Muon Capture, Phase Rotation, and Precooling in Pressurized RF Cavities

Bright muon beams are required for muon colliders, neutrino factories, and intense muon sources. To create the beams, high energy protons hit a target to generate pions that decay into a diffuse cloud of muons. The muon cloud is then: (1) captured in strong magnetic fields, (2) bunched, (3) phase-energy rotated by strong radio frequency (RF) electric fields, and (4) pre-cooled by passing the beam through a low-Z energy absorber. Because these four processes are done sequentially, the process is inefficient, requires extra length and expense, and suffers large muon losses. Pressurized RF cavities will enable higher gradient magnetic fields than is possible with evacuated cavities, thus allowing more options for the initial stages of a muon cooling channel. This project will develop techniques for using pressurized RF cavities for the simultaneous capture, phase rotation, bunching, and precooling processes. In Phase I, an experimental study of the RF breakdown of metals in dense gas was conducted. An RF cavity, pressurized with dense hydrogen gas, was operated for the first time in a strong magnetic field, demonstrating the feasibility of pressurized cavities for muon cooling applications. Simulations were used to demonstrate the combined cooling and bunching provided by the pressurized cavities. In Phase II, experiments will be conducted to systematically study the breakdown properties of gases and metals in beams of ionizing radiation and in strong magnetic fields. Simulations will be used to design the capture, phase rotation, and precooling systems of muon beams in pressurized cavities. Techniques will be developed for the creation of bunch structures appropriate for coalescing at high energy for muon collider applications. Commercial Applications and other Benefits as described by the awardee: The generation of intense muon beams with small emittances should benefit many branches of science, where such beams are needed for muon colliders, Higgs and neutrino factories, bright muon sources, and studies of muon-catalyzed fusion.