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Topic

Development of In-Situ Plasma Processing on 1.3 GHz Superconducting Radiofrequency Cavities at TRIUMF

Department of Physics and Astronomy

Date & location

• Tuesday, April 22, 2025
• 1:00 P.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Tobias Junginger, Department of Physics and Astronomy, University of Victoria (Co-Supervisor)
• Mr. Robert Laxdal, Department of Physics and Astronomy, UVic (Co-Supervisor)

External Examiner

• Dr. David Longuevergne, Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJCLab)

Chair of Oral Examination

• Dr. Timothy Iles, Department of Pacific and Asian Studies, UVic

Abstract

Superconducting Radio Frequency (SRF) technology is a key component in many particle accelerators operating in a continuous wave, or high duty cycle, mode. The on-line performance of SRF cavities, as defined by the accelerating gradient and the unloaded quality factor, Q₀, is negatively impacted by the gradual increase of particulate contamination, furthering field emission. Conventional cleaning procedures are both time- and resource-exhaustive as they are done ex-situ. Plasma processing is an emerging in-situ method of cleaning which chemically removes hydrocarbon-based field emitters by the ignition of a plasma in the cavity volume. An R&D program is underway at TRIUMF with the goal of developing fundamental power coupler (FPC) driven plasma processing of the installed 1.3 GHz nine-cell cavities in the ARIEL 30MeV SRF eLINAC.

Processing recipes have been systematically studied in one single-cell and two multicell cavities off-line. Cavities were first artificially contaminated using a Helium-Methane plasma. In most of the tests, the removal of hydrocarbons was verified through the byproduct responses on a Residual Gas Analyzer (RGA). A plasma recipe with a cavity pressure of 80 mTorr, and a gas ratio of 95% Helium to 5% Oxygen was found to remove the largest abundances of hydrocarbon byproducts from each of the tested cavities. Cavity performance changes were tested cryogenically before and after conditioning with this particular recipe. These experiments were unable to recover the cavity performance but did provide insight toward the plasma processing testing procedure and apparatus needed for assembled cavities.

Multi-cell testing was also conducted to identify plasma locations for the various modes in the fundamental TM₀₁₀ passband. Here, a predictive model was developed to compare frequency shift data resulting from a plasma ignition with field behavior collected from beadpull distributions through a least-squares minimization. The results presented show the estimated plasma locations and movements due to power increases.