In Situ Plasma Processing of Superconducting Cavities at Jefferson Lab

TL;DR

Jefferson Lab developed an in situ plasma processing technique using argon-oxygen gas to clean superconducting cavities, aiming to enhance their performance by reducing hydrocarbons and secondary emission.

Contribution

This paper introduces a novel in situ plasma processing method for superconducting cavities at Jefferson Lab, focusing on system design and initial results.

Findings

Hydrocarbon removal improves cavity surface quality.

Post-processing results show increased operational gradients.

Method reduces secondary emission coefficients.

Abstract

Jefferson Lab began a plasma processing program start-ing in the spring of 2019. Plasma processing is a common technique for removing hydrocarbons from surfaces, which increases the work function and reduces the sec-ondary emission coefficient. Unlike helium processing which relies on ion bombardment of the field emitters, plasma processing uses free oxygen produced in the plasma to break down the hydrocarbons on the surface of the cavity. The residuals of the hydrocarbons in the form of water, carbon monoxide and carbon dioxide are re-moved from the cryomodule as part of the process gas flow. The initial focus of the effort is processing C100 cavities by injecting RF power into the HOM coupler ports. We will then start investigating processing of C50 cavities by introducing RF into the fundamental power coupler. The plan is to start processing cryomodules in the CEBAF tunnel in the mid-term future, with a goal of improving the operational gradients and the energy mar-gin of the linacs. This work will describe the systems and methods used at JLAB for processing cavities using an argon oxygen gas mixture. Before and after plasma pro-cessing results will also be presented.