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.

Biomagnetism: The First Sixty Years

“…They were even used to assess an axon damaged at its end [50]. A limitation of this technique is that the nerve needs to be cut and threaded through the toroid, but Mark Leifer and Wikswo developed an openable toroid that avoided any need for dissection [51]. Toroids have been used to evaluate the continuity of a nerve during surgery [52,53].…”

A Superconducting Quantum Interference Device Magnetometer with a Room-Temperature Pickup Coil for Measuring Impedance Magnetocardiograms

“…11) Although the magnetometer (current probe) was optimized, 12) the noise level (6 pA Hz À1=2 at 1 kHz) of the probe was higher than that of a semiconductor amplifier probe (2.8 pA Hz À1=2 at 1 kHz). 13) Thus, the sensitivity of a current probe using a SQUID in the low frequency band is insufficient for measuring the action potential.…”

Magnetic Field Mapping with a Squid Device

“…Two excellent papers are available that discuss various aspects of the problem. The first concentrates on the design of room-temperature pickup coil systems 6 and optimization of the SIN ratio, while the second 7 discusses the problems of matching the superconducting flux transformer coil to the SQUID.…”