A Path to Higher Q0 with Large Grain Niobium Cavities

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.