No interface energy barrier and increased surface pinning in low temperature baked niobium

Daniel Andrew Turner; Graeme Burt; Tobias Junginger

doi:10.1038/s41598-022-09023-0

No interface energy barrier and increased surface pinning in low temperature baked niobium

Sci Rep. 2022 Apr 1;12(1):5522. doi: 10.1038/s41598-022-09023-0.

Authors

Daniel Andrew Turner^{1

2}, Graeme Burt^{3

4}, Tobias Junginger^{5

6}

Affiliations

¹ Cockcroft institute, STFC Daresbury Laboratory, Warrington, WA4 4AD, UK. daniel.turner@cockcroft.ac.uk.
² Lancaster University, Engineering, Lancaster, LA1 4YW, UK. daniel.turner@cockcroft.ac.uk.
³ Cockcroft institute, STFC Daresbury Laboratory, Warrington, WA4 4AD, UK.
⁴ Lancaster University, Engineering, Lancaster, LA1 4YW, UK.
⁵ Department of Physics, University of Victoria, Victoria, BC, V8P 5C2, Canada.
⁶ TRIUMF, Accelerator division, Vancouver, BC, V6T 2A3, Canada.

Abstract

Superconducting Radio-Frequency cavities are currently made out of niobium. Niobium cavities are limited by the magnetic field on the cavity walls due to the entry of vortices at the field of first vortex penetration, H[Formula: see text]. Low temperature baking in vacuum or low pressure gas atmosphere removes the strong decrease of the quality factor with accelerating gradient (high field Q-slope). Some cavities reach surface magnetic field above the lower critical field H[Formula: see text]. One hypothesis for this performance increase is that the outer layer affected by the treatments acts as a barrier for vortex penetration (effective bilayer). Using a vibrating sample magnetometer the field of first flux penetration (H[Formula: see text]) was measured for Nb ellipsoids with various low temperature treatments. All H[Formula: see text] values were found to be consistent with the lower critical field, H[Formula: see text], as predicted for clean niobium. This led to the conclusion that a metastable flux free state above H[Formula: see text] cannot be observed in DC magnetometry for low temperature baked niobium unlike for bilayers consisting of two superconductors as previously published. The effect of flux pinning differed significantly between treatments, suggesting that the high field Q-slope mitigation might be related to vortex pinning in the surface of the cavities.