Improvement of diamagnetic loop measurements in stellarators using digital signal processing techniques

Plasma energy in fusion devices is usually measured by determining the diamagnetically excluded flux, which is proportional to the energy, i.e. the normalized plasma pressure beta. Flux changes are measured by means of magnetic loops encircling the whole plasma in a poloidal cross-section. Since the magnitude of the diamagnetic flux change is of the order of the plasma beta the influence of external field changes must be suppressed to the order of 10−4. Usually, compensation coils are used to achieve a rough suppression. In this paper a refined compensation based on a linear signal model using digital signal processing is proposed. The method was implemented and tested at the WENDELSTEIN W7-AS stellarator. An example of an enhanced diamagnetic signal is shown.