Electrons which run away in a fusion reactor

Particles in a plasma interact via the Coulomb force at relatively long distances. One consequence of this is that the friction force experienced by individual electrons from their neighbours decreases the faster the particle moves, which allows in particular electrons to "run away" to near the speed of light should a sufficiently strong accelerating force arise. Such situations can be found in a range of plasmas, including in solar eruptions, lightning discharges, and in fusion devices. In particular fusion devices of the tokamak type regularly see runaway electrons form and cause problems, and the runaway electron problem has become a main focus in international fusion research.

In my research I have studied a number of aspects of runaway electrons in order to understand how they arise and can be controlled. Much of the research centers around the code DREAM which can solve a set of coupled partial differential equations for the plasma, and can be used to predict how many runaway electrons that can arise in a tokamak. Also the code SOFT, which can be used to simulate camera images and light spectra from runaway electrons, has been central to my research and enabled a range of experimental studies. With the help of these codes I have, together with colleagues from across the world, optimized the number of runaway electrons that are expected to arise in the next generation of fusion reactors (ARC, ITER, SPARC, and STEP), studied specific physical mechanisms which affect runaway electrons (e.g. scrape-off of runaway electrons during plasma vertical motion, injection of electron cyclotron waves for heating, and current relaxation caused by plasma instabilities), as well as modelled and deduced the distribution function of the runaway electrons in experiments at ASDEX Upgrade, JT60-SA, and TCV. The overarching aim of all this work is to develop models which can be applied in the design of fusion reactors to prevent runaway electrons from damaging reactor components.