Mitigation of parasitic losses in the quadrupole resonator enabling direct measurements of low residual resistances of SRF samples

TL;DR

This paper presents a method to reduce parasitic RF losses in the quadrupole resonator, enabling more accurate measurements of very low residual resistances in superconducting RF samples.

Contribution

The study introduces niobium coating on flange surfaces to mitigate parasitic losses, improving the accuracy of residual resistance measurements in the QPR.

Findings

Parasitic RF losses cause measurement bias at higher modes.

Niobium coating significantly reduces parasitic losses.

Residual resistance below 5 nΩ measured at 2 K and 413 MHz.

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 are 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. Besides the well-known high resolution of the QPR, a bias of measurement data towards higher values has been observed, especially at 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 micrometer 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 n$\Omega$ 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.