The role of radial particle flow on power balance in DIII-D

Abstract

The importance of radial particle flow on the power flowing across the last closed flux surface (separatrix) in DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), Vol. I, p. 159] is considered. The perpendicular thermal diffusivity at the separatrix is near 1 m2/s in low confinement operation (L-mode), and 0.1 m2/s in high confinement (H-mode). The particle diffusivity is about one-fourth of the thermal diffusivity producing radial particle fluxes of the order of kilo-amperes. The particle flux is 10 to 100 times the particle input from neutral beam sources, consistent with core fueling being dominated by neutral recycling. The radial particle flux scales with the neutral pressure in the private flux region, suggesting the core is fueled predominantly from neutrals which recycle from the divertor, through the private flux, and into the core near the singular point where the poloidal field is zero (X-point). There is significant core power loss associated with the large particle flux across the separatrix. The electron temperature measured at the top of the edge pedestal in H-mode operation scales inversely with the particle flux. In turn, the core energy confinement scales with the pedestal temperature, and hence inversely with the particle flux. The results presented here indicate the global particle confinement time is between 0.5 and 2 times the global energy confinement time.