Improved high-gradient performance for medium-velocity superconducting half-wave resonators: Surface preparation and trapped flux mitigation

TL;DR

This work targets high-gradient, high-$Q_0$ operation for FRIB's medium-velocity HWRs by combining advanced surface preparation (electropolishing with post-EP baking) and flux-trapping mitigation. EP+LTB substantially improves high-field performance, reaching up to ~15 MV/m before quench, while addressing residual losses; further gains require suppressing thermo-current–driven flux trapping via demagnetization strategies and cooling control. The authors demonstrate that local magnetic shielding, uniform cooldown, and active field cancellation can dramatically reduce trapped-flux–related losses, achieving $Q_0=2.8\times10^{10}$ at 12 MV/m, exceeding the target by a comfortable margin. These results have significant practical implications for the design and operation of FRIB cryomodules, underscoring thermo-current management and improved thermal conduction as essential paths to higher gradients and lower cryogenic load. The study also situates these findings within the broader SRF literature, suggesting potential benefits for other medium-velocity cavities and future nitrogen-doping or infusion strategies.

Abstract

A development effort to improve the performance of superconducting radio-frequency half-wave resonators (SRF HWRs) is underway at the Facility for Rare Isotope Beams (FRIB), where 220 such resonators are in operation. Our goal was to achieve an intrinsic quality factor (Q0) of >= 2E10 at an accelerating gradient (Ea) of 12 MV/m. FRIB production resonators were prepared with buffered chemical polishing (BCP). First trials on electropolishing (EP) and post-EP low temperature baking (LTB) of FRIB HWRs allowed us to reach higher gradient (15 MV/m, limited by quench) with a higher quality factor at high gradient, but Q0 was still below our goal. Trapped magnetic flux during the Dewar test was found to be a source of Q0 reduction. Three strategies were used to reduce the trapped flux: (i) adding a local magnetic shield (LMGS) to supplement the ``global'' magnetic shield around the Dewar for reduction of the ambient magnetic field; (ii) performing a ``uniform cool-down'' (UC) to reduce the thermoelectric currents; and (iii) using a compensation coil to further reduce the ambient field with active field cancellation (AFC). The LMGS improved the Q0, but not enough to reach our goal. With UC and AFC, we exceeded our goal, reaching Q0 = 2.8E10 at Ea = 12 MV/m.

Figures (17)

• Figure 1: (a) Isometric sectional view of a FRIB $\beta$ = 0.53 HWR. The helium jacket is shown in green. (b) Photograph of a $\beta = 0.53$ cold mass during clean-room assembly.
• Figure 2: Jacketed $\beta = 0.53$ HWR (a) installed in the horizontal rotational EP system and (b) undergoing a low-temperature bake.
• Figure 3: (a) Measured quality factor and (b) corresponding temperature measurements as a function of accelerating gradient ($E_\mathrm{acc}$) for $\beta = 0.53$ HWRs at $\sim 2$ K after EP and after additional LTB. Cyan squares and magenta triangles: initial measurements on S53-159. Blue and red: final measurements on S53-159. Light green: additional cases. Dashed line: $\lambda$ point temperature. $B_p$ = peak surface magnetic field.
• Figure 4: Comparison of measurements at $\sim 2$ K on $\beta = 0.53$ HWRs after BCP or EP+LTB: (a) quality factor and (b) X-rays as a function of gradient. BCP cases: cavities certified and installed into a cryomodule (SCM517) near the end of FRIB production. Silver star: goal for FRIB production cryomodules. Gold star: goal for present effort.
• Figure 5: Comparison of measurements at $\sim 2$ K on $\beta = 0.53$ HWRs after BCP or EP+LTB: (a) quality factor and (b) X-rays as a function of gradient. BCP cases: FE-free cavities certified in the latter third of FRIB $\beta = 0.53$ production.