Determination of time constant at different positions above superconducting cables

摘要

We suggest a simple method for detecting the existence of additional time constants in twisted superconducting structures, in addition to the "main" time constant due to coupling currents between the superconducting strands. The method is based on the simplified procedure for the determination of time constant (which is proportional to the coupling or eddy current losses at low frequencies) from the measurements of the magnetic field above the centre of superconducting cables. Very good agreement was achieved between experimental results and the simplified theory. We show, that measuring the characteristic time t_m, at which the maximum of the difference between the magnetic induction and the induction of the applied magnetic field appears, at another points of the cable, enables to decide whether an additional time constant in the structure is determining the flux entry and the coupling or eddy current losses. We derive the relation between t_m and the time constant % of superconducting cables or normal metals at different positions above the cable. The maximum signal for the relative difference of the magnetic field at the measuring point and the applied magnetic field, crucial for the practical applicability of the method, is also given. The method can be useful also for determining whether some inhomogeneities are present in the structure.