Safety Factor and Turbulence Dynamics Dependence of the L-H Power Threshold on DIII-D

The L-H transition power threshold (P_LH) is found to have a significant but previously unidentified dependence on the plasma current, or q₉₅, at mid-density (n_e~3.2e19 m^-3) on DIII-D. Comprehensive 2D turbulence and flow measurements in the plasma edge reveal the co-existence of two frequency bands of broadband modes across the L-H transition with higher flow shear at lower current, which can help explain the linear decrease in P_LH as the plasma current is reduced from 1.4 MA to 1 MA. At lower density, n_e~1.5e19 m^-3, there is little dependence of P_LH on the plasma current. Density fluctuation measurements by beam emission spectroscopy (BES) show that the lower frequency band (<20 kHz) of the broadband modes propagates in the ion diamagnetic direction at low plasma current, whereas the higher frequency band (> 20 kHz) propagates in the electron diamagnetic direction (in the lab frame). This bimodal turbulence structure has been previously observed in plasmas with lower P_LH, such as those with ion grad-B drift towards X-point and in the isotope dependence (D lower than H). Nonlinear simulations with BOUT++ shows similar dual-mode with experiments, with the most unstable modes being resistive ballooning modes. Additionally, the amplitude of long wavelength (K_perpRho_I<1) density fluctuations is higher at lower currents, implying a higher drive for a Reynolds Stress driven zonal flow. This is consistent with the higher (turbulence driven) flow shear observed at low current, and is qualitatively consistent with the lower zonal flow collisional damping at higher q (lower current). Understanding the physics behind the P_LH dependence on multiple parameters and accurately predicting P_LH is important to ensuring that future burning plasma experiments such as ITER can access high confinement modes of operation.