Fusion conditions – Particle simulation studies of divertor plasmas

“Nuclear fusion” is the melting of light nuclei into heavier ones, a process that according to the laws of physics releases enormous amounts of energy. For the past 50 years many scientists have sought ways of harnessing this fusion reaction under controlled reactor conditions as a safe, clean and practically inexhaustible source of energy. Siegbert Kuhn and his team at the Institute of Theoretical Physics at Innsbruck University are making a major contribution to these efforts and positioning Austrian nuclear fusion research at the forefront of international activities in this field by carrying out particle simulation studies of divertor plasmas sponsored by the Austrian Science Fund (FWF) and in cooperation with international research groups.

In order to obtain an adequate number of nuclear fusion reactions for practical energy production, the particles involved must be made to collide with sufficient frequency and sufficient energy. In principle, this can be most readily achieved in an extremely hot hydrogen gas (approx. 100 million degrees) at appropriate density. At these temperatures the gas is fully “ionised”, meaning that the gas molecules, which are electrically neutral under normal conditions, are split into positively charged nuclei (“ions”) and negatively charged “electrons”. “Such a gas is called a `plasma` and the plasma state is commonly referred to as the `fourth state of matter`”, Kuhn goes on explaining that plasma is the stuff that stars are made of: “Only imagine it: 99.99 % of all matter in the universe is in the plasma state!”. Hot plasma is confined in a ring-shaped vessel (torus) by a magnetic field of suitable structure. The most promising configuration to date is termed “tokamak”. The next ambitious aim of international fusion research is the construction of the “International Thermonuclear Experimental Reactor (ITER)”, which will be the first reactor to work with a plasma largely heated by the fusion reaction itself and which will come very close to the concept of a future commercial fusion reactor in terms of plasma physics.

In a tokamak a distinction is made between the hot “core plasma”, in which the energy-producing nuclear fusion reactions take place, and the cooler “edge plasma” through which the high-energy plasma particles diffusing from the core plasma are passed to the baffle plates of the divertor. “Since there are strict technical limits to the amounts of energy to which divertor plates can be subjected, questions relating to the contact between the plasma and the divertor wall count among the most important scientific and technical challenges of modern fusion research”, explains Kuhn. He has obtained important results for a better understanding of the divertor plasma in his project. Existing models and simulation programmes, for example, have been greatly improved and the strong influence of secondary and fast electrons on the edge layer was clearly shown and quantified. Kuhn: “We were also able to make a major contribution to understanding the forming and effects of fast particles which occur during the heating of the tokamak plasma through wave injection and which can seriously damage the divertor plates. In a next step, our results can be directly used for modelling and optimising existing and planned tokamaks.”

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