Improved Superconducting Direct Current Meter for 25–50 kA

Abstract

An accurate meter to measure direct currents in closed superconducting circuits exposed to high magnetic fields js presented. A toroidal superconducting (sc.) transformer, which consists of a sc. Rogowski pick-up coil and sc. compensation windings, encloses the unknown current The second part of the sc. transformer circuit is a sensing coil around a Hall probe, both perfectly shielded from AC and DC magnetic stray fields. The Hall voltage is amplified and fed back to the compensation current in order to keep the Hall voltage constant. This small compensation current is measured using a shunt and forms an accurate measure for the unknown current. It is used in a second feedback loop to control the unknown current. An additional compensation circuit corrects for undesired magnetic coupling between the current source and the toroidal pick-up coil. With this arrangement a short term accuracy of 0.1{\%} of the full range is obtained. The current is extremely stable, the drift is below 0.25 A per minute. This improved current meter facilitates accurate determination of voltage-current characteristics of superconductors, carrying currents in the range of 25--50 kA and being exposed to high magnetic fields.