3-DIMENSIONAL SIMULATION OF DEL-TI-DRIVEN TURBULENCE AND TRANSPORT

The anomalous transport of ion thermal energy in a heated plasma slab confined by a uniform, straight magnetic field B(z) has been investigated in three-dimensional (3-D) fluid simulations. Convection flows are driven unstable by DELTA-T(i) and nonlinearly develop into narrow streams which carry cold edge plasma into the hot center. The convective flows undergo a sharp transition from laminar to turbulent behavior as the thermal energy confined in the slab is increased beyond a critical level. This transition is reminiscent of similar behavior in Rayleigh-Benard convection in heated fluids. The convective thermal transport increases sharply as this turbulence threshold is exceeded. The structure of the flow patterns and associated transport also depend strongly on the physical dimensions of the confined plasma (L(x),L(y),L(z)) = (a,2-pi-a,2-pi-R) compared with the ion Larmor radius rho-i. For alpha = a2/R-rho-i > 1, the dominant flows have k(y-rho-i approximately a/R and produce an anomalous cross field thermal transport chi-perpendicular-to-i that scales as upsilon(i)a2/R. In the opposite limit alpha = a2/R-rho-i < 1, dominant flows have k(y)a approximately 1 and the transport is given by chi-perpendicular-to-i approximately rho-i-upsilon-i, the Bohm scaling. Close to marginal stability, the transport is greatly reduced. These simulations imply that ion thermal transport in any straight field system such as a tandem mirror or a stellarator with weak shear and nearly rational fields will be strongly anomalous if the DELTA-T(i) threshold for instability is exceeded.