Controlling Plasma Material Interfaces with BIT

The First Grand Challenge

The leading design of magnetic confinement fusion experiments is axisymmetric toroidal devices called tokamaks. This includes the international ITER project, currently under construction in France, and DEMO, whose role will be to prove the feasibility of fusion energy. One of the key challenges in this context is to control the interactions between the extremely hot plasma and the surrounding material walls. Although the plasma is magnetically confined, it can still escape to some degree, and transient heat loads can exceed 10 MW/m², which is comparable to a spaceship on re-entry into the Earth’s atmosphere and borders on what most materials can take. In modern tokamaks, this issue is addressed by setting up an intermediate layer of plasma between the solid surface and the core plasma, called the scrape-off-layer (SOL). Careful designs are necessary to ensure that only limited amounts of impurities enter the plasma, while the material is damaged as little as possible.

How can exascale simulations solve this grand challenge? There are several approaches to modelling tokamak SOL plasmas, depending on the fidelity of the description. The traditional approach is based on fluid models, but recent investigations demonstrate that for the next generation of fusion devices, fluid models must at least be complemented by kinetic ones4. Among the main kinetic effects to be studied are:

● the contribution of non-Maxwellian super-thermal electrons to the divertor heat fluxes;

● effects of the de-magnetisation of the plasma–wall interaction layer;

● time-dependent heat fluxes to the divertors during the so-called Edge Localised Modes (ELM).

At present, BIT1 is the only kinetic code contributing to all of these studies.