S-band high-gradient low β single-periodic magnetically coupled standing-wave accelerating structure for a proton therapy linac

Abstract

S-band high-gradient accelerating structures were proposed to accelerate protons from 30 to 230 MeV for a compact proton therapy linac in Institute of Modern Physics. A backward traveling wave structure and a single-periodic magnetically coupled standing wave structure were designed, and differences between these two structures were analyzed to select a more suitable one for the proton therapy linac. In this paper, a novel single-periodic magnetically coupled standing-wave accelerating structure was studied, with an optimized nose cone that related to a smaller peak surface electric field and higher shunt impedance, with the reduced number of coupling holes and mechanical von Mises stress thus making it possible for higher duty factor and longer rf pulse operation, also with larger coupling holes and improved cell-to-cell coupling thus making this structure suitable for operating in π mode. An 11-cell high-power prototype for β=0.26 particles was developed with an active length of 143 mm. This prototype cavity is supposed to operate at an accelerating gradient of 30 MV/m with 0.1% duty factor, corresponding to a maximum surface electric field of 105 MV/m and a peak rf power of 3.43 MW. Cavity design, optimization, manufacture, rf measurement, and high-power test will be discussed in this paper.