Runaway electron generation during plasma shutdown by killer pellet injection

Abstract

Abstract. Tokamak discharges are sometimes terminated by disruptions that may cause large mechanical and thermal loads on the vessel. To mitigate disruption-induced problems it has been proposed that "killer" pellets could be injected into the plasma in order to safely terminate the discharge. Killer pellets enhance radiative energy loss and thereby lead to rapid cooling and shutdown of the discharge. But pellets may also cause runaway electron generation, as has been observed in experiments in several tokamaks. In the present work, runaway dynamics in connection with deuterium or carbon pellet-induced fast plasma shutdown is considered. A pellet code which calculates the material deposition and initial cooling caused by the pellet is coupled to a runaway code, which determines the subsequent temperature evolution and runaway generation. In this way, a tool has been created to test the suitability of different pellet injection scenarios for disruption mitigation. If runaway generation is avoided, the resulting current quench times are too long to safely avoid large forces on the vessel due to halo currents.

Evaluation of the Dreicer runaway generation rate in the presence of high- impurities using a neural network

“…1 Above the second separatrix, where fast particles are decelerated due to for example radiation reaction losses, the particles will slow down again. This eventually creates a bump-on-tail (Hirvijoki et al 2015;Decker et al 2016;Guo, McDevitt & Tang 2017), which prevents further runaway growth through the Dreicer mechanism, but the involved energies and time scales makes this effect irrelevant except for near the threshold E c . An exception is with strong synchrotron radiation, temperatures of several keV and an electric field approaching the effective critical electric field; in this case a steady-state Dreicer generation rate cannot clearly be determined and a full kinetic simulation may be necessary.…”

“…In such fluid models, the runaway-electron density evolves by analytical generation rates describing Dreicer, hot-tail and avalanche generation, as well as tritium decay and Compton scattering of γ -rays (which can be emitted by the activated wall in the nuclear phase of tokamak operation). This approach has been used in the past to gain insight into the runaway-electron dynamics and electric-field diffusion; some examples are the GO code (Smith et al 2006;Gál et al 2008;Fehér et al 2011;Papp et al 2013) and the work by Martín-Solís, Loarte & Lehnen (2017).…”

Simulation of runaway electron generation during plasma shutdown by impurity injection in ITER

“…The cooling process due to impurity radiation is described in a one dimensional cylindrical model [11] by the following energy balance equations 3 2…”

“…The simulations presented in this paper are performed with a 1D runaway code, which was initially presented in [17,18] and developed further in [11,19], where it was applied to impurity injection scenarios in JET-like plasmas. The code determines the temperature evolution by taking into account radiation, Ohmic heating, heat diffusion and collisions between different particle species.…”

Runaway dynamics in the DT phase of ITER operations in the presence of massive material injection

“…go has been used in the past for evaluating material injection scenarios (Gál et al. 2008; Fehér et al. 2011), and for interpretative modelling of experiments (Papp et al.…”