Design and experimental validation of an eddy currents probe

Eddy currents induced in passive structures are of paramount importance for the functioning of a tokamak. First of all, they are the key ingredient in slowing down plasma instabilities from Alfvén to electromagnetic time scales, allowing active magnetic control and hence the actual possibility of having a long-lasting discharge. Moreover, during a disruption, large eddy currents are induced in the vessel and other conducting structures, giving rise to significant electromagnetic forces that may be even challenging for the integrity of the device. It is hence very important to have a reliable indication of such eddy currents; this is usually done in an indirect way, by measuring the magnetic field in proximity of the vessel and using suitable modelling assumptions to retrieve the value of current density. In this paper we present the design and experimental validation of a probe directly measuring eddy currents flowing in a conductor. The proposed probe is a dipole-like sensor, in contact with the conductor under analysis. In the past, dipole-like sensors have been already proposed for current measurement, especially in MRI applications, but also in fusion devices. An experimental campaign has been carried out on a dedicated test facility, designed and built at the LAMI Lab, Univ. of Cassino. Magnetic field time derivatives of the order of 100 T/s and induced current densities of the order of several MA/m2 on EUROFER sheets have been achieved, thus demonstrating experimentally that the proposed probe is a viable solution for the measurement of eddy currents in ITER.