounted on ridges with supporting stems. The effects of intrinsic and random field distortions in a practical design of the 4-Mev/u 200-MHz IH-APF linac are considered. The intrinsic field distortions in the IH-cavity are caused by an asymmetry of the gap fields due to presence of the stems and pair of ridges. The random field distortions are caused by drift-tube misalignments and non-regular deviations of the gap voltages from programmed values. The RF fields in the IH-cavity have been calculated using Microwave Studio (MWS) code. The effects of field distortions on beam dynamics have been simulated numerically. The intrinsic field distortions are negligible for this IH-APF linac. The random field distortions cause a serious degradation of the beam transmission. The levels of permissible errors for the drift-tube longitudinal and transversal displacements are about ±200 µm and ±100 µm, respectively. The fluctuations of the gap voltages should be minimized to the levels of about ±3%. At this level of tolerances the beam transmission is decreased about 10 % (standard-deviation value) comparing to the beam transmission for the ideal structure. A possible way to relax the level of permissible errors is to shorten the IH-APF structure by increasing the transition energy between RFQ and IH-APF linacs. A modified linac layout is proposed and discussed.