Cooling Dynamics Through Transition Temperature of Niobium SRF Cavities Captured by Temperature Mapping

TL;DR

This study investigates how the cooling rate of niobium SRF cavities affects their superconducting transition, revealing two distinct transition behaviors that influence magnetic flux expulsion and trapping, impacting cavity quality.

Contribution

First experimental temperature mapping of SRF cavity cool-down through transition temperature, identifying two different transition dynamics based on cooling rate.

Findings

Fast cool-down results in sharp transition and better flux expulsion.

Slow cool-down causes nucleation phase transition and flux trapping.

Transition dynamics significantly affect cavity quality factors.

Abstract

Cool-down dynamics of superconducting accelerating cavities became particularly important for obtaining very high quality factors in SRF cavities. Previous studies proved that when cavity is cooled fast, the quality factor is higher than when cavity is cooled slowly. This has been discovered to derive from the fact that a fast cool-down allows better magnetic field expulsion during the superconducting transition. In this paper we describe the first experiment where the temperature all around the cavity was mapped during the cavity cool-down through transition temperature, proving the existence of two different transition dynamics: a sharp superconducting-normal conducting transition during fast cool-down which favors flux expulsion and nucleation phase transition during slow cool-down, which leads to full flux trapping.