Frequency dependence of trapped flux sensitivity in SRF cavities

TL;DR

This study investigates how the sensitivity of trapped magnetic flux in SRF niobium cavities varies with frequency and surface treatment, revealing a non-monotonic relationship influenced by mean-free-path and enabling tuning of vortex response regimes.

Contribution

It provides the first detailed analysis of frequency-dependent trapped flux sensitivity across different surface treatments and demonstrates how to tune vortex response regimes via frequency and mean-free-path adjustments.

Findings

Higher frequency increases flux sensitivity in SRF cavities.

N-doped cavities show intermediate sensitivity due to their mean-free-path.

Vortex response can be tuned from pinning to flux-flow regimes.

Abstract

In this letter, we present the frequency dependence of the vortex surface resistance of bulk niobium accelerating cavities as a function of different state-of-the-art surface treatments. Higher flux surface resistance per amount of trapped magnetic field - sensitivity - is observed for higher frequencies, in agreement with our theoretical model. Higher sensitivity is observed for N-doped cavities, which possess an intermediate value of electron mean-free-path, compared to 120 C and EP/BCP cavities. Experimental results from our study showed that the sensitivity has a non-monotonic trend as a function of the mean-free-path, including at frequencies other than 1.3 GHz, and that the vortex response to the rf field can be tuned from the pinning regime to flux-flow regime by manipulating the frequency and/or the mean-free-path of the resonator, as reported in our previous studies. The frequency dependence of the trapped flux sensitivity to the amplitude of the accelerating gradient is also highlighted.