Magnetic control strategies to reduce first wall heat loads in ITER

Abstract

First wall heat load control is one of the functionalities that will be implemented in the ITER Plasma Control System to maintain plasma heat fluxes and the consequent first wall (FW) temperatures within tolerable limits during a plasma discharge. As part of this strategy, the magnetic controller for the plasma shape will act as an inner loop for first wall heat load control, guaranteeing a first line of defense. In this work we propose to use a FW temperature controller (FWTC) designed as a virtual actuator, modifying the reference plasma equilibrium in order to limit the excess of FW temperature in real-time. A fixed number of plasma equilibrium variations, or FWTC patterns, is first optimized to achieve a plasma heat flux reduction over several FW regions at a reasonable poloidal field coil current control cost, including the second x-point area. Subsequently, an on/off multi-input–multi-output FW temperature control with hysteresis is designed to request a linear combination of FWTC patterns to the magnetic controller. The control scheme implements an integral action with limited authority on gap and active coil current requests, including a hysteresis logic to avoid chattering. The control problem is particularly challenging because of the nonlinear relationship between plasma position and FW heat fluxes featuring magnetic shadowing of neighboring first wall panels. The flexibility of the presented algorithm is illustrated in control-oriented simulations, covering both the ITER Start of Research Operations (controlling the predicted temperature of the inertially cooled FW) and the planned Deuterium-Tritium campaign (controlling the instantaneous temperature of the actively cooled FW).