The double-peaked emission profile has been identified by tomographic reconstruction from the signals of the tangentially viewing visible-light imaging system on EAST. In the upper-single-null discharge with neon seeding, two light emission peaks are located near the X-point and in the high-field-side (HFS) scrape-off layer (SOL) on the poloidal cross-section while the outer divertor target is still attached. The double-peaked emission phenomenon has not been reported for other tokamaks. In particular, the existence of the emission peak near the X-point under the attached condition is unexpected because it is usually an indication of X-point multifaceted asymmetric radiation from the edge under the detached condition. Therefore, it is of fundamental importance for the divertor physics to make clear the mechanism of the double-peaked emission phenomenon. The computational simulations with SOLPS-ITER and DIVIMP are performed in this work. The simulated results show the contribution of W impurity to the double-peaked emission profile is negligible with the upper tungsten divertor. Considering the existence of a lower carbon divertor in EAST, the influence of the possible deposition of the carbon on the upper divertor is further investigated. The simulating CIII (465 nm) and CII (514 nm) emissions, which make a significant contribution to the visible-light emission in channel B (blue) and G (green), respectively, are found to be similar tothe experimental results. The emission peak near the X-point is related to the carbon deposited on the dome, while that in the HFS SOL is contributed by the carbon on the inner baffle. The ionization source of the carbon impurity and the net force pointing to the X-point are analyzed to understand the concentration of the carbon impurity near the X-point, which in turn causes the emission peak.

Doubly peaked density distribution is expected not only to affect the plasma-wetted area at divertor plates, but also to correlate with the upstream density profile and hence characteristics of MHD activities in tokamak plasmas [H. Q. Wang et al., Phys. Rev. Lett. 124, 195002 (2020)]. Clarifying its origination is important to understand the compatibility between power/particle exhausts in divertor and high-performance core plasmas which is required by present-day and future tokamak devices. In this paper, we analyzed the double-peak density profile appeared in the modeling during the physics design phase of the new lower tungsten divertor for EAST by using comprehensive 2D SOLPS-ITER code package including full drifts and currents, with concentrations on unfavorable magnetic field (ion B×∇B drift is directed away from the primary X-point). The results indicate that E×B drift induced by plasma potential gradient near the target, which is closely related to the divertor state, plays essential roles in the formation of double-peak profile at the target: (1) Large enough radial Ep×B drift produces a broadened high-density region; (2) Strong poloidal Er×B drift drives a significant particle sink and creates a valley on the high-density profile. Thus, the simulation results can explain why this kind of doubly peaked density profile is usually observed at the high-recycling divertor regime. In addition, features of the double-peak ion saturation current distribution measured in preliminary experiments testing the new lower tungsten divertor are qualitatively consistent with the simulations.

The double-peaked distribution of particle deposition at the divertor targets has been observed in various tokamaks, and is considered as a potential approach for mitigating the divertor particle and heat load in future fusion reactor. Recently, the systematical analysis of the double-peaked distribution behavior during EAST experiments show that, the appearance of the double-peaked profile is related to the line-average density and heating power. In order to understand the general trends and related mechanisms, the influences of the upstream density (ne,sep) and power into scrape-off layer (PSOL) on the double-peaked density profile are investigated by SOLPS-ITER simulations with full drifts and currents. It is found that, the ne peak near the strike point is mainly contributed by the strong ionization source close to the target, and the ne peak in far-SOL region is caused by the synergetic effects of poloidal and radial E×B drifts along the SOL. The double-peaked distribution is affected by the PSOL and impurity seeding by increasing or decrease the whole profile of the electron temperature at the target (Tet). When the peak value of Tet (Tet,peak) is fixed, the density peak in the far-SOL is increased for higher ne,sep, by reducing the Tet in the far-SOL region on the lower-field side under unfavorable BT, and by the upstream-extended ionization source due to the geometry effect on the high-field side under favorable BT. Statistical analysis of the simulated results shows that the scaling expression of the peak ratio is ~Tet,peak -(1.4~2.1) ne,sep 1.6~1.8. In addition to upper boundary found in the analysis of EAST experiment, a lower boundary of the region where double-peaked feature appears on the PSOL-ne,sep plane is identified by simulations and preliminarily confirmed according to the measurements in several EAST discharges.

A novel double-peaked emission profile has been observed by the visible light imaging system in EAST. In the upper divertor region, two emission peaks in the high-field side (HFS) scrape-off layer (SOL) and near the X-point are identified by tomographic reconstruction. In our previous SOLPS-ITER simulation work, the emission peaks in the HFS SOL and near the X-point are attributed to the carbon deposition on the HFS baffle and the dome of the upper tungsten divertor, respectively. The appearance of the emission peak near the X-point when the outer target is attached was understood by analysing the carbon transport in the private flux region (PFR). However, the simulated intensity of the emission peak in the HFS SOL is lower by one order of magnitude than that in the PFR, while they are at the similar level in the experiment. In this work, the influence of the drifts is further studied by SOLPS-ITER simulations. Under the favourable toroidal field condition, the HFS SOL plasma transits to the detachment regime. The chemical sputtering of the carbon deposited on the inner baffle increases significantly, as well as the emission intensity of the peak in the HFS SOL. Meanwhile, the C 2+ density peak in the PFR decreases due to the E × B drift towards inner target. The emission intensity in the PFR near the X-point, therefore, decreases moderately. As a result, the intensities of the two emission peaks become comparable, which matches better with the experiment.