Collisionality and magnetic geometry effects on tokamak edge turbulent transport. II. Many-blob turbulence in the two-region model

Abstract

A two-region model, coupling the outboard midplane and the X-point region, was proposed in Paper I [J. R. Myra, D. A. Russell, and D. A. D’Ippolito, Phys. Plasmas 13, 112502 (2006)] to study the effects of collisionality and magnetic geometry on electrostatic turbulent transport in the edge and scrape-off layer of a diverted tokamak plasma by filamentary coherent structures or “blobs.” Attention was focused on the properties of isolated blobs. That study is extended here to the many-blob, turbulent saturated state driven by a linearly unstable density profile. The evolution of the density profile is included. It is demonstrated that turbulent density transport increases with collisionality but decreases with enhanced magnetic field-line fanning and shear in this model. Field-line shear induces poloidal velocity in isolated blob propagation and de-correlates the electrostatic potentials in the two regions in the turbulent regime. Probability density functions of density flux resemble those of experimental probe data: both are insensitive to magnetic field geometry and collisionality. It is shown that blobs are born where the skewness of density fluctuations vanishes and the logarithmic pressure gradient is maximized. The simulations show increased particle fluxes with increased plasma resistivity, which are due to increases in both blob velocity and creation rate (or spatial “packing fraction”). A wavelet-type Gaussian-fitting analysis is used to study the dependence of blob velocity on blob size. It is found that streamers, which dominate the simulations, move faster than circular blobs when the two regions are electrically disconnected.