UEDGE simulations of X-point radiator regime in DIII-D

The DIII-D X-point radiator regime is reproduced in UEDGE to support the interpretation of dedicated experiments in DIII-D. Accurate modeling of the X-point radiator regime is important for future devices, as it represents a promising scenario for achieving core-edge integration. Using a fixed-fraction model in UEDGE, the X-point radiator regime is performed with both carbon and nitrogen. Consistent with experimental observations, it shows that radiation is concentrated just inside the LCFS, within the normalized flux surface range of PsiN = 0.98–1.0. This cooling reduces the pressure at the X-point from upstream by a factor of ~3 for nitrogen and ~5 for carbon. However, discrepancies remain between the simulations and experimental observations: in the model, access to a deep X-point radiator regime occurs via a bifurcation-like transition from a deeply detached state, whereas experimentally, the transition appears to evolve more continuously; Additionally, the simulated radiation is more poloidally extended and the radiation front is located farther upstream compared to experimental measurements.

The impact of varying particle and thermal transport coefficients, D and χ, respectively, on the radiation front location in the X-point radiator regime is investigated. Lower values of D/χ near the separatrix tend to produce more poloidally extended radiation and shift the radiation front radially closer to the separatrix, whereas higher D/χ values lead to a more localized front farther from the separatrix.