Turner Daniel Andrew, Burt Graeme, Junginger Tobias
Cockcroft institute, STFC Daresbury Laboratory, Warrington, WA4 4AD, UK.
Lancaster University, Engineering, Lancaster, LA1 4YW, UK.
Sci Rep. 2022 Apr 1;12(1):5522. doi: 10.1038/s41598-022-09023-0.
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.
超导射频腔目前由铌制成。由于在第一涡旋穿透场(H_{p})处涡旋的进入，铌腔受到腔壁上磁场的限制。在真空或低压气体气氛中进行低温烘烤可消除品质因数随加速梯度（高场(Q)斜率）的强烈下降。一些腔体的表面磁场超过了下临界场(H_{c1})。对于这种性能提升的一种假设是，受处理影响的外层充当了涡旋穿透的屏障（有效双层）。使用振动样品磁力计测量了经过各种低温处理的铌椭球体的第一磁通穿透场(H_{p})。发现所有(H_{p})值都与清洁铌所预测的下临界场(H_{c1})一致。这导致得出结论，与先前发表的由两种超导体组成的双层不同，在低温烘烤铌的直流磁测量中无法观察到高于(H_{c1})的亚稳态无磁通状态。不同处理之间的磁通钉扎效应差异显著，这表明高场(Q)斜率的缓解可能与腔体表面的涡旋钉扎有关。
