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 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.
The COMPASS upgrade tokamak (Panek et al 2017 Fusion Eng. Des. 123 11–16) will be a tokamak of major radius R 0 = 0.894 m with the possibility to reach high field (B t ∼ 5 T) and high current (I p ∼ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4 MW of neutral beam injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius R tan = 0.65 m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast ions in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver (Jaulmes et al 2014 Nucl. Fusion 54 104013). We have implemented a Coulomb collision operator similar to that of NUBEAM (Goldston et al 1981 J. Comput. Phys. 43 61) and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code (Artaud et al 2018 Nucl. Fusion 58 105001) yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code (Cunningham 2013 Fusion Eng. Des. 88 3238–3247) calculates the equilibrium and a Biot–Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on Suzuki et al (1998 Plasma Phys. Control. Fusion 40 2097). We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral densities.
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