Analysis of pedestal plasma transport

An H-mode Edge Pedestal (HEP) plasma transport Benchmarking Exercise (BE) was undertaken for a single DIII-D pedestal. Transport modeling codes used include 1.5D interpretive (ONETWO, GTEDGE), 1.5D predictive (ASTRA) and 2D ones (SOLPS, UEDGE). The particular DIII-D discharge considered is 98889, which has a typical low density pedestal. Profiles for the edge plasma are obtained from Thomson and CER data averaged over the last 20 % of the average 33.53 ms repetition time between Type I ELMs. The modeled density of recycled neutrals is largest in the divertor X-point region and causes the edge plasma source rate to vary by a factor∼ 10 on the separatrix. Modeled poloidal variations in the density and temperatures on flux surfaces are small on all flux surfaces up to within about 2.6 mm (ρN > 0.99) of the mid-plane separatrix. For the assumed Fick’s-diffusion-type laws, the radial heat and density fluxes vary poloidally by factors of 2–3 in the pedestal region; they are largest on the outboard mid-plane where flux surfaces are compressed and local radial gradients are largest. Convective heat flows are found to be small fractions of the electron ( < ∼ 10 %) and ion ( < ∼ 25 %) heat flows in this pedestal. Appropriately averaging the transport fluxes yields interpretive 1.5D effective diffusivities that are smallest near the mid-point of the pedestal. Their “transport barrier” minima are about 0.3 (electron heat), 0.15 (ion heat) and 0.035 (density) m/s. Electron heat transport is found to be best characterized by ETG-induced transport at the pedestal top and paleoclassical transport throughout the pedestal. The effective ion heat diffusivity in the pedestal has a different profile from the neoclassical prediction and may be smaller than it. The very small effective density diffusivity may be the result of an inward pinch flow nearly balancing a diffusive outward radial density flux. The inward ion pinch velocity and density diffusion coefficient are determined by a new interpretive analysis technique that uses information from the force balance (momentum conservation) equations; the paleoclassical transport model provides a plausible explanation of these new results. Finally, the measurements and additional modeling needed to facilitate better pedestal plasma transport modeling are discussed. PACS numbers: 52.55.Fa, 52.25.Fi, 52.55.Dy, 52.55.Rk Submitted to: Nuclear Fusion Analysis of pedestal plasma transport 2