Abstract
The quadrupole resonator (QPR) is a dedicated sample-test cavity for the RF characterization of superconducting samples in a wide temperature, RF field, and frequency range. Its main purpose is high resolution measurements of the surface resistance with direct access to the residual resistance, thanks to the low frequency of the first operating quadrupole mode. In addition to the well-known high resolution of the QPR, a bias of measurement data toward higher values has been observed, especially in higher harmonic quadrupole modes. Numerical studies show that this can be explained by parasitic RF losses on the adapter flange used to mount samples into the QPR. Coating several micrometers of niobium on those surfaces of the stainless steel flange that are exposed to the RF fields significantly reduced this bias, enabling a direct measurement of a residual resistance smaller than 5 nΩ at 2 K and 413 MHz. A constant correction based on simulations was not feasible due to deviations from one measurement to another. However, this issue is resolved given these new results.
Highlights
Superconducting radio frequency (SRF) cavities are a key component for many state-of-the-art particle accelerators
We presented an extensive numerical study quantifying the impact of parasitic losses at the normal conducting adapter flange for quadrupole resonator (QPR) measurements together with experimental data
These parasitic losses cause a systematic bias of surface resistance measurement data, mainly affecting the extracted residual resistance
Summary
Superconducting radio frequency (SRF) cavities are a key component for many state-of-the-art particle accelerators. The quadrupole resonator (QPR) is a dedicated sample-test cavity, providing high resolution measurements in a wide parameter space of temperature and RF field at three different frequencies.. With the transmitted power Pt measured at the pickup antenna with coupling Qt. The calibration constant c is known from simulations, giving the ratio of stored energy to the integral of the RF field on the sample surface. The calibration constant c is known from simulations, giving the ratio of stored energy to the integral of the RF field on the sample surface With this calorimetric compensation technique, any heating occurring in the thermal system of the sample assembly is interpreted as the surface resistance of the sample. We provide measurement data of a niobium film sample tested first using a standard stainless steel adapter flange and exhibiting a residual resistance of about 29 nΩ in the first quadrupole mode. The observed reduction of measured RS is in good agreement with numerical simulations, proving a significant suppression of the bias and boosting the accuracy of measurement data to an unprecedented level
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