Speaker
Jon Christian Rost
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1094.pdf
Observations of sheared turbulence in the H-mode Er well by phase
contrast imaging on DIII–D∗
J.C. Rost1 , A. Marinoni1 , E.M. Davis1 , M. Porkolab1 , K.H. Burrell2
1 Massachusetts Institute of Technology, Cambridge, USA
2 General Atomics, San Diego, USA
Phase Contrast Imaging (PCI) has been used on DIII–D to measure turbulent density fluc-
tuations in several H-mode regimes, observing highly sheared turbulence in the Er well. Two
sources are identified: instabilities in the pedestal that extend into the Er well and instabilities
located in the well itself. PCI has a high bandwidth 10 kHz < f < 2 MHz and wavenumber-
resolved measurements over 1 < k < 25 cm−1 , with a beam geometry that results in enhanced
sensitivity to turbulence distorted by velocity shear. The sheared edge turbulence resolves into
two frequency ranges with well-defined lab-frame phase velocities.
Studies of the medium frequency f < 800 kHz turbulence in the Quiescent H-mode regime
(QH-mode) scanned the plasma edge through the PCI beam, allowing the radial structure of the
sheared edge turbulence to be reconstructed, revealing turbulence with kr < 0 on the inner half
of the Er well and with kr > 0 on the outer half. Varying the injected torque in QH-mode plasmas
shows that the lab-frame phase velocity of this turbulence varied directly with the E×B velocity
at the top of the pedestal. In combination, these observations suggest that an instability located
at the top of the pedestal extends into the Er well, where the shear distorts the turbulence.
The high frequency, high phase velocity turbulence is, in contrast, observed to change on sub-
ms time scales with changes in the Er well, forming within 100 µs of the L-H transition, and
appearing and vanishing as the Er well collapses and reforms during Limit-Cycle Oscillations
(LCO) and at an ELM. The lab-frame phase velocity is seen to vary with VE×B at the center
of the well. The instability is sensitive to the shape of the Er well, being absent in the very
narrow well seen in QH-mode but often present in the wider well seen in ELM-free H-mode
and wide-pedestal QH-mode.
The research presented here characterizes highly-sheared density turbulence in the pedestal
and Er well of non-ELMing H-mode regimes with the ultimate goal of understanding the role
of turbulence in determining the structure in these regimes.
∗ Work supported in part by the US Department of Energy under DE-FG02-94ER54235, DE-
FC02-04ER54698, and DE-FC02-99ER54512.
