Extended theory of diamagnetic measurements with account of the wall currents in tokamaks

Abstract

Effect of the poloidal current induced in the tokamak vessel wall during transient events is explicitly incorporated into the formulas for the diamagnetic signal. The analysis is based on the equation linking this current to two drivers, the evolving plasma inside and the varying toroidal field produced by currents outside. The wall itself is considered resistive and serves as a damper. The main part of the described analytical approach is applicable to any situation of interest, but the emphasis is put on the case with diamagnetic loop placed on the inner side of the wall, as it is planned in ITER [R. Fresa, et al., Fusion Eng. Des. 100, 133 (2015)]. Such a loop will actually measure the voltage equal to the poloidal current induced in the wall multiplied by the wall poloidal resistivity. With this current as a main detectable quantity, a refined algorithm is needed for determining the plasma energy from the magnetic measurements. Theory-based recommendations are given to improve the diagnostics, especially for the rapid events like thermal quenches and ELM (edge localized mode) crashes.