Showing papers by "T.D. Rognlien published in 2004"

Driving mechanism of SOL plasma flow and effects on the divertor performance in JT-60U

Abstract: The measurements of the SOL flow and plasma profiles both at the high-field-side (HFS) and low-field-side (LFS), for the first time, identified the SOL flow pattern and its driving mechanism. 'Flow reversal' was found near the HFS and LFS separatrix of the main plasma for the ion ∇B drift direction towards the divertor. 'Flow reversal' at the main SOL was reproduced numerically using the UEDGE code with the plasma drifts included although Mach numbers in measurements were greater than those obtained numerically. Particle fluxes towards the HFS and LFS divertors produced by the parallel SOL flow and Er × B drift flow were evaluated from the measured profiles of Mach numbers, the density and the radial electric field. The drift flux in the private flux region was also evaluated, and it was found that its contribution to the HFS-enhanced asymmetry of the divertor particle flux was larger than the ion flux from the HFS SOL. The ion flux for the intense gas puff and divertor pump ('puff and pump') was investigated, and it was found that both the Mach number and density were enhanced, in particular, at the HFS. Ion flux at the HFS SOL can be enhanced so as to become larger than the drift flux in the private flux region.

Multi‐fluid code simulations including anomalous non‐diffusive transport of plasma and impurities in the tokamak SOL

Abstract: Fast intermittent transport has been observed in the scrape-off layer (SOL) of major tokamaks including Alcator C-Mod, DIII-D, and NSTX. This kind of transport is not diffusive but rather convective. It strongly increases plasma flux to the chamber walls and enhances the recycling of neutral particles in the main chamber. We discuss the anomalous cross-field convection (ACFC) model for impurity and main plasma ions and its relation to intermittent transport events, i.e. plasma density blobs and holes in the SOL. Along with plasma diffusivity coefficients, our transport model introduces time-independent anomalous convective velocities across the confining magnetic field. In the discharge modeling, diffusivity coefficients and ACFC velocity profiles are adjusted to match a set of representative experimental data. We use this model in the edge plasma physics code UEDGE to simulate the multi-fluid two-dimensional transport for these three tokamaks. We present simulation results suggesting the dominance of anomalous convection in the far SOL transport. These results are consistent with the hypothesis that the chamber wall is an important source of impurities and that different impurity charge states have different magnitude and sign of their anomalous radial convective velocities.

Correlation of Density Pedestal Width and Neutral Penetration Length

Abstract: Pedestal studies in DIII-D and MAST find good correlation between the width of the H-mode density barrier and the neutral penetration length [1, 2]. These results suggest that the width may be set by the combined effects of neutrals and plasma transport. This paper is a report on fluid simulations of boundary plasma using the BOUT code[3] with a neutral source added. Thus both neutral and plasma physics are treated. The plasma transport is self-consistently driven by boundary turbulence due to the resistive X-point mode, while neutrals are described by a simple analytic model. The plasma profiles are evolved on the same time scale as the turbulence for the given heat source from the core plasma and particle source from the neutrals. For prescribed neutral profiles, we find the formation of a density pedestal inside the separatrix in the L-mode even though the calculated plasma diffusion coefficients are almost radially constant and without the formation of a temperature pedestal. These results support the hypothesis that particle fueling can provide the dominant control for the size of the H-mode density barrier. The width of the density barrier decreases as the pedestal density increases which is also consistent with experimental data.

Turbulence in the divertor region of Tokamak edge plasma

Abstract: Results of recent modeling of tokamak edge plasma with the turbulence code BOUT are presented. In previous studies with BOUT, the background profiles of plasma density and temperature were set as flux surface functions. However, in the divertor region of a tokamak, the temperature is typically lower and density is higher than those at the mid-plane. To account for this in the present study, a poloidal variation of background plasma density and temperature is included to provide a more realistic model. For poloidally uniform profiles of the background plasma, the calculated turbulence amplitude peaks near outer mid-plane, while in the divertor region the amplitude is small. However, present simulations show that as the background plasma profiles become more poloidally non-uniform, the amplitude of density fluctuations, n i , starts peaking in the divertor. It is found that in the divertor region the amplitude of n i fluctuations grows approximately linearly with the local density of the background plasma, n i0 , while the amplitudes of T e and Φ fluctuations are positively correlated with the local electron temperature, T e0 . Correlation analysis shows that plasma turbulence above and below the x-point is largerly uncorrelated.

Navier‐Stokes Neutral and Plasma Fluid Modelling in 3D

Abstract: The 3D finite volume transport code BoRiS is applied to a system of coupled plasma and neutral fluid equations in a slab. Demonstrating easy implementation of new equations, a new parallel BoRiS version is tested on three different models for the neutral fluid - diffusive, parallel Navier-Stokes and full Navier-Stokes - and the results are compared to each other. Typical effects like density enhancement by ionization of recycled neutrals in front of a target plate can be seen and differences are linked to the neutral models in use.