A Path to Higher Q0 with Large Grain Niobium Cavities

TL;DR

This paper explores heat treatment of large grain niobium SRF cavities at high temperatures to significantly improve their quality factor Q0 at medium gradients, potentially reducing cryogenic costs.

Contribution

It demonstrates that high-temperature heat treatment of large grain niobium cavities in an all-niobium furnace enhances Q0, offering a cost-effective alternative to fine-grain niobium.

Findings

Achieved residual resistance of 1-5 nΩ in large grain niobium cavities.

Q0 values up to 4.6×10^10 at 90 mT at 2 K.

Q0 around 2×10^11 at 1.5 K.

Abstract

The improvement of the quality factor Q0 of superconducting radio-frequency (SRF) cavities at medium accelerating gradients (20-25 MV/m) is important in order to reduce the cryogenic losses in continuous wave (CW) accelerators used for a variety of applications. In recent years, SRF cavities fabricated from ingot niobium have become a viable alternative to standard high-purity fine-grain Nb for the fabrication of high-performing SRF cavities with the possibility of significant cost reduction. Recent studies demonstrated the improvement of Q0 at medium field in cavities heat treated at 800-1200 {\deg}C without subsequent chemical etching [ ]. To further explore this treatment procedure, a new induction furnace with an all-niobium hot-zone was commissioned [ ]. A single-cell 1.5 GHz cavity fabricated from ingot material from CBMM, Brazil, with RRR 200, was heat treated in the new furnace in the temperature range 800-1400 {\deg}C for several hours. Residual resistance value of 1 - 5 n\Omega have been consistently achieved on this cavity Q0-values as high as 4.6\times1010 at 90 mT peak surface magnetic field at 2 K. Q0 values of about ~2\times1011 have been measured at 1.5 K.