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

A novel approach to fabricating Nb
₃
Sn 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 Nb
₃
Sn 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 T
_c
onset and transition width (ΔT
_c
) of the ingots initial/starting Sn content to the desired stoichiometric Nb
₃
Sn phase, where the lower starting Sn content coupons resulted in lower T
_c
's with broader transition widths and the higher starting Sn coupons resulted in higher T
_c
's with narrower ΔT
_c
's. The best samples had T
_c
onsets ∼17–18 K and ΔT
_c
's < 2 K. Two samples were further tested for RF surface resistance (R
_s
) 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 R
_s
were at least two orders of magnitude higher than similar high quality Nb
₃
Sn 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.