Transitions of turbulence in plasma density limits

A series of BOUT [X. Q. Xu et al., Phys. Plasmas 7, 1951 (2000)] simulations is conducted to investigate the physical processes which limit the density in tokamak plasmas. Simulations of turbulence in tokamak boundary plasmas are presented which show that turbulent fluctuation levels and transport increase with collisionality. At high edge density, the perpendicular turbulent transport dominates the parallel classical transport, leading to substantially reduced contact with divertor plates and the destruction of the edge shear layer, and the region of high transport then extends inside the last closed flux surface. As the density increases these simulations show resistive X-point $\mathrm{mode}\to \mathrm{resistive}$ ballooning modes. The simulations also show that it is easier to reach the density limit as the density increases while holding pressure constant than holding temperature constant. A set of 2D transport simulations with increasingly large radial outboard transport, as indicated by BOUT for increasing density, shows that such transport can lead to an X-point multifaced asymmetric radiation from the edge when impurity radiation is included, which is a common symptom of density-limit related disruptions. BOUT further demonstrates that the local transport scaling with the current is similar to the global low-confinement-mode (L-mode) transport model ${\mathrm{(\tau }}_E{\mathrm{\propto I}}_p)$ (by fixing q profiles). This current scaling appears on a plot of discharge current versus density as abruptly large radial transport once the Greenwald density is approached or exceeded. All of these results indicate that rapid edge cooling due to large radial transport is a key for the physics of the tokamak density limit.