W7-X magnetic diagnostics: Performance of the digital integrator

Abstract

In long pulse magnetic fusion experiments the use of conventional magnetic diagnostics is a challenge due to the so-called integrator problem. Magnetic sensors based on pickup coils and flux loops measure the time derivative of magnetic fluxes, and therefore, such signals have to be integrated in time to obtain flux signals. Although alternative methods of magnetic field measurement exist, magnetic flux measurements cannot be substituted due to the required measure of the vector potential integral along the area boundary. In addition, many of the alternative methods are not suited for usage in the harsh environment of a burning plasma experiment. An integrator for 1800s pulse operation in Wendelstein 7-X has been developed based on a chopper input stage and digital integration. Tests have been performed using a 25 turn diamagnetic loop at the WEGA stellarator, which has the capability of running long discharges. An important integrator property, in particular, for coping with radiation induced effects as they will occur in ITER, is the common mode rejection. At present, the integrator appears to be stable within 100μVs∕1000s when the common mode input voltage varies within ±1V.