A strategy for the optimal choice of the magnetic sensors for the estimation of plasma parameters with fault tolerance in the ITER tokamak

In a tokamak, magnetic sensors are the primary diagnostics used to estimate the quantities controlled in feedback by the plasma current, position and shape controller. As a consequence, the number and position of the magnetic sensors has a large impact on the performance of the magnetic control system and on the tokamak configuration. In this paper, we consider the problem of choosing an optimal set of magnetic sensors for the ITER tokamak. As performance index we consider the capability of reconstructing the first plasma current density moments. Indeed a good reconstruction of these parameters guarantees also a good reconstruction of the parameters usually controlled by the magnetic control system (e.g. plasma-wall gaps, position of the current centroid, plasma current, etc.). The problem of choosing the best set of magnetic sensors is converted in a suitable convex optimization problem. Our approach allows us also to evaluate the level of redundancy in a given set of magnetic sensors and hence it gives information on how to behave in case of sensor faults. Numerical simulations are included in order to show the effectiveness of the approach.