Instrumentation & Measurement Magazine 24-9 - 19

point out (Fig.1) that both fc
and ρff
are instrumental in assessing
the suitability of SCs for these kinds of applications. Fig. 5
shows a summary of such framework for several potentially
interesting superconductors, including the results obtained on
S1, S2, S3, S4 and S5 samples. As an example, FeSe0.5
Te0.5
(S4),
belonging to the new family of the iron based SCs still unexplored
for high-frequency applications, exhibits a large fc
also a particularly large ρff due to its complex electronic states
structure, while MgB2 (S5) is only slightly better than Nb3
How ρff can be tuned (in particular on FeSe0.5
but
Sn.
Te0.5) is still a matter
of fundamental research. As it can be seen from Fig. 5, a
reliable determination of both fc
and ρff
is essential. From these
observations and from the experimental results, it emerges
that none of the SC candidates for RF applications in a high dc
magnetic field are fully optimized yet: by limiting the discussion
to Nb3
Sn and YBCO, while Nb3Sn would benefit from a
higher density of PCs, YBCO requires PCs that affect less the
microscopic scattering mechanisms.
Conclusions
The surface impedance Z measurement of superconductors
(SCs) in the mixed state is a fundamental step in the test and development
of materials with always increased performances.
This type of measurement provides useful information
both in view of RF technological applications, such as the
aforementioned beam screens for particle accelerators or the
realization of haloscopes for dark matter particles detection
experiments, and for scientific aspects related to the study of
the physics of SCs. Given the emerging technological interest,
a metrological study of the performance of these techniques is
essential to obtain reliable measurements. In this framework,
multifrequency dielectric resonators sided with suitable data
analysis procedures are powerful tools for a correct evaluation
of the vortex motion parameters in SCs. A comparison of the
parameters as measured in different materials allows margins
for improvement which can be achieved through the engineering
of the materials to be identified.
It should be mentioned that, in particular for particle accelerators
beam screens, studies such as the present one are
preliminary since only the linear response is addressed. Measurements
of Z at frequencies closer to operation (~ 1 GHz)
and, more importantly, at higher RF power will be needed to
assess the feasibility of superconducting beam screens. In this
framework, the need to perform accurate and precise Z measurements
at cryogenic temperatures and high frequency, high
dc fields and large RF powers, is a challenging metrological
journey that has only begun.
Acknowledgement
This work has been partially carried out within the framework
of the EUROfusion Consortium and has received funding from
the Euratom Research and Training Programme 2019-2020
under grant agreement No 633053. The views and opinions expressed
herein do not necessarily reflect those of the European
Commission. The work has been also partially supported by
MIUR-PRIN project " HIBiSCUS " - grant no. 201785KWLE.
December 2021
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IEEE Instrumentation & Measurement Magazine
19
The authors warmly thank T. Spina, from the Fermi National
Accelerator Laboratory (USA), and R. Flükiger, from the University
of Geneva (Switzerland), for having provided the
Nb3
Sn sample; G. Celentano, V. Pinto, A.A. Armenio, from the
Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo
economico sostenibile - ENEA (Italy), for the YBCO samples;
V. Braccini, from Consiglio Nazionale delle Ricerche, SuPerconducting
and other INnovative materials and devices institute,
CNR-SPIN (Italy), for the FeSe0.5
Te0.5 samples; and A. Crisan,
from the National Institute of Materials Physics (Romania), for
the MgB2
samples.
https://indico.cern.ch/event/845054/

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