Dr. Stylianos Varoutis

Karlsruhe Institute of Technology (KIT), Germany
Dr. Stylianos Varoutis graduated in 2004 from the Department of Mechanical Engineering, University of Thessaly, Greece. He received his M.Sc. and Ph.D. degrees in Rarefied Gas Dynamics from University of Thessaly in 2006 and 2009, respectively. In 2009, he moved to the Institute for Technical Physics, at Karlsruhe Institute of Technology (KIT), Germany, as an EFDA Fusion Researcher Fellow. Since then, he has been working at KIT as a Senior Research Scientist in the field of Vacuum Gas Dynamics. His research activities include plasma physics as well as numerical investigation of rarefied gas flows, by the use of stochastic and deterministic kinetic approaches.

Talk title: The role of vacuum gas dynamics in the particle exhaust of stellarator and tokamak fusion devices
The particle exhaust of a nuclear fusion reactor is one of the major problems that significantly affects the plasma conditions and is related to a wide range of operational and safety aspects. The effective control of the exhaust process provides a route to high-density regimes required for high performance and hence high-energy gain. 

The main components that determine the particle exhaust of a fusion device, are the divertor and the associated vacuum systems. Their main goal is to remove efficiently gas fuel, namely deuterium and tritium, the helium ash, which is a product of the fusion reaction, as well gas impurities, as Argon, Neon, Xenon etc. The design and optimization of such a system requires the exploitation of a simulation tool capable of dealing with very complex geometries and of describing the vacuum flow conditions, in the whole range of the Knudsen number. 

The most effective numerical tool, which is capable of dealing with the above-mentioned challenges, is the Direct Simulation Monte Carlo (DSMC) method [1], which is currently the main numerical approach, within the European Fusion program, for modelling the vacuum gas dynamics in the particle exhaust of tokamak [2,3] and stellarator fusion devices [4]. 

The focus of the present talk is mainly twofold. First, the 3D simulations of the particle exhaust of the Wendelstein 7-X stellarator, will be highlighted. This modelling activity is characterized by high geometrical complexity and highcomputational effort. The implementation of the DSMC method allowed for quantifying all the macroscopic parameters i.e. pressure, number density, temperature etc, as well as overall quantities i.e. pumped fluxes and gas leakages in the divertor area. Additionally, comparisons between numerical and experimental results, have been performed. Moreover, based on the 3D simulations, design optimization strategies of the particle exhaust configuration, have been identified. 

 The second part of this talk is devoted to the modelling and optimization of the divertor pumping system of a tokamak type demonstration power plant (DEMO) [5], which represents conceptually the first commercial fusion reactor for wide energy production. By applying the DSMC method coupled with corresponding plasma codes, the optimal position of the pumps is investigated for various operational conditions (plasma scenarios) and pumping speeds. The presented results include the estimation of all macroscopic quantities of practical interest as well as the qualitative behavior of the gas flow inside the DEMO divertor region. 

References 
[1] G. A. Bird, Molecular Gas Dynamics and the Direct Simulation of Gas Flows, Oxford University Press, Oxford, UK (1994). 
[2] S. Varoutis, et al., “Simulation of neutral gas flow in the JET sub-divertor”, Fusion Engineering and Design, 2017, 121, pp. 13-21. [3] C. Tantos, et al., “3D numerical study of neutral gas dynamics in the DTT particle exhaust using the DSMC method”, Nuclear Fusion, 2024, 64(1), 016019.
[4] S. Varoutis, et al., “Numerical simulation of neutral gas dynamics in the W7-X sub-divertor”, submitted to Nuclear Fusion.
[5] S. Varoutis, et al., “Optimization of pumping efficiency and divertor operation in DEMO”, Nuclear Materials and Energy, 2017, 12, pp. 668-673.