Measurement and calculation of residual magnetic field in a Bi2223/Ag magnet

Residual magnetic field in a high temperature superconductor (HTS) magnet is a factor which should be avoided when the field of the magnet is being operated under a high uniformity and high accurate control. In this paper, the residual field in a Bi2223/Ag pancake magnet at 77 K is systematically studied. The amplitude of the residual field as a function of maximum charging current and the relaxation of the residual field are measured. For a magnet with an inner diameter of 60 mm, an outer diameter of 80 mm and a height of 30 mm, the maximum residual field is about 14.5 G after ramping the charging current up to 15 A, followed by dropping the current to zero. A simple model is proposed, from which the residual field can be estimated as along as the configurations of the magnet and the critical current (I-c) of the original short sample are given.