Hydrocarbon Transport and Chemistry in the Tokamak Edge

Controlled nuclear fusion is one of the potentially most promising energy sources of the future, but its realisation on earth is a formidable problem. The physical and technological feasibility is to be demonstrated by the planned fusion experiment ITER, which is one of the most important co-operative research projects of the beginning 21st century. From the technological point of view ITER will be a tokamak, similar to the biggest existing machine JET (Joint European Torus) in Culham (GB), where the gas of deuterium and tritium fusion fuel particles is magnetically confined and heated up to over 100 million K (fully ionized plasma) at sufficiently high densities and for a sufficiently long time. With fusion plasmas approaching reactor conditions, one has to deal with the problem of finding materials that can sustain an incident flow of hot plasma (10 4 - 105 K) at a flux about 1025 particles/s onto the divertor target blades of the tokamak for a long time. Especially in ITER the power loads could be very high under certain conditions ("ELMs"), leading to substantial erosion of the divertor target blates. One of the major issues for next-generation tokamaks like ITER will be the problem of co-deposition of tritium via hydrocarbons in the divertor target materials, if the carbon option is kept open for fusion. Hydrocarbons are likely to be produced at divertor target blades by chemical erosion processes. For studying these hydrocarbons the Monte Carlo Code EIRENE has been extended with the so called trace ion module, which now allows to study not only the chemistry of the hydrocarbons, but also the transport of the related molecular ions. Within the present project we propose to extend the EIRENE code further with new physics modules for anomalous transport and finite Larmor radius effects. Finite Larmor radius effects are especially important for highly energetic ions, which are produced during neutral beam heating. The extended code package will then be applied to simulate the catabolism of methane during gas puffs in the existing tokamaks MAST and TEXTOR. Finally, predictive simulations for studying the hydrocarbon catabolism and carbon migration in the next generation tokamak ITER will be performed. The proposed project will significantly contribute to a better understanding of the transport and molecular processes which hydrocarbons undergo in the divertor region of a tokamak and hence of the tokamak device as a whole.

In this project essential numerical developments on computational models have been pursued to simulate the transport of trace ions simultaneously with neutral particles at the edge of plasmas in fusion experiments. The "trace ion module" (TIM) developed for this purpose at University of Innsbruck for use with the established neutral particle simulation code EIRENE has been applied to modelling of topical scenarios in tokamak fusion experiments (JET, ASDEX Upgrade, MAST). This enables an improved understanding of the kinetics of impurity particles in fusion plasmas and a better predictability of future experiments like ITER. Impurities essentially arise from contact of the hot hydrogen plasma with wall materials like carbon or tungsten. The trajectories of hydrocarbon molecular ions and partly highly charged tungsten ions, that originate from plasma-wall interactions, can be simulated with TIM in high precision statistically by Monte-Carlo methods. Comparison of the simulation results with spatially highly resolved measurements at the MAST tokamak showed good agreement in the transport pattern of hydrocarbon trace ions. In order to more precisely cover the rich hydrocarbon chemistry in fusion plasmas, relevant cross sections recently measured at University of Innsbruck have been analysed and processed for inclusion into the HYDKIN simulation data base. The newly developed TIM code has further been applied by a research group at the Max-Planck-Institute for Plasma Physics in Garching on simulation of tungsten transport in the ASDEX Upgrade experiment, and is now generally available as a numerical tool in context of the EIRENE model for evaluation of impurity transport properties of wall materials for future fusion experiments like ITER.