First Experimental Evidence of Turbulence-driven Main Ion Flow and ExB Flow Triggering the L-H Transition

Simultaneous measurements of main ion flow, $E\times B$ flow, and turbulence level $\tilde{n}/n$ inside the separatrix (LCFS) show for the first time that the initial turbulence collapse preceding the L-H transition is due to turbulence-driven ion flow and $E\times B$ flow in the ion diamagnetic direction, opposing the pressure-gradient-driven equilibrium $E\times B$ flow in the L-mode phase. Low to high confinement (L-H) transitions characterized by limit cycle oscillations (LCO, [1]) allow probing the trigger dynamics and synergy of turbulence-driven meso-scale flows, and pressure-gradient driven flows with high spatio-temporal resolution. A density/plasma current scan indicates that the LCO is triggered at a critical value of turbulence-driven flow shear. Near the minimum of the electric field well, turbulence-driven flow in the electron diamagnetic direction is observed. The radial flow (shear) reversal is consistent with the direction of the ($\tilde{n}$, $E_r$) limit cycle observed just inside the LCFS in DIII-D (and recently in LCO-H-mode transitions in HL-2A and JFT-2M), and the reversed limit cycle direction observed in the inner shear layer. Causality of shear-flow generation has been established: early during LCO, the $E\times B$ shearing rate leads the ion pressure gradient increase; during the final phase of the LCO, the edge pressure gradient and ion diamagnetic flow are modulated and increase, and the shearing rate lags the ion pressure gradient. Pressure-gradient-driven shear then becomes sufficiently large to secure the final LCO-H-mode transition. A two-predator, one-prey model, similar to a previously developed model [2] but retaining arbitrary polarity of turbulence-driven flow with to respect pressure-gradient-driven $E\times B$ flow, captures essential aspects of the transition dynamics, including the magnitude and direction of the driven poloidal main ion flow.\par \vskip6pt \noindent [1] L.~Schmitz, et al., Phys.\ Rev.\ Lett.\ {\bf 108}, 155002 (2012).\par \noindent [2] K.~Miki and P.H.\ Diamond, Phys.\ Plasmas {\bf 19}, 092306 (2012).