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

Yuting Wu, Kenji Saito, Alex Taylor, Andrei Ganshyn, Chris Compton, Ethan Metzgar, Kyle Elliott, Laura Popielarski, Sam Miller et al.

Sang-hoon Kim, Spencer Combs, Taro Konomi, Ting Xu, Walter Hartung, Wei Chang, and Yoo-Lim Cheon

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Phys. Rev. Accel. Beams 29, 033102 – Published 10 March, 2026

DOI: https://doi.org/10.1103/nh3z-zwz3

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 ≥2×1010 at an accelerating gradient (Eacc) of 12  MV/m. FRIB production resonators were prepared with buffered chemical polishing. First trials of electropolishing (EP) and post-EP low-temperature baking 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 cooldown” (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=2.8×1010 at Eacc=12  MV/m.

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Outline

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Phys. Rev. Accel. Beams 29, 033102– Published 10 March, 2026
  • Received 8 April 2025
  • Revised 12 December 2025
  • Accepted 13 February 2026
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