Induced voltages and currents in copper and stainless steel core mineral insulated cables due to radiation and thermal gradients

To reconstruct the shape and position of the plasma boundary of ITER, in-vessel magnetic coils will be employed, which will be exposed to radiation and temperature gradients. A study was undertaken to assess their impact on magnetic coil measurements and on other tokamak diagnostics employing mineral insulated (MI) cables. Thermally induced electromotive force voltages in MI cables with Cu and stainless steel (SS) cores were measured using a scanning oven, establishing a well-defined temperature profile. Next, the MI cables were irradiated in the BR2 reactor, with the cables guided through a double-wall tube to create significant temperature gradients during the irradiation. The core-to-sheath currents were measured and interpreted. No correlation was observed between the core-to-core-voltages and the core-to-sheath current asymmetry. Significant core-to-core voltages were observed, which could be interpreted as due to the Seebeck effect with increasing Seebeck coefficients during the irradiation. The coefficient of the Cu core cable increased proportionally to the neutron fluence, which could be attributed to transmutation of Cu to Ni. For the SS core cable the coefficient was found to saturate at a local fluence of about 1019 n/cm2 and depended on the local temperature during irradiation.