Extracting superconducting parameters from surface resistivity by using inside temperatures of SRF cavities

TL;DR

This paper presents a method using computer simulations to accurately determine the inside surface temperature of SRF cavities, enabling precise extraction of superconducting parameters from surface resistivity measurements, especially at high RF-fields.

Contribution

It introduces a combined simulation approach to accurately find the inside temperature, improving parameter extraction over traditional approximations.

Findings

Assuming surface temperature equals helium bath temperature causes at least 10% error at high RF-fields.

The combined HEAT and SRIMP code effectively determines the inside temperature for better parameter analysis.

Significant errors occur when neglecting the temperature difference at high RF-fields.

Abstract

The surface resistance of an RF superconductor depends on the surface temperature, the residual resistance and various superconductor parameters, e.g. the energy gap, and the electron mean free path. These parameters can be determined by measuring the quality factor Q0 of a SRF cavity in helium-baths of different temperatures. The surface resistance can be computed from Q0 for any cavity geometry, but it is not trivial to determine the temperature of the surface when only the temperature of the helium bath is known. Traditionally, it was approximated that the surface temperature on the inner surface of the cavity was the same as the temperature of the helium bath. This is a good approximation at small RF-fields on the surface, but to determine the field dependence of Rs, one cannot be restricted to small field losses. Here we show the following: (1) How computer simulations can be used to determine the inside temperature Tin so that Rs(Tin) can then be used to extract the superconducting parameters. The computer code combines the well-known programs, the HEAT code and the SRIMP code. (2) How large an error is created when assuming the surface temperature is same as the temperature of the helium bath? It turns out that this error is at least 10% at high RF-fields in typical cases.