Reducing the L-H Power Threshold in ITER - What Can We Learn from Microscopic Transition Physics?*

We demonstrate for the first time that fast electric field transients triggering the L-H transition are quantitatively consistent with the combined radial polarization (displacement) currents due to Reynolds stress, thermal ion orbit loss, and ion viscosity. These $E_{\mathrm{r}}$ transients (typically 0.05-1 ms) can produce large \textbf{\textit{E}}x\textbf{\textit{B}} shear and can trigger L-H transitions when the L-mode ``equilibrium'' shear flow due to the ion pressure gradient is insufficient to suppress edge turbulence. Typical examples are plasmas with unfavorable grad-$B$ drift direction and/or strong toroidal co-current rotation. Edge turbulence is suppressed once the transient \textbf{\textit{E}}x\textbf{\textit{B}} shearing rate exceeds the plasma frame turbulence decorrelation rate [1]. \quad Initial experiments indicate that the L-H transition power threshold $P_{\mathrm{LH}}$ can be reduced at low ion collisionality via Neoclassical Toroidal Viscosity (NTV) from applied n$=$3 non-resonant magnetic fields (NRMF). CER data confirm that the applied NTV counter-current torque locally reduces L-mode edge toroidal co-rotation, increasing the shear in the $\mbox{v}_{\phi } B_{\theta } $ term in the radial ion force balance. The well-known increased $P_{\mathrm{LH}}$ with unfavorable grad-$B$ drift direction is attributed to reduced shear flow in the outer shear layer due to higher (intrinsic) edge co-rotation. This increase is often mitigated in ITER-similar-shape plasmas in DIII-D via localized rotation reversals in the inner shear layer, triggered by sawteeth or transport avalanches. \quad These new insights can open up paths for reducing $P_{\mathrm{LH}}$ during the initial ITER hydrogen campaign with limited auxiliary power, by generating edge NTV [via the planned (partial) 3-D coil set], by exploiting edge magnetic topology modifications due to MHD modes, or by localizing power deposition to critical edge layers. [1] L. Schmitz et al., Phys. Rev. Lett. \textbf{108}, 155002 (2012).