Simulation of beam-induced plasma in gas-filled rf cavities

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.

Figure 1

Normalized $E_z$ distributions in HPRF cavity along the axis (blue dashed line) and at radius $r=2.29\mathrm{mm}$ (green dotted line), and its analytic fit in space code (solid red line).

Figure 2

Schematic diagram of the computational domain of the HPRF simulations.

Figure 3

Comparison of simulated and experimental values of $dw$ for HPRF cavity filled with pure hydrogen gas at 100 atm. Simulation $dw$ values represent average values over all macroparticles and averaged in time over one rf cycle.

Figure 4

Comparison of simulated values of the magnitude of the external electric field with experimental values including error estimates in the HPRF cavity filled with pure hydrogen gas at 100 atm.

Figure 5

Comparison of values of power, averaged in time over one rf cycle, in the HPRF cavity filled with pure hydrogen gas at 100 atm obtained from experiments and simulations using formula (11).

Figure 6

Simulated values of number of electrons in the HPRF cavity filled with pure hydrogen gas at 100 atm.

Figure 7

Charges in the HPRF cavity filled with 20.4 atm hydrogen gas and 1% dry air dopant. (a) Evolution of the total number of electrons in the cavity, (b), (c), and (d) show distribution of electrons, hydrogen ions, and dopant ions, respectively, in the center of the HPRF cavity.

Figure 8

Comparison of simulations and experiments for HPRF cavity filled with 20.4 atm hydrogen gas with 1% dry air dopant.

Figure 9

Schematic diagram of the ionization algorithm.

Figure 10

Schematic diagram illustrating ionization algorithm with variable representing number of plasma macroparticles.