UBC Theses and Dissertations
Muon spin rotation characterization of superconducting niobium for applications in high field superconducting radio frequency cavities Buck, Terry J.
μSR was used to investigate the effect of sample preparation on the magnetic field penetration of high purity niobium to improve the fabrication and preparation of superconducting niobium radio frequency (RF) cavities for use in particle acceleration. The sample preparations tested were (1) electro-polish etching, (2) buffered chemical polish etching, (3) nitrogen doping, (4) plating with Nb₃Sn, (5) baking at 120⁰C, (6) baking at 800⁰C, and (7) baking at 1400⁰C. Three different sample geometries and two different applied magnetic field orientations were used in order to observe the effect of sample shape on the μSR measurements and to minimize the effect of the demagnetization factor on the results. The results showed that etching caused flux to enter the center of the samples at a lower applied magnetic field; however, a 120⁰C bake caused the etched samples to reach higher field before experiencing flux penetration. These results correlate with RF cavity test results using the same treatment method. Higher heat treatments caused a reduction in the pinning strength of the niobium samples and caused flux to enter the center of the sample at lower applied magnetic fields. Impurities and vacancies in a sample were suspected of acting as pinning centers and increasing the pinning strength; certain impurities and vacancies are also thought to prevent hydride formation in samples and prevent high field RF losses in cavities. If the same impurities that prevent RF losses in cavities also create pinning centers in the μSR samples, it could explain why the DC field μSR measurements are showing similar results to AC field RF cavity tests. The perpendicular field results for the Nb₃Sn plated and nitrogen doped samples showed no difference compared with regular niobium samples that had undergone similar heat treatments; however, the parallel field measurements of the Nb₃Sn plated sample show an increase in the field of first flux entry. Parallel field measurements are less affected by pinning strength than the perpendicular field measurements and give a better indication of when the sample first experiences flux entry. Plating niobium with Nb₃Sn could increase the effective HC1 and thereby accelerating gradient of cavities.
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