Advancing the Maximum Accelerating Gradient of Niobium-3 Tin Superconducting Radiofrequency Accelerator Cavities: RF Measurements, Dynamic Temperature Mapping, and Material Growth

Abstract

Niobium-3 Tin (Nb3Sn) is the most promising alternative material for Superconducting Radiofrequency (SRF) particle accelerator cavities. Current SRF accelerators use superconducting niobium accelerator cavities, which are nearing their theoretical limits of performance. Nb3Sn promises increased quality factors, twice the operational temperature (4.2 K instead of 2 K}), and almost twice the theoretical accelerating gradient--96 MV/m in a TESLA elliptical style cavity. These advances can reduce the size and complexity of particle accelerators while simultaneously making them more efficient. The capability of operating at 4.2 K enables the creation of small-scale superconducting accelerators that are run off cryocoolers and could be used in research and industrial applications. Current Nb3Sn cavities achieve quality factors of 2*10^10 at 4.2 K. The accelerating gradient, however, is limited far below the theoretical potential of this material, with the best recorded reaching 24 MV/m. In this work we present studies on what is limiting the maximum accelerating gradients in these cavities. We study cavity performance under RF testing, make dynamic measurements of cavity heating during operation, study samples with microscopy, and develop models of Nb3Sn material growth. In the process we develop new diagnostic tools for SRF development: a dynamic/high speed temperature mapping system that measures the spatial heat distribution on a cavity at 50 ksps, and high-power test system for measuring the ultimate critical fields (theoretical limit of the accelerating gradient) of new materials. We explore models of cavity losses/heating that limit the accelerating gradient and propose modifications to the material growth process to create Nb3Sn cavities with higher accelerating gradients and quality factors. We conclude with results from a new cavity coating wherein we have suppressed multi-gap superconductivity that has been seen in these cavities and was a limitation to the quality factor.