Runaway electron generation during plasma shutdown by killer pellet injection
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 this 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.
Runaway electron dynamics in tokamak disruptions
In the light of the current understanding of the adverse effect of CO2 on the climate of our planet and the expected increase in energy demand over the next decades, is it of great importance that we develop new clean energy sources. Fusion energy promises to be such a source if the formidable physics and technical problems that are associated with it can be overcome. The most promising reactor concept in fusion research, the tokamak, has one severe drawback. To confine the plasma, i.e. the fuel, it has to maintain a very high current which makes the plasma susceptible to disruptions. Disruptions are events in which the plasma in the reactor is terminated through a sudden loss of plasma energy to the wall resulting in a very fast cooling of the plasma and consequently a large increase in plasma resistivity. The resulting fast decrease in current induces a high electric field in which a part of the electron population can be accelerated to very high energies, the so called runaway electrons. In large tokamaks up to two-thirds of the pre-disruption current can be converted into runaway current, which amounts to about 2 MA in current day devices. These beams of relativistic electrons can do serious damage to the reactor when they are lost from the plasma. Observations from different tokamak experiments show that there is no runaway electron generation for magnetic fields below 2 Tesla, but a theoretical explanation is still lacking. In this work the dynamics of runaway generation during disruptions are studied with the aim of identifying a mechanism that can explain the experimental results. For this purpose we use a one-dimensional model of runaway electron dynamics, called RUN, that includes the various generation processes and losses and the electric field diffusion self-consistently. One of the loss mechanisms is the resonant interaction of the runaway electrons with the magnetosonic whistler wave. The growth rate of the wave is inversely proportional to the magnetic field. If the wave is excited, the interaction will increase the pitch angle of the runaway electrons which will result in more synchrotron radiation being emitted. The resulting force will slow the electrons down. It is shown that synchrotron radiation alone cannot reduce the number of runaway electrons sufficiently to explain the 2 Tesla threshold, which was confirmed by the results obtained with RUN. Runaway electrons can also be lost from the plasma through diffusion induced by magnetic perturbations. Both the case in which the whistler wave induces the perturbations and the effect of a constant perturbation were investigated with RUN. In the case of whistler wave induced perturbations there was inverse relation between the runaway current and the magnetic field strength. The results for a constant magnetic perturbation showed a small proportionality between perturbation strength and runaway current for small perturbations (up to δB/B = 10−5) and a very strong negative relation for δB/B > 10−4. Interaction with whistler waves and losses due to magnetic perturbations are not likely to provide a sufficient explanation for the observed magnetic field dependence of runaway electron generation in tokamaks. The runaway current in the later phases of the discharge is strongly dependent on the number of runaway electrons generated in the early stages of the disruption. Only simulations with a very high density showed negligible numbers of primary runaway electrons, suggesting that an explanation for the experimental results will have to be found by looking at the influx of impurities during the disruption and the subsequent increase in electron density. The strength of the magnetic field could affect the penetration of impurities towards the plasma center, explaining the observed B-field dependence.
The softening of current quenches in JT-60U
Current quenches caused by density limit disruptions have been investigated in JT-60U divertor plasmas in order to develop general methods of reducing the current decay rate and of suppressing the generation of runaway electrons. The reduction of the impurity influxes at an energy quench and the direct neutral beam (NB) heating of the plasma core during a current quench were beneficial for reducing the speed of the current quench. The low stored energy just before the energy quench and the high safety factor at the plasma edge had the advantage of decreasing the impurity influxes at the energy quench. The detached plasma state was useful for degrading the energy confinement for both joule and NB heated plasmas within a short time period. Runaway electrons were not generated at plasma densities of more than 5*1019 m-3 measured just before the energy quench. Fast controlled plasma shutdown with energy quenches but without a current quench was demonstrated successfully from 2 MA to zero, with a value of dIp/dt of -6 MA/s. This shutdown was obtained by lowering the stored energy within a short time period of 20 ms, actively using the detached plasma state produced by intense helium gas puffing