Comparison of a simple recycling model and a complex recycling model in edge plasma transport calculations

Abstract

The results of a significantly more efficient model (molecular model) for computing particle and energy sources from recycling in edge plasma transport codes, such as b2, are compared with results from more detailed calculations using the b2 code coupled with the neutral transport code, DEGAS. The molecular model considers the reflux both of cold molecules and of energetic backscattered atomic neutrals from the divertor target plates and has been implemented to function intemally within the b2 code. The molecular model calculations are shown to be in reasonable agreement with more detailed calculations done with the coupled b2/DEGAS codes for seven cases typical of conditions in the scrape-off layer of the Doublet III (DIII-D) tokamak. Because the molecular model is a far less expensive computational tool, it represents a significant improvement over the existing (coupled) edge transport recycling model. Thus, it may find wider application in design studies for future devices, such as the International Thermonuclear Experimental Reactor (ITER).

Effect of edge neutrals on the low-to-high confinement transition threshold in the DIII-D tokamak

“…The version of the edge transport code that we use, B2.5, contains a semianalytic multiple-species neutrals model that includes the transport of molecules, Franck-Condon atoms, and energetic reflected atoms. 44 This model has been compared with coupled B2/DEGAS calculations, with good agreement over a broad range of plasma conditions. Particle and energy reflection coefficients are those used in the DEGAS code.…”

“…Spatial diffusion of the recycling neutrals is simulated with an analytical model. 44 Particle balance in edge transport calculations is maintained with the use of wall and/or plate recycling coefficients that are less than unity, i.e., the particles that enter the edge/ SOL by diffusion and convection from the core are mostly removed at the divertor and outer walls rather than by ionization of the recycled neutrals inside the separatrix. A consequence of this is that stand-alone B2.5 simulations have traditionally been limited to a domain that extends only a centimeter or so inside the separatrix ͑i.e., to regions where the transport is dominated by parallel flow͒.…”

Kinetic neutral transport effects in the pedestal of H-mode discharges in the DIII-D tokamak

“…[7,8]. The B2.5 code uses an internal neutrals model [11] that has been benchmarked against DEGAS, rather than being linked with the Monte Carlo code itself. As discussed above, in order to ensure global consistency of the plasma and neutrals simulations, the B2.5 and DEGAS calculations are iterated until the core fueling rate, from divertor recycling (DEGAS) and neutral beams, is consistent with the core plasma particle efflux calculated with B2.5.…”

A review of ELMs in divertor tokamaks

“…When an ELM occurs, the heat pulse can reattach the divertor plasma, pushing the ionization region back near the plate; Ha drops along one chord and rises along another. In DIII-D, the inner divertor leg is often detached, even without extra gas puffing [62], and the H a emission as viewed from a chord along this inner leg drops during the ELM pulse while other channels simultaneously rise. This spatial shift in the location of the radiating region can be seen with TV cameras viewing visible lines of carbon emission as well [63].…”