Reconstruction and interpretation of edge line radiation asymmetry from first-principles kinetic simulation*

With kinetic edge simulations, it is shown that the observed ionization asymmetry on DIII-D is due to reversal of ion flows which modifies the primary recycling location from either the outer or inner divertor plates. Here, we present the first successful reproduction of observed Lyman-alpha brightness on DIII-D directly from first-principles simulations. The XGC total-f particle-in-cell software suite is now fully coupled with the  DEGAS2 neutral transport solver and was used to predict the plasma structure in the pedestal and SOL self-consistently with recycling neutrals. DEGAS2 and its associated synthetic diagnostics act as a validation mechanism for XGC and provides physical insight for the scrape-off layer and ionization fueling.

 

Experiments have previously discovered a striking reversal of inboard/outboard asymmetry in the Lyman-alpha emission when the toroidal magnetic field is reversed. This asymmetry is caused by a combination of ExB and parallel flow reversal, while turbulence is responsible for the overall signal brightness. The simulated parallel stagnation point is close to the target, and this is interpreted as a kinetic effect in these discharges, which have marginal collisionality in the scrape-off layer. While neoclassical simulations capture the asymmetry, they under-predict the Lyman-alpha brightness by an order of magnitude consistently across all channels. With turbulence included either via self-consistent physics (using XGC1) or artificial anomalous diffusion (with XGCa), both the overall signal strength and the high/low-field side asymmetry are well reproduced with the synthetic diagnostics for both directions of the toroidal magnetic field.