Study of edge transport near density limit with BOUT++ simulations*

As a first step to understand edge plasma behaviors, especially near the density limit before disruptions, a series of BOUT++ simulations are carried out to study the edge turbulence and transport prior disruptions. We start with a simulation of a DIII-D lower single-null density-limited shot right before density ramping. Strongly inboard-outboard asymmetric fluctuation indicates that the dominate turbulence driver is the resistive ballooning mode. As the density gradually increases turbulence and radial transport are dramatically enhanced likely due to the increasing resistivity. The broadened heat flux width on outboard mid-plane are observed in our simulations. We also find that radial transport starts to dominate parallel transport near the density limit which could potentially retract edge plasma profile, induce detachment and x-point MARFE formation, and eventually lead to disruption.