Runaway electron beam stability and decay in COMPASS

Abstract

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.

Runaway electron seed formation at reactor-relevant temperature

“…Central to this concern is the expected large avalanche multiplication factor of any RE 'seed', which scales exponentially with plasma current (I P ) [7,8] and may be enhanced by interactions with high-Z impurities [9,10]. A robust program of experimentation on existing tokamaks is presently ongoing to tackle this challenge [11][12][13][14][15][16][17][18][19][20][21][22]. When considering the extrapolation of RE dynamics observed on present devices to fusion-grade plasmas, a key sensitivity expected from first principles is the pre-disruption electron temperature (T e ).…”

Overview of the COMPASS results *

“…The physics of RE interaction with mitigation species and materials, loss mechanisms, transport, interaction with naturally present or artificially created magnetic perturbations, and the RE impact on plasma-facing components was addressed in recent dedicated experimental campaigns in COMPASS [39][40][41]. The improved knowledge of RE behaviour was applied in the worldwide unique RE radial feedback control algorithm [42] and advanced beam detection with an extended set of dedicated diagnostic systems [43][44][45].…”

Generation and suppression of runaway electrons in MST tokamak plasmas

“…The RMP can introduce stochasticity in the plasma, enhancing electron transport and minimizing the energy that any runaway electrons can achieve. This was shown in ASDEX-Upgrade [15], COMPASS [16] TEXTOR [17,18], DIII-D [19], JT-60U [20], and RFX-mod [21], and has been investigated in MST reversed-field pinch plasmas [22]. It was also initially predicted for ITER [23,24], although more recent work suggests that it may not be a viable solution due to a limit on the current that can be driven in the ELM suppression coils [25,26].…”