Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity

Abstract

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

The importance of the electron mean free path for superconducting radio-frequency cavities

“…However, under certain circumstances, R BCS has been observed to decrease with increasing field strength. 4,5 This field-dependent reduction in surface resistance causes an increase in Q 0 ; throughout the remainder of this work, we will use the term "positive Q-slope" to differentiate it from the traditional negative Q-slope in the medium-and high-field regions (the positive Q-slope effect is also commonly referred to as the "anti-Q-slope"). 4,6,7 Figure 1 shows examples of this positive Q-slope, medium-field Q-slope, and high-field Qslope.…”

“…In the field of SRF, the positive Q-slope phenomenon was discovered in 2013 by Grassellino et al at Fermilab in niobium doped with nitrogen and by Dhakal et al at Jefferson Laboratory. 4,5 In this context, "doping" entails treating an SRF material in a furnace with a low-pressure dopant gas atmosphere (nitrogen-doped bulk niobium is the only material considered in this work). Impurity-doping SRF cavities in this way causes both this reverse dependence of R BCS on the field strength and a decrease in the low-field surface resistance, making the technology highly appealing for new accelerators requiring very efficient SRF cavities.…”

Maximum screening fields of superconducting multilayer structures

“…4 High-field rf performance of the Nb cavities can be boosted by depositing not only materials with higher H s but also alloyed Nb-I-Nb multilayers which can increase H m and benefit from a significant raise of Q(H) with H in a wide field region. [17][18][19][20] A polycrystalline Nb multilayer may be…”

“…The lack of radiation losses and vortex dissipation in the Nb cavities (clean Nb has the highest lower critical field H c1 ≃ 180 mT among type-II superconductors) enables one to probe the high-field nonlinear quasiparticle conductivity 17 which can be tuned by alloying the surface with impurities. [18][19][20] The screening field limit of Nb could be exceeded by using s-wave superconductors with higher H c and the critical temperature T c , but such materials are prone to the dissipative penetration of vortices at H ≃ H c1 < H N b c1 . To address this problem it was proposed to coat the Nb cavities with multilayers of thin superconductors (S) with high H c > H N b c separated by dielectric (I) layers 11 (see Fig.…”

“…tuned by alloying and heat treatment to reduce the residual resistance, 3,17,18 and is also less prone to the current-blocking grain boundaries than the A-15 or pnictide compounds. 31 Enhancement of the vortex penetration field by a dirty Nb/Al 2 O 3 bilayer deposited onto the Nb cavity was observed in Ref.…”