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

摘要

The plasma flow in the scrape-off-layer (SOL) plays an important role in 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 outer plate side to the SOL middle side in the outer SOL region of tokamaks. In order to study these SOL flow patterns by fully taking account of the kinetic effects, a full particle code, PARASOL, is applied to a tokamak plasma with the upper-null point (UN) or lower-null point (LN) divertor configuration for the downward ion ▽B drift. PARASOL simulations for the medium aspect ratio (A = 5.5) reveal the variation of the flow pattern. For the UN case with the ion ▽B drift away from the null point, the flow velocity V_‖ parallel to the magnetic field is formed almost in-out symmetrically. In the inner SOL region V_‖ is directed to the inner divertor plate and in the outer SOL V_‖ is directed to the outer plate. The stagnation point (V_‖ = 0) is located symmetrically at the bottom. On the other hand for the LN case with the ion ▽B drift towards the null point, V_‖ in the outer SOL region has a backward flow pattern. The stagnation point moves below the mid-plane of the outer SOL and V_‖ in the mid-plane outer SOL is directed to the inner plate. 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. The trapped-ion effects can be stronger than the electric-field effects for the standard tokamaks with A < 5.