Interplay of Turbulence, MARFE Dynamics, and Density Limit in Negative Triangularity Plasmas on DIII-D

This study investigates the microscopic physics of the density limit in negative triangularity plasmas in the DIII-D tokamak. By varying the input power up to 12 MW, we examined the relationship between maximum achievable density and power input, noting a modest positive correlation. Detailed measurements highlighted the behavior of turbulence and shear flows, particularly as the density limit is approached. Initially, turbulence at the low-field-side (LFS) increases prior to the emergence of the X-point multifaceted asymmetric radiation from the edge (MARFE). Subsequently, this MARFE migrates towards the high-field-side (HFS), intensifying HFS turbulence and precipitating significant edge cooling. The appearance of HFS MARFE correlates with the saturation of edge equilibrium density (ρ>0.95) at a level below the Greenwald limit, while the core density continues to increase, surpassing the Greenwald density. However, the growth of core density starts to cease when mid-plane line-averaged density fluctuations surpass a critical threshold, consistent with a collapse in the mean shear layer and reduced confinement time. Moreover, the adiabaticity parameter drops below unity when disruptive events are observed. These findings offer novel insights into the complex interplay between MARFE dynamics, turbulence, and the density limit in tokamak plasmas.