Kavli Affiliate: Ting Xu
| First 5 Authors: Yuting Wu, Yuting Wu, , ,
| Summary:
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.
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