Superconducting screening of flux linked between a SQUID pick-up coil and a coaxial cylindrical sample

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Abstract
An expression is derived for the screened magnetic flux linked between a semi-infinite source solenoid and a coaxial loop, for small axial separations. The screen is a coaxial, infinitely long, superconducting cylinder. For large axial separations the equations of Osterman and Williamson (1983) are appropriate. Both expressions are integrated to give the flux linked between two semi-infinite solenoids, and the instrument function for a second-order gradiometer is constructed by superposition. A comparison of the screened instrument function with experimental measurements yields deviations of less than 2% compared with 6% for the unscreened function.