High-Q operation of SRF cavities: The potential impact of thermocurrents on the RF surface resistance

TL;DR

This study investigates how thermocurrents during cooldown can generate magnetic flux that increases RF surface resistance in SRF niobium cavities, affecting their performance, supported by measurements and simulations.

Contribution

It provides extensive experimental and simulation evidence that thermocurrents can significantly impact the RF surface resistance of SRF cavities.

Findings

Thermocurrents generate magnetic flux affecting cavity Q factor.

Thermal cycling reduces residual resistance by manipulating thermocurrents.

Non-symmetric current distribution arises from temperature-dependent material parameters.

Abstract

For many new accelerator applications, superconducting radio frequency (SRF) systems are the enabling technology. In particular for CW applications, much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Starting in 2009, we suggested a means of reducing the residual resistance by performing a thermal cycle [1], a procedure of warming up a cavity after initial cooldown to about 20K and cooling it down again. In subsequent studies [2], this technique was used to manipulate the residual resistance by more than a factor of 2. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a non-symmetric current distribution. Hence a significant amount of magnetic flux can be generated at the RF surface.