Two-port CW measurements on RF cavities: Notes on self-consistency assessment and indirect methods

TL;DR

This paper introduces indirect methods and a graphical self-consistency check for CW RF cavity measurements, improving accuracy in power dissipation and quality factor estimation amid systematic errors.

Contribution

It derives new formulae for indirect calculation of cavity parameters and evaluates a graphical method for self-consistency, addressing measurement uncertainties.

Findings

Derived formulae for indirect cavity parameter estimation.

Validated a graphical self-consistency assessment method.

Applied methods to superconducting cavity cold tests.

Abstract

In the case of a radio-frequency (RF) cavity with a mismatched input coupler, a direct calculation of the power dissipation in the cavity and the intrinsic quality factor from continuous-wave (CW) measurements may have uncertainty due to systematic errors. Formulae for an indirect calculation of these quantities are derived for the case of a cavity with two couplers of fixed coupling strength. In this approach, the signal from the pickup coupler is used to infer the amplitude of the "emitted wave" from the input coupler. A graphical method for self-consistency assessment is evaluated. The impact of frequency offsets is considered. Applications of these methods are presented, drawing on cold tests of superconducting cavities produced for the Facility for Rare Isotope Beams.