Lower L-H transition power threshold via enhanced turbulence Reynolds stress and flow shear in favorable magnetic geometry

The L-H transition power threshold (P$_{\mathrm{LH}})$ in favorable magnetic geometry (ion $\nabla $B drift direction towards X-point) is up to a factor of 2 lower than in the unfavorable magnetic geometry (ion grad-B drift direction away from X-point) on multiple tokamaks. In a systematic experiment on DIII-D, the ion $\nabla $B drift direction was changed continuously from unfavorable to favorable configurations during the plasma shot. In the process the input power was kept constant at the value above the P$_{\mathrm{LH}}$ for favorable configuration, but lower than the P$_{\mathrm{LH}}$ for unfavorable configuration. Toroidal field and plasma current were also kept constant and there is little change in the edge n$_{\mathrm{e}}$ and T$_{\mathrm{e}}$ profiles. The density fluctuation amplitude, measured with BES decreases approaching the transition, while a large increase of turbulence Reynolds stress and turbulence flow shear are simultaneously observed during the slow varying of the ion $\nabla $B drift direction approaching the transition. These measurements demonstrate an important correlation between turbulence and turbulence driven flow and a lowering of P$_{\mathrm{LH}}$, provide insights into the underlying physics behind the hidden parameters and inform a more complete physics-based model of L-H transition power threshold.