A temperature-mapping system for multi-cell SRF accelerating cavities

TL;DR

This paper presents a high-resolution temperature mapping system with nearly two thousand thermometers for multi-cell SRF cavities, enabling detailed analysis of surface losses and quality factors in superconducting accelerators.

Contribution

A novel multi-cell T-mapping system with multiplexing and high resolution was developed and tested on a 9-cell SRF cavity, providing new insights into surface losses and cavity performance.

Findings

Most surface losses occur in the first cell at high gradients.

Hotspots degrade cavity Q0 by approximately 1.65 times.

Hotspots account for about 40% of total power loss.

Abstract

A Temperature mapping (T-map) system for Superconducting Radio Frequency (SRF) cavities consists of a thermometer array positioned precisely on an exterior cavity wall, capable of detecting small increases in temperature; therefore it is a powerful tool for research on the quality factor (Q0) of SRF cavities. A new multi-cell T-mapping system is has been developed at Cornell University. The system has nearly two thousand thermometers to cover 7-cell SRF cavities for Cornell ERL project. A new multiplexing scheme was adopted to reduce number of wires. A 1mK resolution of the temperature increase Delta T is achieved. A 9-cell cavity of TESLA geometry was tested with the T-map system. By converting Delta T to power loss and quality factor, it has been found that for this cavity, most surface losses were generated by the first cell when the accelerating gradient is increased above 15MV/m. The comparison of Q-value between with and without hotspots shows the heating on cavity wall degraded cavity Q0 about 1.65 times. The power loss on the hotspots is about 40% of the total power. Effective and intuitive ways of displaying surface properties of the cavity interior, e.g. the residual resistivity, will be shown.