Accelerating fields up to 49 MV/m in TESLA-shape superconducting RF niobium cavities via 75C vacuum bake

TL;DR

This paper reports a novel surface treatment involving a 75C vacuum bake that significantly enhances the accelerating fields of superconducting niobium cavities to 49 MV/m, surpassing previous limits and indicating potential for higher performance in SRF applications.

Contribution

Introduction of a new 75C vacuum bake step that, combined with existing treatments, achieves unprecedented accelerating fields in TESLA-shaped SRF niobium cavities.

Findings

Achieved 49 MV/m accelerating field in CW operation.

Discovered that a 75C bake step enhances quench fields.

Indicated potential for surpassing current field limitations.

Abstract

In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art.