Shafranov shift in the ‘electron root’ feature at the W7-AS stellarator

Plasma discharges at W7-AS in the ‘electron root’ regime show highly peaked electron temperature- and pressure profiles. A switch-off of the ECRH leads to a fast temperature drop with time constants of less than 1 ms and a corresponding change of the Shafranov shift in the center of the plasma. For a temporally resolved comparison between calculated and measured temperature profiles, a fast method of gaining the mapping of the magnetic field to the radius along the ECE sightline is presented. Using these mappings to compare measured ECE signals with modeled temperatures (assuming plausible profiles of the radial electrical field), the time constant for the decay of the electric field after a ECRH power switch-off was found to be 0.6 ms.