Mid-T heat treatments on BCP’ed coaxial cavities at TRIUMF

Abstract

Mid-T heat treatments in the range from 250°C to 400°C on superconducting radio-frequency (SRF) cavities have been shown to provide high quality factors that rise with applied rf field strength in high frequency, electro-polished (EP), elliptical cavities operating at 2 K, similar to nitrogen doped cavities. The rise in quality factor is attributed to a decrease in the temperature dependent part of the surface resistance RBCS. Until now, no results have been reported for these new treatments on quarter-wave resonators (QWR) and half-wave resonators (HWR). The TRIUMF multi-mode coaxial cavities are dedicated test cavities that allow frequency and temperature resolved performance characterization of treatments without changing environments, therefore providing an excellent test vehicle to test these new treatments with rf frequencies ranging from 200 to 1,200 MHz. In this paper, performance measurements from both QWR and HWR cavities are reported and their performance compared with four different treatments: baseline, a conventional 120°C low temperature bake for 48 h, and two mid-T bakes at 300°C and 400°C for 3 h. In addition, sample analysis using SEM, EDX and SIMS of witness samples is also shown. It is found that the mid-T bakes are not directly transferable to low frequency cavities. In the fundamental modes of the two test cavities, no performance gain over the baseline treatment nor a decreasing temperature dependent component with rising rf amplitude was observed. At frequencies above 1 GHz and low temperatures, the mid-T bakes show a reduced field dependence of RBCS compared to both the baseline and 120°C treatments.

Depth-resolved Characterization of Meissner Screening Breakdown in Surface Treated Niobium

“…As both samples are from the same batch with the same buffered chemical polishing (BCP) etching step applied, it is unlikely that surface roughness variation is the sole origin of the much larger "dead layer" in the "O-doping" sample. Surface composition characterization has been performed on duplicate witness samples with equivalent treatment using time-of-flight SIMS (TOF-SIMS) and energy dispersive X-ray spectroscopy (EDX), and has been reported by Kolb et al 81 . Both characterizations indicate an elevated level of carbon concentration at the surface of the "O-doping" sample, possibly received during the 400 • C-bake.…”

“…The fit to the distribution using a phenomenological distribution function (described in detail in 47 ) is also shown. A 5 nm layer of Nb 2 O 5 on a Nb substrate is assumed in the simulation to represent a typical Nb system (see, e.g., 76,81,100 ).…”