RF R&D for Muon Ionization Cooling Channels

Large emittance muon beam must be reduced or cooled both longitudinally and transversely before it can be accelerated in an accelerator complex for a neutrino factory or a muon collider. In practice, ionization cooling is considered to be one of the most promising transverse cooling schemes for muon beams. When traversing an ionization cooling channel, which consists of absorbers and low frequency (~ 201-MHz) RF cavities and superconducting solenoidal magnets, a muon beam loses its energy in the absorbers first and then re-gains the lost longitudinal energy from the RF cavities, therefore results in a net transverse emittance reduction. In principle, this cooling process can be repeated over many times depending on the cooling requirement. Moreover, muons have finite lifetime (~ 2 micro-second at rest), manipulation of the muon beam, such as the transverse cooling, has to be fast. This requires the normal conducting low frequency RF cavities have the highest possible accelerating gradient. This paper reports the status and progress of RF cavity R&D for muon ionization cooling channels with emphasis on RF cavity conditioning and operation in a strong magnetic field. The research work is supported and conducted in MUCOOL collaboration under the US NFMCC (Neutrino Factory and Muon Collider Collaboration).