Gyrokinetic prediction of the divertor heat-load width for ITER*

The total-f gyrokinetic code XGC has been used across the separatrix and SOL to study the fundamental multiscale physics behind the divertor heat-flux width. XGC shows that in today’s tokamaks, the ion neoclassical orbit physics tends to lead the divertor heat-flux width physics, and agree with the “Eich scaling.”  Sheared ExB-flow across the magnetic separatrix surface is strong, and turbulence is “blobby."    In ITER with full magnetic field, however, simulations find that the mean ExB flow shear is weak due to the small ρ_* effect, that the turbulence pattern across the separatrix becomes of streamer type with long radial correlation length. As a result, the heat-flux width becomes several times greater than the Eich-predicted value. Relation between the up-stream “scrape-off layer width” and the down-stream divertor heat-flux width is not strong, which implies that the length of the divertor leg and the magnetic structure in the divertor chamber could be an important factor in setting the divertor heat-load width in ITER. The first-phase ITER plasma at B=1.8T does not show such enhancement in the divertor heat-flux width over the Eich value, supporting the rho* physics effect, and indicating a bifurcation event.