Fluid simulations of conducting-wall-driven turbulence in boundary plasmas

Abstract

It is clear that the edge plasma plays a crucial role in global tokamak confinement. This paper is a report on simulations of a new drift wave type instability driven by conducting wall (also originally named as a ∇Te instability) [Phys. Fluids B 3, 1364 (1991)]. A 2d(x,y) fluid code has been developed in order to explore the anomalous transport in the boundary plasmas. The simulation consists of a set of fluid equations (in the electrostatic limit) for the vorticity ∇⊥2φ, and the temperature Te in a shearless plasma slab confined by a uniform, straight magnetic field Bz with two divertor (or limiter) plates intercepting the magnetic field. The model has two regions separated by a magnetic separatrix: In the edge region inside the separatrix, the model is periodic along the magnetic field while in the scrapeoff layer (SOL) region outside the separatrix, the magnetic field is taken to be of finite length with model (logical sheath) boundary conditions at diverter (or limiter) plates. The simulation results show that the observed linear instability agrees well with theory, and that a saturated state of turbulence is reached. In saturated turbulence, clear evidence of the expected long-wavelength mode penetration into the edge is seen, an inverse cascade of wave energy (toward both long wavelengths and low frequencies) is observed. The simulation results also show that amplitudes of potential and the electron temperature fluctuations are somewhat above and the heat flux are somewhat below those of the simplest mixing-length estimates. A full inverse cascade of the turbulence indicates that the cross-field transport is not diffusive. A self-consistent simulation to determine the microturbulent SOL electron temperature profile has been done, the results of which reasonably agree with the experimental measurements.