COMPENSATION TECHNIQUES FOR HTS-RF-SQUID MAGNETOMETERS OPERATING IN UNSHIELDED ENVIRONMENTS

We have developed two methods of compensating environmental disturbanc es applicable to high- and low-temperature superconducting quantum int erference device (SQUID) systems, operating in a magnetically unshield ed environment. For testing, we used first- and second-order axial ele ctronic gradiometer setups with rf SQUID magnetometers operating at 77 K, with different baselines between 1.8 and 8 cm. The magnetometers w ere single-layer washer rf SQUIDs, with bulk or thin-film magnetic Bur concentrators and transformers in flip-chip geometry. The tested meth ods resulted in disturbance compensation levels comparable with those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in maximum gradiometric sensiti vities of 13.5 fT/cm root Hz for first-order and 22 fT/cm(2) root Hz f or second-order compensation, with baselines between 7 and 8 cm down t o 10 Hz. Common mode rejection was balanced to, better than 10(4) for homogeneous fields, and better than 200 for gradient fields, with seco nd-order compensation. Magnetically unshielded measurements of biomagn etic signals with different baselines were demonstrated. (C) 1998 Publ ished by Elsevier Science Ltd. All rights reserved.