Optimization of a clip-on SQUID current probe

Abstract

This paper presents details of an unusual clip-on nanoammeter. The device utilizes a toroidal magnetic flux sensor which is split to allow its placement around a continuous current carrying filament, and is coupled to a sensitive superconducting current amplifier. Optimization formulas allow the design of an instrument having the greatest possible signal-to-noise ratio consistent with the physical volume available for the sensing element. A prototype exhibited 125-nA current sensitivity at 25 Hz with a noise level of 40 nA/Hz1/2. The performance of this instrument verified the predictions of the theory. The optimization analysis for this probe demonstrated that the signal-to-noise ratio could have been doubled by a slight change in toroid dimensions. It was shown that SQUID noise is negligible for this instrument, justifying the assumption of a noiseless amplifier in the original calculation. Finally, the conditions under which an impedance matching transformer can reduce the importance of the SQUID noise relative to the Johnson noise are derived, and the required inductances are given.