Runaway electrons (REs) as one of the yet unsolved threats for ITER and future tokamaks are a topic of intensive research at most of the European tokamaks. The experiments performed on COMPASS are complementary to the experiments at JET and MST (Medium-Size Tokamaks), building on the flexibility of the diagnostics setup and low safety constraints at this smaller device. During the past couple of years two different scenarios with the RE beam generation triggered by gas injection have been developed and investigated. The first one is based on Ar or Ne massive gas injection (MGI) into the current ramp-up phase leading to a disruption accompanied by runaway plateau generation [1], while the second uses smaller amounts of gas in order to get runaway current dominated plasmas [2]. The successful generation of the beam in the first scenario depends on various parameters, including the toroidal magnetic field. The generated beam is often radially unstable, and the stability seems to be a function of various parameters, including the value of current lost during the CQ. The second scenario is much more quiescent, with no observable fast current quench and it is highly reproducible. This allows to reasonably diagnose the beam phase and also to apply secondary injections or resonant magnetic perturbations (RMP) to assist the decay of the beam. In this regard, interesting results have been achieved using secondary deuterium injection into a runaway electron beam triggered by Ar or Ne. The current of the RE beam can be controlled at a fixed value, however only using relatively high loop voltage. The radial position is not fully controlled and the request for the stabilising field is changing independently on plasma current which implies that the RE energy plays a key role in correct approach to beam position control. The experiments with elongated beams were also carried out. Last but not least it seems that Ar and Ne behave differently in terms of radiated power and HXR intensity during the beam decay. 1.
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