Superconducting RF materials other than bulk niobium: a review

Abstract

For the past five decades, bulk niobium (Nb) has been the material of choice for superconducting RF (SRF) cavity applications. Alternatives such as Nb thin films and other higher-Tc materials, mainly Nb compounds and A15 compounds, have been investigated with moderate effort in the past. In recent years, RF cavity performance has approached the theoretical limit for bulk Nb. For further improvement of RF cavity performance for future accelerator projects, research interest is renewed towards alternatives to bulk Nb. Institutions around the world are now investing renewed efforts in the investigation of Nb thin films and superconductors with higher transition temperature Tc for application to SRF cavities. This paper gives an overview of the results obtained so far and challenges encountered for Nb films as well as other materials, such as Nb compounds, A15 compounds, MgB2, and oxypnictides, for SRF cavity applications. An interesting alternative using a superconductor–insulator–superconductor multilayer approach has been recently proposed to delay the vortex penetration in Nb surfaces. This could potentially lead to further improvement in RF cavities performance using the benefit of the higher critical field Hc of higher-Tc superconductors without being limited with their lower Hc1.

Microwave Response of Nanostructured High-Tc Superconductor Thin Films

“…Melkov with coauthors (see, e.g., their works [29][30][31][32]). Nevertheless, the problem of superconducting materials which will be optimal for rf applications remains still open [33].…”

Development and Characterization of Nb3Sn/Al2O3 Superconducting Multilayers for High-performance Radio-Frequency Applications

“…where the phenomenological parameter Γ accounts for the broadening of the DOS peaks due to a finite lifetime of quasiparticles, and N0 is the DOS in the normal state. The fit was done with Γ = 0.4 meV and Δ ≈ 3.1 meV, consistent with the conventional gap value for a stoichiometric Nb3Sn 4 . The ratio Γ Δ ≈ 13 ⁄ % in our samples turns out to be about 2-3 times larger than the values observed by tunneling spectroscopy on 1-2 m thick Nb3Sn films for rf applications 27 and Nb coupons 28 .…”

Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

“…The first equation describes perfect conductivity where electrons accelerate continuously due to electric field since no resistance is present. The second London equation in (3), in combination with Maxwell's equation…”

“…In high-T c superconductors, GBs even at low misorientation angles are weak superconducting links that impede the current flow giving rise to the electromagnetic granularity which is one of the serious obstacles for applications of the cuprates and the iron-based superconductors [11]. In planar GBs the critical current density across the GB, J c,gb (α) falls off exponentially when the misorientation angle α exceeds a critical angle α c , where α c can be as little as 3 [3].…”

“…Hence, θ n = −4β 0 k(−1) n /π 4 (2n + 1) 3 , and that can be stepped forward in time using any standard technique. Here the operator "F " denotes the spatial Fourier transform.…”