Investigations Towards Ultra-Low Cost Nb3Sn SRF Cavity Fabrication Via Melt Casted Bronze Route Processing

Abstract

A novel approach to fabricating Nb3Sn SRF cavities and other RF components using an ultra-low-cost melt casting fabrication process has been investigated. This simple, low cost melt casting technique has the potential to be used to fabricate nearly any superconducting Nb3Sn structure using either the Bronze Route (BR), Internal Tin (IT), External Tin (ET) processes as well as normal conducting pure copper (Cu) cavities. Most of the heat treated samples that were examined using SEM/EDX seemed to show a reasonable correlation of Tc onset and transition width (ΔTc) of the ingots initial/starting Sn content to the desired stoichiometric Nb3Sn phase, where the lower starting Sn content coupons resulted in lower Tc's with broader transition widths and the higher starting Sn coupons resulted in higher Tc's with narrower ΔTc's. The best samples had Tc onsets ∼17–18 K and ΔTc's < 2 K. Two samples were further tested for RF surface resistance (Rs) and Quality Factor (Q) at JLAB. These RF measurements were performed at 7.4 GHz using a calorimetric technique and showed two transitions, one at ∼8 K and another at ∼14 K; values of Rs were at least two orders of magnitude higher than similar high quality Nb3Sn films directly deposited substrates by JLAB and with another superconducting transition close to the superconducting transition temperature of niobium. Substantial improvements in the processing variables ranging from higher quality of the initial ingots with higher Sn content, to better electro-polishing and reaction heat treatment regimens will be necessary to realize improved RF performance metrics.