Evolution of the runaway electron decay in response to the application of magnetic perturbations, mitigation gases and active antenna

The COMPASS tokamak provided unique conditions to study Runaway Electrons (RE) physics in the ITER-like geometry. Twelve experimental campaigns were dedicated to identify limiting thresholds for safe termination or efficient suppression of RE beam [1] and to thoroughly explore the possible advantages of alternative mitigation strategies. High flexibility and reproducibility of COMPASS RE scenarios allowed the execution of extensive scans devoted, for example, to decoupling effects of individual factors acting on RE dynamics. The studied factors include resonant magnetic field perturbations (RMP), radiation drag, instabilities, and the newly developed unique RE radial position feedback [2]. The main principle of the RE radial feedback was successfully used as a fast and robust method to obtain information about the RE energy in parallel to HXR radiation and unique calorimetry measurements. These methods are used to show the impact of different mitigation materials, externally triggered RMPs and the feedback itself on the RE beam energy and its current decay rate. The experimentally observed effects of RMPs are supported by a full orbit RE tracer developed at COMPASS, MARS-F [3] and ORBIT [4] codes. First tests with an active launch of low power 0.5 GHz waves into a RE populated low density plasma were performed. Preliminary results from measurements of electromagnetic waves in the 0.1-1.5 GHz range during this scenario are presented.