Improvement of 2D plasma identification by preliminary treatment of 3D measurement

Axially-symmetric plasma equilibria are frequently altered by several 3D elements including active coils, vessel and first wall geometry [1]. Note that also flux and field probes, fastened to vessel or mechanical supporting structures, are influenced by any lack of symmetry. Plasma position and shape identification are mainly based on magnetic measurements. As a consequence, it is highly advisable to perform a preliminary signal treatment aimed to identify and reject, as much as possible, spurious effects due to possible lack of symmetry and, in addition, trying to recover equivalent measurements in axially-symmetric configuration. The paper deals with this problem by introducing, in order to describe the actual probes position, a description base for possible 3D geometries, characterized by small deformations with respect to nominal (axially-symmetric) configuration, examples being shifts or rotations along orthogonal axes. The projection of the measured data in the deformation base provides hints about possible deformations of the geometry and then an equivalent set of measurements is recovered by subtracting the effect of deformation. In addition, the procedure takes advantage from a comparative analysis between measurements with or without plasma. The paper will present a possible problem formulation and its implementation in a parallel computing environment, suited for real time identification [2]. A parametric analysis will be used to show the actual relevance of the issue in practical applications.