Impact of heat treatments on the performance of low-frequency superconducting quarter-wave resonators at 4.3 K

Abstract

We applied heat treatments to 80.5 MHz quarter-wave resonators made from bulk niobium and prepared with buffered chemical polishing BCP. We evaluated their performance at 4.3 K. We found that a 48 hour, 120 C bake-out ("low-temperature bake out") reduces the surface resistance by a factor of 2 to 3, stemming from a reduction in the Bardeen-Cooper-Schrieffer contribution, consistent with previous findings. This decrease leads to a 38% decrease on average in the medium-field Q-slope when compared to cavities which had only BCP. Mechanisms for the change in quality factor with low-temperature baking have been explored. We observed no improvement in cavity performance after a 3-hour bake-out at 350 C ("medium-temperature bake out"), in contrast to observations for higher-frequency cavities.

Figures (11)

• Figure 1: Intensity maps of the magnitude of the electric field (left) and magnetic field (right) for the FRIB $\beta = 0.085$ QWR. The field amplitudes are normalized to a stored energy of $U = 1$ Joule.
• Figure 2: Cavity intrinsic quality factor ($Q_0$) as a function of accelerating gradient ($E_\mathrm{acc}$) for several $\beta = 0.085$ QWRs after BCP (black) or BCP+LTB (red) at 4.3 K (solid lines) and 2 K (dashed lines). The peak surface magnetic field ($B_\mathrm{pk}$) is also shown. The star represents the FRIB 2 K cryomodule goal.
• Figure 3: FRIB QWR in the UHV furnace (top) and undergoing in-situ LTB (bottom).
• Figure 4: Isometric view of the FRIB $\beta = 0.085$ QWR and schematic of the RF and vacuum seals with a double indium gasket. Image originally published in Ref. ZHANG2021165675.
• Figure 5: Inferring $R_\mathrm{BCS}$ as a function of field: $R_s$ at 4.3 K (red), $R_s$ at 2 K (blue), and difference (purple). Results for several FRIB $\beta = 0.085$ QWRs that underwent BCP + LTB are included.