Two-dimensional Full Particle Simulation of the Flow Patterns in the Scrape-off-layer Plasma for Upper- and Lower- Null Point Divertor Configurations in Tokamaks

The plasma flow in the scrape-off layer (SOL) plays an important role for the particle control in magnetic fusion reactors. The flow is expected to expel Helium ashes and to retain impurities in the divertor region, if it is directed towards the divertor plate. It has been experimentally observed, however, that the flow direction is sometimes opposite; from the plate side to the SOL middle side in the outer SOL region of tokamaks. A full particle code, PARASOL, is applied to a tokamak plasma with the upper-null-point (UN) or lower-nullpoint (LN) divertor configuration for the downward ion grad-B drift. PARASOL simulations for the medium aspect ratio reveal the variation of the flow pattern: For the UN case, the flow velocity V// parallel to the magnetic field is directed to the diverter plate both in the inner and outer SOL regions and the stagnation point (V// = 0) is located symmetrically at the bottom. On the other hand for the LN case, V// in the outer SOL region has a backward flow pattern. The stagnation point moves below the mid-plane of the outer SOL. These simulation results are very similar to the experimental results. Simulations are carried out by changing the aspect ratio and by artificially cutting the electric field. It is found that the banana motion of trapped ions is very important for the formation of the flow pattern in addition to the self-consistent electric field.