First Experimental Characterization of Plasma Parameters and Carbon Decontamination Rates in a Microwave Resonator Used in Particle Accelerators

TL;DR

This study provides the first direct measurements of plasma parameters and carbon decontamination rates in a microwave resonator used for particle accelerators, offering insights to optimize plasma cleaning of SRF cavities.

Contribution

It presents the first experimental characterization of plasma parameters and cleaning rates in a microwave resonator for SRF cavity decontamination.

Findings

Plasma parameters vary significantly with experimental conditions.

Effective cleaning rates depend on plasma parameters and processing conditions.

Guidelines for improving plasma processing in SRF cavities are proposed.

Abstract

In-situ plasma processing of superconducting radio frequency (SRF) cavities is a performance recovery technique used to mitigate the field emission limiting phenomenon. It has been proved very effective at major particle accelerator facilities such as SNS, CEBAF, FRIB, FNAL and C-ADS. This technique is based on the ignition of a noble-gas/oxygen plasma inside the cavity over several hours to remove hydrocarbon-based contamination, responsible for the parasitic field emission degradation observed after several years of operation. Despite a large experimental R\&D effort from the community, plasma parameters and cleaning rates under various experimental conditions have never been directly evaluated. In this study, plasma parameters were measured using a Langmuir probe and cleaning rates thanks to a quartz crystal microbalance (QCM) coated with an amorphous carbon film to simulate a carbon-based contamination. In this article, the main results from a large parameter space are discussed along with guidelines for improving the plasma processing effectiveness in SRF cavities. The encountered technical challenges are also discussed, as the SRF cavity is by design not intended to be a plasma reactor.