Physics of the Scrape-Off Layer

The Scrape-Off Layer (SOL) forms the outermost region of a tokamak plasma, connecting the core plasma to the reactor wall. Plasma particles and energy flow along magnetic field lines in this region to specific wall components, the divertor target plates. To prevent damage to the divertor target plates and other wall components, heat fluxes in the SOL must be carefully controlled. Additionally, the SOL influences key properties of the core plasma, such as plasma particle density and impurity concentration.

The research group focuses on the following areas using experimental measurements and complex computer simulations:

• Plasma and neutral particle transport in the SOL: These transport processes determine the energy deposition onto the wall and the particle balance in the plasma.
• Plasma scenarios with high radiation fraction: In these scenarios, impurity atoms such as nitrogen, argon, or neon are deliberately introduced into the plasma to radiate power within the SOL before they reach the divertor target plates. This significantly reduces the heat load on the divertor target plates.
• Transport of impurities: In addition to deliberately introduced impurities, impurities are also generated by the fusion process itself (helium) and by erosion of the wall material (e.g., tungsten). In the plasma core, the impurity concentration must be kept as low as possible, as higher levels would reduce the fusion performance.
• Use of alternative divertor concepts: Alternative divertors use special magnetic field configurations in the divertor region to optimise power and particle control. The ASDEX Upgrade tokamak was extensively modified with additional magnetic field coils, allowing for the experimental investigation of alternative divertor concepts, such as the snowflake divertor, under fusion-relevant conditions.