Further improvement of medium temperature heat treated SRF cavities for high gradients

TL;DR

This paper investigates medium temperature heat treatments on superconducting RF cavities, demonstrating that low-temperature bakes can cure high-field Q-slope issues and significantly improve quality factors and maximum gradients.

Contribution

It introduces a low-temperature bake procedure that effectively cures high-field Q-slope in SRF cavities previously treated with medium temperature heat treatments.

Findings

Low-T bake cures high-field Q-slope in SRF cavities.

Q values exceeding 3×10^{10} at 16 MV/m achieved.

Maximum gradients of 32 to 40 MV/m demonstrated.

Abstract

The application of heat treatments on 1.3 GHz TESLA type cavities in ultra-high vacuum at 250{\deg}C to 350{\deg}C is called medium temperature or mid-T heat treatment. In various laboratories such treatments on superconducting radio frequency (SRF) cavities result reproducible in three main characteristic features for the quality factor $Q_0$ in dependency of the accelerating electric field strength $E_{acc}$. First, comparing mid-T heat treatment with a baseline treatment, a significant increase of $Q_0$ up to $5\cdot10^{10}$ at 2K can be observed. Second, with increasing accelerating gradient $E_{acc}$ the $Q_0$ increases up to a maximum around 16 to 20 MV/m. This effect is known as anti-Q-slope. The third observation for a mid-T heat treatment compared to a baseline treatment is an often reduced maximum gradient $E_{acc}$. The appearance of a high-field-Q-slope (HFQS) was reported after mid-T heat treatments of 3 hours at 350{\deg}C or of 20 hours at 300{\deg}C at DESY. Using the heating temperature and the heating time taken from the temperature profile of the furnace effective oxygen diffusion lengths $l$ were calculated. In the follow-up study presented here, a set of three single-cell cavities with diffusion lengths $l$ above 1700 nm, showing HFQS, were treated with an additional so-called low-T bake of 24-48 hours at 120{\deg}C to 130{\deg}C. The subsequent reproducible Q(E) -performances results indicate that the low-T bake procedure cures the HFQS like for cavities treated with the EuXFEL recipe of EP and following low-T treatments. As presented in the following, Q values of more than $3\cdot10^{10}$ at 16 MV/m and accelerating gradients of 32 to 40 MV/m are achieved.