Exploration of Parameters That Affect High Field Q-Slope

TL;DR

This study investigates how trapped magnetic flux and hydride concentration influence high field Q-slope in superconducting cavities, aiming to improve understanding and mitigation of HFQS to enhance cavity performance.

Contribution

It provides experimental insights into the effects of trapped flux and hydride growth on HFQS, highlighting parameters that influence high-gradient superconducting cavity performance.

Findings

Trapped magnetic flux correlates with HFQS onset.

Hydride concentration increase worsens HFQS behavior.

Cooldown procedures affect hydride formation and flux trapping.

Abstract

The onset of high field Q-slope (HFQS) around 25 MV/m prevents cavities in electropolished (EP) condition from reaching high quality factors at high gradients due to the precipitation of niobium hydrides during cooldown. These hydrides are non-superconducting at 2 K, and contribute to losses such as Q disease and HFQS. We are interested in exploring the parameters that affect the behavior of HFQS. We study a high RRR cavity that received an 800 C by 3 hour bake and EP treatment to observe HFQS. First, we explore the effect of trapped magnetic flux. The cavity is tested after cooling slowly through Tc while applying various levels of ambient field. We observe the onset of the HFQS and correlate this behavior with the amount of trapped flux. Next, we investigate the effect of the size/concentration of hydrides. The cavity is tested after holding the temperature at 100 K for 12 hours during the cooldown to promote the growth of hydrides. We can correlate the behavior of the HF QS with the increased hydride concentration. Our results will help further the understanding of the mechanism of HFQS.