Runaway Electron Beam Stability and Decay in COMPASS

The paper presents two scenarios used for generation of runaway electron (RE) beam in the COMPASS tokamak with a focus on the decay phase and control of the beam. The first scenario consists of argon massive gas injection (MGI) into the current ramp-up phase leading to a disruption accompanied by runaway plateau generation. In the second scenario, injection of smaller amount of gas is used in order to isolate runaway electron beam from high temperature plasma. The current control and radial and vertical position feedback control performances in the second scenario were experimentally studied and analysed. The role of RE energy in the radial position stability of the RE beam seems to be crucial. A comparison of the decay phase of RE beam in various amounts of Ar or Ne was studied using AXUV tomography and HXR intensity measurement. Argon clearly leads to higher HXR fluxes for the same current decay rate compared to neon, while radiated power based on AXUV measurements is larger for Ne in the same set of discharges.