Test cavities to characterize superconductor samples are of great interest for the development of materials suitable for superconducting radio frequency (SRF) accelerator systems. They can be used to investigate fundamental SRF loss mechanisms and to study the material limitations for accelerator applications. Worldwide, this research is based on only few systems that differ in operating frequency, sample size and shape, and the accessible parameter space of frequency, temperature, and RF field strength. For useful performance predictions in future accelerators, it is important that the operating parameter range is close to that employed in accelerating systems. Since 2014, the Helmholtz-Zentrum Berlin has operated such a system built around a redesigned Quadrupole Resonator (QPR). It is based on a system originally developed at CERN. Important new design modifications were developed, along with new measurement techniques and insight into their limitations. In the meantime, an increasing number of laboratories are adopting the QPR for their measurement campaigns. This paper provides a comprehensive overview of the state-of-the-art, the wide spectrum of measurement capabilities, and a detailed analysis of measurement uncertainties, as well as the limitations one should be aware of to maximize the effectiveness of the system. In the process, we provide examples of measurements performed with Nb3Sn and bulk niobium.

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.

No 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 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.

No abstract