An adaptive method to identify the plasma magnetic contour from magnetic and polarimetric measurements

The identification of a plasma magnetic contour can be cast as an inverse problem in which a set of equivalent currents, representing (the effect of) the actual plasma current distribution, have to be determined in order to best fit a set of external measurements. The diagnostic systems are typically provided with arrays of magnetic measurements, including both flux and field probes, positioned outside the plasma chamber, and some non magnetic measurements, such as motion Stark effect or multichord FIR polarimetry. In the framework of the method discussed, the basic external (magnetic) measurements have been integrated with internal (polarimetric) measurements to improve the identification process and gain information about the internal plasma current profile. The inverse problem can be formulated as the pseudo-inversion of the transfer matrix linking the unknown equivalent currents to the magnetic and polarimetric measurements. In this paper, the equivalent currents method is improved by means of a procedure that adaptively tries to allocate the EC to obtain the best estimate of the plasma contour. The adaptive strategy is presented in detail and its performance evaluated against two ITER equilibrium plasma configurations.