Pressurized rf cavities in ionizing beams

TL;DR

This study investigates the performance of high pressure gas-filled RF cavities under intense particle beams, focusing on plasma effects and gas interactions, crucial for muon collider and Higgs factory development.

Contribution

It provides experimental data on plasma evolution, electron-ion recombination, and gas interactions in RF cavities under various conditions, validating this technology for particle accelerators.

Findings

Gas type influences plasma behavior and energy dissipation.

Dopants like SF6 and O2 aid in plasma electron removal.

Recombination rates vary with gas pressure and electric field.

Abstract

A muon collider or Higgs factory requires significant reduction of the six dimensional emittance of the beam prior to acceleration. One method to accomplish this involves building a cooling channel using high pressure gas filled radio frequency cavities. The performance of such a cavity when subjected to an intense particle beam must be investigated before this technology can be validated. To this end, a high pressure gas filled radio frequency (rf) test cell was built and placed in a 400 MeV beam line from the Fermilab linac to study the plasma evolution and its effect on the cavity. Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally, sulfur hexafluoride and dry air were used as dopants to aid in the removal of plasma electrons. Measurements were made using a variety of beam intensities, gas pressures, dopant concentrations, and cavity rf electric fields, both with and without a 3 T external solenoidal magnetic field. Energy dissipation per electron-ion pair, electron-ion recombination rates, ion-ion recombination rates, and electron attachment times to $SF_6$ and $O_2$ were measured.