Hydrogen Filled RF Cavities for Muon Beam Cooling

Muon beams used in high energy physics research must be cooled as quickly as the short muon lifetime requires. This cooling is accomplished by ionization cooling, which requires low-Z energy absorbers immersed in a strong magnetic field and high-gradient, large-aperture RF cavities. However, RF cavities that operate in vacuum are vulnerable to dark-current-generated breakdown, which is exacerbated by strong magnetic fields; thus, extra safety windows are required to separate RF regions from hydrogen energy absorbers. These windows degrade the cooling. This project will develop RF cavities that are pressurized with dense hydrogen gas that will absorb the dark currents and allow the cavities to operate in strong magnetic fields. The same real estate will be used to provide the energy absorber and the RF acceleration needed for ionization cooling. To accomplish this development: (1) systematic measurements of the operation of a hydrogen-filled cavity will be made as a function of the external magnetic field and charged particle beam intensity; (2) a pressurized RF cavity will be designed, built, and tested; and (3) beam-induced gas breakdown will be studied by improving simulation models. Commercial Applications and other Benefits as described by the awardee: Commercial uses for bright muon beams include screening cargo containers for homeland security, low-dose radiography, and muon-catalyzed fusion. Scientific uses include low energy beams for rare process searches, muon spin resonance, muon beams for neutrino factories, and muon colliders such as Higgs factories or energy-frontier discovery machines.