Abstract
Processes occurring in a radio-frequency (rf) cavity, filled with high pressure gas and interacting with proton beams, have been studied via advanced numerical simulations. Simulations support the experimental program on the hydrogen gas-filled rf cavity in the Mucool Test Area (MTA) at Fermilab, and broader research on the design of muon cooling devices. SPACE, a 3D electromagnetic particle-in-cell (EM-PIC) code with atomic physics support, was used in simulation studies. Plasma dynamics in the rf cavity, including the process of neutral gas ionization by proton beams, plasma loading of the rf cavity, and atomic processes in plasma such as electron-ion and ion-ion recombination and electron attachment to dopant molecules, have been studied. Through comparison with experiments in the MTA, simulations quantified several uncertain values of plasma properties such as effective recombination rates and the attachment time of electrons to dopant molecules. Simulations have achieved very good agreement with experiments on plasma loading and related processes. The experimentally validated code SPACE is capable of predictive simulations of muon cooling devices.
Highlights
Using muons is an attractive choice for realizing a multiTeV lepton collider and producing a well-defined intense neutrino beam for neutrino experiments
In the rf cavity filled with pure hydrogen gas, ionization and electron-ion recombination are the main processes in the plasma
The vacuum gap between the computational domain boundary and the rf cavity wall is used to eliminate the effect of the numerical boundary condition
Summary
Using muons is an attractive choice for realizing a multiTeV lepton collider and producing a well-defined intense neutrino beam for neutrino experiments. Because muons are tertiary particles in the production process, the phase space volume of a muon beam needs to be shrunken to fit into the accelerator optics. Ionization cooling is a viable method to quickly cool down the beam temperature within the muon lifetime [1]. Muons propagate through an ionization material with strong magnetic focusing and lose their kinetic energy via ionization processes. The lost energy is immediately and adiabatically recovered by rf cavities. Better cooling efficiency is obtained with higher rf gradient. The achievable rf gradient is limited by the presence of a static magnetic field in a vacuum cavity
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