"First Direct Evidence of Turbulence-Driven Ion Flow Triggering the L- to H-Mode Transition," by Lothar Schmitz (UCLA)

Date: 
Friday, December 12, 2014 - 1:00pm to 2:00pm
Series: 
Plasma Seminars

Plasma Seminar

Physics and Astronomy Building (PAB) Room 4-330
Friday, December 12, 2014
1-2pm
Refreshments at 12:30pm

Speaker: Lothar Schmitz (UCLA)

Talk Title: "First Direct Evidence of Turbulence-Driven Ion Flow Triggering the L- to H-Mode Transition"

Abstract:

H-mode operation is almost certainly required for burning tokamak plasmas. However, a physics-based model of the L- to H-mode transition, and the threshold power scaling is still lacking. We report recent progress in mapping the detailed formation dynamics of the edge transport barrier at the L- to H-mode transition via Doppler Backscattering in DIII-D. Transitions near the power threshold exhibit limit cycle oscillations (LCO), where the E×B flow (shear) and edge turbulence are periodically modulated, expanding the transition timescale. LCO demonstrate that both turbulence-driven flow (via the perpendicular Reynolds stress) and diamagnetic flow (via the increasing pressure gradient) act synergistically in securing the H-mode transition. For the first time, high resolution main ion CER measurements in a helium plasma show that the initial turbulence collapse preceding the L-H transition is triggered via turbulence-generated poloidal ion flow opposing the L-mode edge plasma equilibrium E×B flow. A dipolar meso-scale flow pattern subsequently evolves. The periodic reduction of edge turbulence and transport enables a gradual increase of the edge pressure gradient and ion diamagnetic flow, eventually sustaining strong equilibrium E×B shear and locking in H-mode confinement. A predator-prey model, retaining opposite polarity of the turbulence-driven and pressure-gradient-driven E×B flow, captures essential aspects of the transition dynamics. 

Location: 
PAB 4-330