Characterizing materials for superconducting radiofrequency applications—A comprehensive overview of the quadrupole resonator design and measurement capabilities

Abstract

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.

An Innovative Approach of Surface Polishing for SRF Cavity Applications

“…As mentioned earlier, the best way to evaluate surface quality after metallographic polishing is to measure surface resistance under radio-frequency waves at cryogenic temperatures. However, this type of test is expensive, technically difficult, and shows a very low availability [32][33][34][35]. The final optimization of polishing parameters is not pos- Before producing a new sample for the RF test at a cryogenic temperature additional studies on Q-disease were conducted on the samples after the metallographic procedure.…”

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

“…For further details, the reader is referred to Ref. 4. The QPR sample chamber assembly consists of a top-hat-shaped superconducting part and a stainless steel adapter flange as shown in Fig.…”

“…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. [1][2][3][4] With a first operating quadrupole mode at about 415 MHz, R BCS at 2 K is typically smaller than 2 nΩ, enabling direct measurements of Rres. The operational experience of the QPR indicates a bias of measurement data toward systematically overestimated R S , limiting the measurement accuracy at low R S and hence especially impacting Rres.…”

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

“…In the following, the QPR design is described only briefly. For further details the reader is referred to 4 . The QPR sample chamber assembly consists of a top-hatshaped superconducting part and a stainless steel adapter flange as shown in Fig.…”

“…Hence, for R&D on materials, coatings or surface treatments aiming at application in SRF cavities, precision measurements of the surface resistance are required. 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 [1][2][3][4] . With a first operating quadrupole mode at about 415 MHz, R BCS at 2 K is typically smaller than 2 nΩ, enabling direct measurements of R res .…”