Resonance Frequency Shift Measurements of SRF Cavities at DESY

TL;DR

This paper presents a new measurement setup for analyzing resonance frequency shifts in SRF cavities at DESY, revealing insights into superconducting properties and novel phenomena related to interstitial atom modifications.

Contribution

Development and validation of a precise frequency-shift measurement system for SRF cavities, enabling detailed study of superconducting transition dynamics and anomalous effects.

Findings

Established a framework for determining electron mean free path in cavities.

Observed an anomalous dip in frequency shift near the critical temperature.

Enhanced measurement accuracy and reproducibility with recent system upgrades.

Abstract

The variation of the resonance frequency and intrinsic quality factor of superconducting radio-frequency cavities during the transition from the superconducting to the normal-conducting state provides essential insight into the fundamental superconducting properties of the cavity material. Investigating these transition dynamics is crucial for the continued advancement of niobium cavities whose near-surface regions are intentionally modified through the controlled introduction of interstitial atoms, such as oxygen and nitrogen, leading to the emergence of several novel behaviors whose underlying mechanisms are not yet fully understood. This work reports on the development and commissioning of a dedicated frequency-shift measurement setup. In its initial implementation, the system establishes a precise framework for determining the electron mean free path within both the superconducting penetration depth and the normal-conducting skin depth. It further enables investigation of an anomalous dip in the temperature dependence of the frequency shift near the critical temperature in cavities containing interstitial atoms in the near-surface lattice, a novel phenomenon previously reported in the literature. A recent upgrade, currently in the final stage of validation, significantly improves measurement accuracy and reproducibility. The improved setup enables comprehensive studies of the frequency shift and quality factor over the full temperature range above 7 K, contributing to a deeper understanding of the superconducting properties.