Runaway electron experiments at COMPASS in support of the EUROfusion ITER physics research

Abstract

The role of the COMPASS tokamak in research of generation, connement and losses of Runaway Electron (RE) population is presented. Recently, two major groups of experiments aimed at improved understanding and control of the REs have been pursued. First, the eects of the Massive Gas Injection (MGI, ∼ 10 21 Ar/Ne particles) and impurity seeding (∼ 10 18 particles) were studied systematically. The observed phenomena include generation of the post-disruption RE beam and current conversion from plasma to RE. Zero loop voltage control was implemented in order to study the decay in simplied conditions. A distinctive drop of background plasma temperature and electron density was observed following an additional deuterium injection into the RE beam. The loop voltage control the parametric dependence of the current decay rate dI/dt can be studied systematically and possibly extrapolated to larger RE experiments at COMPASS in support of the EUROfusion research 2 facilities. Second, recent results of experiments focused on the role of the magnetic eld in physics of RE were analysed. In this contribution, special attention is given to the observed eects of the Resonant Magnetic Perturbation (RMP) on the RE population. The benets of the RE experiments on COMPASS was reinforced by diagnostic enhancements (fast cameras, Cherenkov detector, vertical ECE etc.) and modelling eorts (in particular, coupling of the METIS and LUKE codes).

Runaway electron beam stability and decay in COMPASS

“…Despite the fact that ITER ELM coils are probably too weak to affect the RE orbits in the confined beam as they are too far from the plasma, these experiments will contribute to the understanding of the beam behavior in the perturbed fields and to validation of theoretical predictions and models. These results as well as other experiments related to the two scenarios described below are published in [2] that is currently in press. The most urgent question is what situation is considered to be a successful beam mitigation and how to reliably achieve it.…”

“…From the ITER point of view, the worst danger is that the beam can be lost in very localized manner and concentrated flux of high energy particles may damage first wall including the cooling elements or other important structures exposed directly to the plasma. The unmitigated RE beam termination may occur on much shorter timescales than a typical plasma disruption, at COMPASS, terminations of extremely low-density plasmas in the slide-away regime on microsecond timescales were recorded [2] due to radial position instability. These terminations also caused very localized hotspots.…”

“…1 where Bt=1.15 T. The scenario includes carefully tuned fuelling waveform, injection at early times (low currents), optimised position reference and argon MGI with at pressure 1-3 bars and short valve opening time. Despite the relatively low reproducibility, a systematic analysis brought very interesting results [1,7], more recently the crucial role of the toroidal magnetic field has been confirment in a dedicated scan [2]. No RE beams were created in the very same scenario at fields lower that 1.1 T despite the fact that the beam was reliably produced at higher fields.…”

“…Due to the low reproducibility of ramp-up scenario, which would be problematic especially in the RE beam decay experiments, an alternative, more quiescent scenario has been developed. In these discharges, the current flattop is reached and the fueling is turned off which leads to the decay of the thermal plasma density and a rise of the RE current in several tens of ms. Ar or Ne injection is then introduced using piezo valve puff or MGI (which leads to much faster decay, see [2]). With a short delay (5 ms) after the injection, the derivative of the current in the primary windings (which creates the external loop voltage) is set to zero, see FIG.…”

“…2 for the detailed overview. During this stage, additional puffs or RMPs [2,9] may be applied to investigate the influence on the natural decay of the beam. The puff causes the quench of the thermal plasma while the REs are practically unaffected, the thermal quench (TQ) is very slow in the case of piezo gas-puff lasting roughly 5 ms (see the profile evolution in FIG.2) while in the case of MGI, it seems to last much less than 1 ms.…”