#
62nd Annual Meeting of the APS Division of Plasma Physics

## Volume 65, Number 11

##
Monday–Friday, November 9–13, 2020;
Remote; Time Zone: Central Standard Time, USA

### Session GI02: Invited: Magnetic Fusion: Pedestal and Transport

9:30 AM–12:30 PM,
Tuesday, November 10, 2020

Chair: Amanda Hubbard, MIT

### Abstract: GI02.00006 : Reducing the L-H Power Threshold in ITER - What Can We Learn from Microscopic Transition Physics?*

12:00 PM–12:30 PM
Live

Preview Abstract
Abstract

####
Author:

Lothar Schmitz

(University of California, Los Angeles)

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).

*This work was supported by the US Department of Energy under DE-FG02-08ER54984, DE-FG02-08ER 54999, and DE-FC02-04ER54698.