Critical fields of Nb₃Sn prepared for superconducting cavities

Abstract

Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.

Superfluid flow in disordered superconductors with Dynes pair-breaking scattering: depairing current, kinetic inductance, and superheating field

“…Besides, we have derived the formulas of ∆, n s , λ, and j s at T = 0, taking the effects of Γ into account [Eqs. ( 21)- (29)]. The formulas for T ≃ T c have also been obtained, which have the similar forms as the usual GL results except that T c depends on Γ.…”

“…To understand the nonlinear L k for small current regions, we use the approximate formulas, Eqs. ( 27)- (29). After some calculations, we find (see Appendix D):…”

“…The surface magnetic field that induces j d is coincident with the superheating field H sh , which is the stability limit of the Meissner state. H sh is thought to define the upper limit of the accelerating field [9][10][11] and is one of the main interests in fundamental SRF studies [28,29].…”

A Field-Enhanced Conduction-Cooled Superconducting Cavity for High-Repetition-Rate Ultrafast Electron Bunch Generation

“…Such a configuration allows for a dissipated power of 1.0 W considering a cryocooler operating at a constant temperature of T k = 3.4 K. For the case of pure niobium the temperature at rod tip is 6.5 K; see Fig. 5(a) and the peak axial electric and surface magnetic fields are respectively 18.2 MV/m and 18.3 mT (below B c1 = 170 mT for Nb 19 ). The corresponding cavity quality factor is estimated to be Q 2.9 × 10 8 .…”

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

“…The depairing current density J d is related to the bias current of SSPD and is the maximum current that SRF cavities and superconducting cables can support. The screening current density on the inner surface of the cutting-edge Nb cavity reaches a current density close to J d , and SRF researchers study next-generation cavities using alternative materials with higher J d theoretically [64][65][66][67][68][69] and experimentally [70][71][72][73][74][75][76][77][78][79][80].…”