Nb₃Sn superconducting radiofrequency cavities: fabrication, results, properties, and prospects

Abstract

A microns-thick film of Nb3Sn on the inner surface of a superconducting radiofrequency (SRF) cavity has been demonstrated to substantially improve cryogenic efficiency compared to the standard niobium material, and its predicted superheating field is approximately twice as high. We review in detail the advantages of Nb3Sn coatings for SRF cavities. We describe the vapor diffusion process used to fabricate this material in the most successful experiments, and we compare the differences in the process used at different labs. We overview results of Nb3Sn SRF coatings, including CW and pulsed measurements of cavities as well as microscopic measurements. We discuss special considerations that must be practised when using Nb3Sn cavities in applications. Finally, we conclude by summarizing the state-of-the-art and describing the outlook for this alternative SRF material.

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

“…With the substantially higher critical temperature (T c ~18 K) of Nb 3 Sn compared to Nb [3][4][5], 1.3 GHz SRF cavities demonstrated a Q 0 (quality)-factor of ~10 10 at 4.2 K, where Q 0 -factor is defined by the geometric factor (G) over the surface resistance (R s ) of a cavity as G/R s , at useful accelerating fields; a value much higher than has been previously achieved with Nb SRF cavities under the same conditions (frequency, temperature, etc.) [6][7][8].…”

“…The coherence length () of Nb 3 Sn is significantly shorter than metallic Nb ((Nb 3 Sn) ~3 nm vs (Nb) ~50 nm) [1,8,11], potentially making Nb 3 Sn more vulnerable to small scale defects in the Nb 3 Sn layer.…”

“…Several studies [12][13][14][15] have been performed to investigate the structural defects in Nb 3 Sn coatings, such as abnormally thin-grains, compositional variations, and Sn-deficient Nb 3 Sn phases [1,8,12], which may be responsible for the Q 0 -slope and quenching, which is a loss of superconductivity. Orientation relationships between Nb substrates and Nb 3 Sn films were measured and the Nb 3 Sn/Nb heterophase interfaces are demonstrated to play a critical role in the formation of thin-grains and Sn-deficient regions [15].…”

Record high-gradient SRF beam acceleration at Fermilab

“…The facility is also capable to supporting a broad spectrum of advanced beam studies and experiments [47,48]. In particular, it will be quite suitable for the beam tests of a number of novel methods to further significantly increase the accelerating gradients up to 90 MV m −1 in pulsed SRF cavities, such as those now actively developed, using of Nb 3 Sn rather than pure Nb cavities [49][50][51] or based on impurity addition to the cavity surface or layered structures of insulating and superconducting films [52,53].…”

Photocathode quantum efficiency of ultrathin Cs2Te layers on Nb substrates

“…A replaceable, superconducting plug cathode would be particularly attractive for a compact SRF linac as it has a simple design [2,3]. Recent advances in Nbbased SRF cavities, including record high Q values at 15-20 MV=m via a nitrogen doping process [4], as well as the successful in situ growth of higher T C Nb 3 Sn on the inside surface [5,6] suggests that a compact electron linac operating at 4.2 K is feasible in the future. A limiting factor is that the present choices of superconducting photocathode have relatively low quantum efficiencies (QE), e.g., for Nb QE < 0.01% and for Pb QE < 0.1% at 248 nm wavelength [3].…”