Operation of normal-conducting rf cavities in multi-Tesla magnetic fields for muon ionization cooling: A feasibility demonstration

Abstract

Ionization cooling is the preferred method for producing bright muon beams. This cooling technique requires the operation of normal conducting, radio-frequency (rf) accelerating cavities within the multitesla fields of dc solenoid magnets. Under these conditions, cavities exhibit increased susceptibility to rf breakdown, which can damage cooling channel components and imposes limits on channel length and transmission efficiency. We report, for the first time, stable high-vacuum, normal-conducting cavity operation at gradients of 50 MV/m in an external magnetic field of three tesla, through the use of beryllium cavity elements. This eliminates a significant technical risk that has previously been inherent in ionization cooling channel designs.