H-infinity Estimation of Eddy Currents in a Tokamak

The estimation of induced “eddy” currents in the metallic structures surrounding the plasma chamber of a tokamak can be very important to have a precise estimation of plasma quantities as toroidal current and shape. In facts, plasma shape identification is mainly based on magnetic measurements (fields and fluxes), which are affected by the presence of these currents and subject to the usual sensor noise affecting electromagnetic probes and to drifts induced by the nuclear environment. The presence of a dynamic observer may help to wash out such persistent contribution and isolate the effect of eddy currents on magnetic measurements. Moreover the on-line estimation of the eddy currents allows to have an on-line prediction of the mechanical stresses induced by the electromagnetic loads in the metallic structures during transients. Although the eddy current dynamics can be well modeled by a linear system, even in the tokamak axisymmetric approximation, the high number of states, the uncertainties, and the process and measurement noise, makes the estimation problem challenging. The present paper shows how the use of a H-infinity observer can mitigate these problems.