In the previous simulations on the divertor heat fluxes of C-Mod and EAST, the inverse proportional to the plasma current or poloidal magnetic field trend of Eich's Scaling is well reproduced by the BOUT+ + simulations. However, the simulated SOL width is only half of the value realized in EAST measurements. The reason is believed to be the lack of RF heating scheme in the simulations. The RF heating on EAST, especially lower hybrid wave (LHW), is considered to change the boundary topology and increase the flux expansion. To prove the topology change effects of LHW in the scraped-off layer (SOL), a modeled helical current filament (HCF) due to LHW in SOL, which has the same amplitude of 13 kA to the experiments, is added as the force-free form into the 6-field 2-fluid module of BOUT++. The radial magnetic field induced by this HCF could be much smaller than the perturbed field induced by the edge tubulence or ELM, but it is able to force the perturbations of the turbulence or ELM with the same toroidal mode number to grow up at the start of the linear phase during the simulations. This forced mode is effective to compete with the spontaneous fluctuations and change their linear properties, which leads to the suppression of the divertor heat flux and the broadening of SOL width. The modeling results imply that the HCF with the toroidal mode number is able to increase the SOL width by ∼, and the peak parallel heat flux towards divertor target is decreased by ∼. The scan of the toroidal mode number of HCF indicates that the smaller could generate the broader SOL width and lower peak heat flux. The modeled broadening of the particle flux by HCF clearly shows the secondary striate filaments on divertor target, which is similar to the splitting of the strike point observed in the measurements by the divertor probes.
Dependence of turbulent transport on the divertor flux expansion in ASDEX Upgrade