Abstract
Nb Superconducting Radio-Frequency (SRF) cavities are observed to break down and lose their high-Q superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues temperature and power dependent local third harmonic response was measured on bulk Nb and Nb thin film samples using a novel near-field magnetic microwave microscope between 2.9K-10K and 2GHz-6GHz. Both periodic and non-periodic response as a function of applied RF field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The RF-current-biased Resistively Shunted Junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The non-periodic response is consistent with vortex semi-loops penetrating into the bulk of the sample once sufficiently high RF magnetic field is applied, and the data can be fit to a Time-Dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of Nb samples suggests that we have captured the generic nonlinear response of air-exposed Nb surfaces.
Highlights
Researchers studying high-energy physics are considering several next-generation accelerator designs, including the International Linear Collider (ILC), where a very large number of Nb superconducting radio-frequency (SRF) cavities will be employed [1,2]
The nonperiodic response is consistent with vortex semiloops penetrating into the bulk of the sample once sufficiently high rf magnetic field is applied and the data can be fit to a time-dependent Ginzburg-Landau (TDGL) model of this process
Measurements are performed on several other bulk Nb and Nb film samples and a very similar periodic response of V3f vs Hrf is observed, showing that this is a generic response of this material (Figs. 6–8 in the Appendix)
Summary
Researchers studying high-energy physics are considering several next-generation accelerator designs, including the International Linear Collider (ILC), where a very large number (approximately 16 000) of Nb superconducting radio-frequency (SRF) cavities will be employed [1,2]. Purity of material [14], dc critical magnetic field or superheating field [15], postmortem microanalysis of hot-cold spots [16,17], or various sophisticated optical inspection tools [5,12] make up the backbone of SRF material science. These efforts have resulted in advancements in cavity treatment recipes that have led to SRF cavities with high gradients and high-quality factors > 3 × 1010 at 1.3 GHz and 2 K. Surface defects are expected to locally suppress the rf critical current, which leads to a higher nonlinear response, making this microscope an excellent defect detector [27,28]
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