Theoretical estimates of maximum fields in superconducting resonant radio frequency cavities: stability theory, disorder, and laminates

Abstract

Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities, especially those made of new higher-Tc materials such as Nb3Sn, NbN, and MgB2. Indeed, beyond the so-called superheating field H sh , flux will spontaneously penetrate even a perfect superconducting surface and ruin the performance. We present intuitive arguments and simple estimates for H sh , and combine them with our previous rigorous calculations, which we summarize. We briefly discuss experimental measurements of the superheating field, comparing to our estimates. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB2? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium?Finally, we discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry.

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications

“…The width of Sn segregation at a GB (~3 nm) is comparable to the superconducting coherence length of Nb 3 Sn [2,10,20]. This suggests that Sn segregation at GBs can indeed possibly provide a path for magnetic flux penetration from the surface.…”

“…In contrast, the magnetic-field direction in SRF cavities changes on a nanosecond time-scale, leading to significant dissipation of heat and a corresponding decrease in the quality factor if the flux penetrates into the superconductor (including penetration into the GBs) [20]. The potential importance of GBs in Nb 3 Sn SRF cavities has been discussed extensively [3,8,10,21,22], but the structure and chemistries of GBs of Nb 3 Sn prepared by the vapor-diffusion process for SRF cavity applications are unknown, and, therefore, the level of vulnerability of GBs remains unclear.…”

Weak-field dissipative conductivity of a dirty superconductor with Dynes subgap states under a dc bias current up to the depairing current density

“…Although j d of dirty-limit superconductors for Γ = 0 was calculated many years ago [36], that for Γ > 0 is still unknown. The value of j d (Γ, T ) is related to the maximum accelerating field that SRF cavities can achieve with the bulk SRF [37][38][39][40][41][42][43] and the thin-film SRF technologies [40,[44][45][46][47], and also related to the threshold current of superconducting nanowire single-photon detectors [5].…”

Effect of inhomogeneous surface disorder on the superheating field of superconducting RF cavities

“…The superheating field depends on the geometry of the vacuum-superconductor interface as well as the spatial distribution of disorder within the region of the screening currents. For a planar half-space geometry the effects of homogeneous disorder and engineered multilayer superconductor-insulator structures has been studied [2][3][4][5][6]. The main results are that homogeneous disorder increases the penetration depth, but reduces the critical current, with a modest enhancement of the superheating field at low temperatures [2].…”