Extraction of the plasma current contribution from the numerically integrated magnetic signals in ISTTOK

ISTTOK tokamak operates in AC mode allowing to have consecutive cycles of positive and negative plasma current. The plasma centroid position is estimated from the reconstruction of the signals obtained from a poloidal array of twelve magnetic probes. In the previous setup, this reconstruction relied on a cylindrical approximation algorithm in which the error fields generated by active coils and currents in the passive structures were not taken into account, leading to some centroid position inaccuracy. Although this approach has been able to produce flat top current AC discharges with reasonable reliability, a better solution was implemented as a preferred plasma position estimator. The signals are currently corrected subtracting the contribution from external magnetic fields following a method that relies on state-space models which use data collected from a systematic vacuum calibration procedure. Also, the plasma centroid position is estimated using a multi-filament current model. Furthermore, the recent implementation of numerical integrators in the real time ATCA based data acquisition system substantially improved the real time conditioning of the magnetic signals measurements. First results demonstrate that this new approach delivers a more reliable estimation of the plasma current centroid position with noticeable improvement on control performance, which is paramount to the success ratio of the AC plasma current switching.