Simulation of divertor heat flux width on EAST by BOUT++ transport code

The BOUT++ edge plasma transport code is applied to study the effects of neutral recycling, drifts and heating scheme on edge plasma profiles and the divertor heat flux width of 2 discharges of the experimental advanced superconducting tokamak (EAST) steady-state H-mode plasmas heated by low hybrid wave (LHW) and neutral beam (NB) respectively. Neutral recycling seems to have played an important role in the plasma density profile. The edge plasma density drops dramatically for case w/o neutral recycling while it can be sustained for case w/ neutral recycling. Drifts are found to have significant influences on edge plasma profiles. Both the amplitude and the width of the divertor heat flux are found to have increased a lot due to drifts. The simulated heat flux width w/ drifts for the two discharges shows reasonable agreement with the experiments, however, the width from the simulation and experiment for the LHW heated discharge is much larger than that of the NB heated discharge. Comparison with Goldston's drift-based model and more detailed analysis on the heat flux contributions from drifts versus turbulence show that drifts are the dominant factor in edge plasma transport for both LHW and NB heated discharges while turbulence may have played a more important role for determining the heat flux width in LHW heated discharges than that in NB heated discharges, which may account for the larger heat flux width in the LHW heated discharge. The magnetic topology and the equilibrium change by the LHW power may also be a potential reason to the larger heat flux width in the LHW heated discharges, which still needs more evidences and studies to support it. Additional SOLPS simulation w/o drifts for the two discharges show reasonable agreement with the counterpart from BOUT++ simulation, suggesting the two are both suitable codes for EAST.