arXiv subjects

Guillaume Rosaz

Publications and source records attributed to Guillaume Rosaz.

3 recordsLinked to original sources

arXiv2026

Design optimization, commissioning, and uncertainty analysis of the quadrupole resonator system at Jefferson Lab for SRF material characterization

A quadrupole resonator (QPR) provides a sample-based platform for characterizing materials for superconducting radio-frequency (SRF) applications under controlled field, frequency, and temperature conditions. This paper presents the design optimization, commissioning, and validation of the Jefferson Lab QPR system, including a quantitative assessment of measurement uncertainty. The resonator geometry was re-optimized from the CERN version-II design to improve quadrupole-mode separation and enable four usable modes at 400, 806, 1221, and 1640 MHz. The measurement system combines self-excited-loop RF operation, cable-loss-corrected power calibration, decay-based external-Q calibration, and RF-DC thermal-substitution calorimetry to determine the peak surface magnetic field Bpk and sample surface resistance Rs. Commissioning measurements on bulk Nb and Nb3Sn-Ta-Cu samples validated the system response over a broad range of frequency, temperature, and RF field. The extracted superconducting energy-gap parameters are consistent with the reported values for Nb and Nb3Sn, as well as with those obtained from single-cell cavity measurements. The commissioned system operates from 1.8 K to near the superconducting transition temperature of the sample, with accessible Bpk values from approximately 5 mT to a sample- and temperature-dependent heater-power-budget limit; a maximum field of 60 mT was demonstrated for bulk Nb at 400 MHz and 4 K. The combined relative standard uncertainties are 8.3% for Bpk and below 18% for Rs when r = PDC2/PDC1 is less than 0.9. The worst-case Bpk resolution at the 95% confidence level is approximately 1.35 mT, while the Rs resolution is below 1 nOhm at 10 mT and 2 K. These results establish the JLab QPR as a calibrated, multi-frequency platform with quantified measurement uncertainty for SRF material characterization.

physics.acc-ph↗

arXiv2024

Microscopic Examination of SRF-quality Nb Films through Local Nonlinear Microwave Response

The performance of superconducting radio-frequency (SRF) cavities is sometimes limited by local defects. To investigate the RF properties of these local defects, especially those that nucleate RF magnetic vortices, a near-field magnetic microwave microscope is employed. Local third harmonic response (P3f) and its temperature-dependence and RF power-dependence are measured for one Nb/Cu film grown by Direct Current Magnetron Sputtering (DCMS) and six Nb/Cu films grown by High Power Impulse Magnetron Sputtering (HiPIMS) with systematic variation of deposition conditions. Five out of the six HiPIMS Nb/Cu films show a strong third harmonic response that is likely coming from RF vortex nucleation due to a low-Tc surface defect with a transition temperature between 6.3 K and 6.8 K, suggesting that this defect is a generic feature of air-exposed HiPIMS Nb/Cu films. A phenomenological model of surface defect grain boundaries hosting a low-Tc impurity phase is introduced and studied with Time-Dependent Ginzburg-Landau (TDGL) simulations of probe/sample interaction to better understand the measured third harmonic response. The simulation results show that the third harmonic response of RF vortex nucleation caused by surface defects exhibits the same general features as the data, including peaks in third harmonic response with temperature, and their shift and broadening with higher microwave amplitude. We find that the parameters of the phenomenological model (the density of surface defects that nucleate RF vortices and the depth an RF vortex travels through these surface defects) vary systematically with film deposition conditions. From the point of view of these two properties, the Nb/Cu film that is most effective at reducing the nucleation of RF vortices associated with surface defects can be identified.

cond-mat.supr-con↗

arXiv2024

Thickness effect on superconducting properties of niobium films for radio-frequency cavity applications

Niobium-coated copper radio-frequency cavities are cost-effective alternatives to bulk niobium cavities, given the lower material costs of copper substrates and their operation in liquid helium at around 4.2 K. However, these cavities historically exhibited a gradual degradation in performance with the accelerating field. This phenomenon, not yet fully understood, limits the application of niobium thin film cavities in accelerators where the real-estate gradient needs to be maximized. Recent studies on niobium films deposited on copper using high power impulse magnetron sputtering (HiPIMS) technique show promising results in mitigating the performance degradation of niobium thin film radio-frequency cavities. This paper examines the effect of film thickness on the superconducting properties of niobium films deposited on copper using HiPIMS. The study provides insights into how the critical temperature, transition width, lower and upper critical fields, and critical current density vary with the film thickness. Increasing the thickness of niobium films deposited through HiPIMS is found to enhance superconducting properties and reduce densities of defects and structural irregularities in the crystalline lattice. This shows potential for enhancing overall performance and potentially mitigating the observed performance degradation in niobium thin film radio-frequency cavities. Additionally, the Ivry's scaling relation among critical temperature, thickness, and sheet resistance at the normal state appears applicable to niobium films up to approximately 4 $μ$m. This extends the previously confirmed validity for niobium films, which was limited to around 300 nm thickness.

physics.acc-ph↗