  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.
