Measurement of plasma centroid position on ISTTOK using cleaned magnetic signals

ISTTOK [1] is a large aspect ratio tokamak operated in Lisbon. One of the most important features of this machine is the capability of performing AC discharges that are much longer than the standard DC ones [1]. On the other hand, especially after the plasma current reversal, the usual measurement of the plasma centroid by magnetic diagnostics was not reliable since the signals were corrupted by the eddy currents induced by the variations of poloidal field coil currents. Up to now, an alternative technique of the plasma centroid position measurement based on the plasma potential value provided by Langmuir probes has been used. The drawback of this alternative method is that the results can be affected by perturbations of the plasma edge. To enable the usage of more standard method based on magnetic diagnostics, a simple, efficient and real-time applicable method of magnetic signal cleaning from the eddy currents effect was proposed. It is based on a fitted state-space model describing the dependency of the magnetic signal on the currents in the poloidal field coils taking into account the conductive structures surrounding the plasma. The fit was performed based on experimental data. Afterwards, an algorithm described in [4] for the plasma centroid position measurement was modified to deal with the ISTTOK geometry.