Impact of pedestal parameters on a controlled H-L back transition in DIII-D plasmas

H-mode is a plasma mode characterized by an increased pressure gradient in the pedestal region, resulting in a transport barrier and increased confinement time. In future commercial reactors, it will be important to control the transitions in and out of H-mode. The H-L back transition can occur in a benign manner (soft transition) or abruptly, producing a large transient (hard transition). During hard transitions, a sudden increase in the Dα light can be seen at the divertor, which can indicate damaging high ion flux to the plasma-facing components. Although superficially similar to ELMs that occur during H-mode, it has been demonstrated that the instability resulting in the H-L transient is not the ideal peeling-ballooning mode responsible for type-I ELMs. Two fluid simulations using the BOUT++ code have shown that the transient associated with hard H-L transition is sensitive to toroidal rotation, which has also been seen in experiment. In the work presented here, BOUT++ simulations are used to match characteristics of pedestal instabilities observed in DIII-D plasmas and characterize mode dependencies on quantities like density, radial electric field, and rotation.