Suppressing multipacting in a 56 MHz quarter wave resonator

Abstract

We propose using a beam excited 56 MHz radio frequency (rf) niobium quarter wave resonator (QWR) to enhance the luminosity and bunching of the RHIC's (relativistic heavy ion collider's) beam. From experience with similar structures, multipacting is expected but is to be avoided in a storage ring; therefore, we undertook extensive simulations with the Multipac 2.1 2D code. They revealed that multipacting occurs in various bands up to a peak surface electric field of $50\text{ }\text{ }\mathrm{kV}/\mathrm{m}$, and is concentrated mostly above the beam gap and on the outer conductor. To resolve this issue, we introduced a ripple structure into the outer conductor and successfully eliminated multipacting.