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Simulations on Plasma-Surface Interactions

Magnetic fusion experiments and modelling to date suggest that it will be a big challenge to keep the heat fluxes incident on the plasma-exposed walls of a fusion reactor down to approximately 10 MWm-2 - about the level briefly experienced by the nose of a space vehicle reenteringthe Earth’s atmosphere. The edge region can strongly influence the performance of the core region of a fusion device because of the rapid particle transport parallel to the magnetic field. A predictive calculation of edge plasma dynamics still remains elusive because of the broad range of spatial and time scales, complex magnetic field structure and multitude of physical and chemical processes involved.

The goal of computational edge plasma science is therefore to provide numerical engineering tools for divertors with sufficient predictive quality upon which to base designs suitable for future fusion reactors.

Instabilities at Plasma-Vacuum Interfaces

Instabilities occur, when a small perturbation of the desired state of a plasma continues to grow over time rather than subside. This can lead to significant changes to the plasma configuration and should if possible be suppressed in a controlled experiment.

In a fusion reactor with a magnetic confinement scheme, the edge region between the core of the hot plasma and the vacuum vessel wall can be susceptible to such unstable behaviour. Here, the magnetic field tries to separate regions of high plasma density from low density regions. Differences in the motion of heavy plasma components (ions) and light components (electrons) can lead to a spatial accumulation of charges. The resulting electric field combined with the confining magnetic field causes a sheared collective drift of the plasma particles. Such a configuration is known to be unstable. In this scenario, the growing perturbations can lead to increased fluxes across the magnetic field.

Simulations can help understand the mechanisms behind these instabilities and find plasma configurations that are less strongly affected by them.



Coupled Modelling of Plasma-Wall Interaction

Due to the very disparate temporal and spatial scales involved, only a combination of new multi-scale algorithms and effective exploitation of HPC will allow this vision to be realized. The codes used and developed here are: EIRENE - a micro-macro-Monte Carlo Transport code, wide-spread in magnetic fusion research, and PEPC-F – a tree code for mesh-free computation of electrostatic fields in 3D kinetic plasma simulations. The aim is to couple these models, forming the basis for an innovative kinetic approach to this multi-scale and multi-physics area of fusion simulation science. Both of these codes already exhibit a high degree of parallel scalability and are therefore promising candidates for exploiting exascale systems.

Sheath FormationDevelopment of a plasma sheath at a conducting wall (blue) with freely floating potential. Thermal plasma ions and electrons are injected from the opposite boundary, where they travel freely up to the wall and are either absorbed or reflected. After a transitional period (displayed here), a quasi-stationary equilibrium is reached yielding quantifiable particle fluxes and damage rates at the wall.