Edge Transport Barrier Evolution and H-mode inhibition in DIII-D Negative Triangularity Plasmas*

Diverted negative triangularity plasmas (NT, $delta_{Ave} < -0.2$) in DIII-D remain resiliently in L-mode, without evidence of edge bifurcation in turbulence and flows, but exhibit H-mode-like thermal confinement and normalized pressure $eta_N$. With counter-$I_{p}$ neutral beam injection, the $ extbf{ extit{E}} imes extbf{ extit{B}}$ velocity in the plasma edge can significantly exceed typical positive triangularity (PT) L-mode values; however, the $ extbf{ extit{E}} imes extbf{ extit{B}}$ shearing rate only exceeds the turbulence decorrelation rate within an extremely narrow (few Gyro-radii wide) edge layer. At weakly negative triangularity ($delta_{Ave}> -0.2$), the shearing rate exceeds the decorrelation rate across a wider edge layer, resulting in limit cycle oscillations (LCO [2]) and/or sustained H-mode. Low and intermediate wavenumber edge density fluctuations are reduced intermittently during LCO. We present the L-H transition power threshold parametric dependence on triangularity, neutral beam torque, and plasma current (safety factor $q_{95}$), and edge turbulence and flow correlation analysis across the L-mode/LCO and H-mode regimes. Results from initial gyrofluid stability analysis of the edge/outer core plasma is also shown.