FLUID SIMULATIONS OF CONDUCTING-WALL-DRIVEN TURBULENCE IN BOUNDARY PLASMAS

It is clear that the edge plasma plays a crucial role in global tokama k confinement. This paper is a report on simulations of a new drift wa ve type instability driven by conducting wall (also originally named a s a del T(e) instability) [Phys. Fluids B 3, 1364 (1991)]. A 2d(x,y) f luid code has been developed in order to explore the anomalous transpo rt in the boundary plasmas. The simulation consists of a set of fluid equations (in the electrostatic limit) for the vorticity del(perpendic ular-to)2 phi, and the temperature T(e) in a shearless plasma slab con fined by a uniform, straight magnetic field B(z) with two divertor (or limiter) plates intercepting the magnetic field. The model has two re gions separated by a magnetic separatrix: In the edge region inside th e 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) boun dary conditions at diverter (or limiter) plates. The simulation result s show that the observed linear instability agrees well with theory, a nd that a saturated state of turbulence is reached. In saturated turbu lence, 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 res ults also show that amplitudes of potential and the electron temperatu re fluctuations are somewhat above and the heat flux are somewhat belo w those of the simplest mixing-length estimates. A full inverse cascad e of the turbulence indicates that the cross-field transport is not di ffusive. A self-consistent simulation to determine the microturbulent SOL electron temperature profile has been done, the results of which r easonably agree with the experimental measurements.