Speaker
Xi Chen
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O2.112.pdf
Wide Pedestal Quiescent H-mode Plasmas in DIII-D Tokamak
Xi Chen1, K.H. Burrell1, T.H. Osborne1, K. Barada2, D. R. Ernst3, C. Chrystal1, B.A.
Grierson4, G.R. McKee5, T. Odstrcil3, C.P. Paz-Soldan1, C.C. Petty1, T.L. Rhodes2, J.C.
Rost3, W.M. Solomon1, T.M. Wilks3, Z. Yan5 and L. Zeng2
1
General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, USA
2
University of California Los Angeles, P.O. Box 957099, Los Angeles, CA 90095 USA
4
3
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Princeton
5
Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543-0451, USA
University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA
A new quiescent H-mode (QH-mode) regime with enhanced pedestal without ELMs has
been discovered in the DIII-D tokamak in ITER relevant low torque and collisionality [1,
2]. The regime was originally discovered in conventional QH-mode when the counter-Ip
neutral beam torque drops to ~2Nm in balanced double null shape. Across the transition,
the pedestal electron pressure increases by 60% and widens by 50% and the plasma
confinement rises by 40%. It is referred to as ‘wide-pedestal QH-mode’ because the
pedestal width exceeds the EPED-KBM limit. The onset of edge broadband MHD modes
and micro-turbulence accompanied with a lower ExB shear in this region is a common
feature of the wide-pedestal QH, instead of the edge harmonic oscillations (EHO). It is
conjectured that the increased transport provided by these edge modes reduces the pedestal
gradients resulting in an enhanced pedestal while still remaining below the ELM-limit.
The wide-pedestal QH-mode has been created and sustained in a range of shapes from
slightly upper single null to lower single null, including the ITER similar shape and for a
range of torques spanning the ITER equivalent range. Wide-pedestal QH has been initiated
and sustained with net zero NBI torque throughout the discharge with good confinement
and terminated only due to hardware constraints. Surprisingly, confinement improvement
has been observed with core electron heating using ECH. Impurity transport is studied with
Silicon and Aluminum injection using laser blow-off diagnostics.
The role of edge magnetic and density fluctuations in forming the wide pedestal is being
studied. The broadband MHD is composed of two counter-propagating low-k branches
while the intermediate-k density turbulence propagates in the electron-direction (lab frame)
and oscillates periodically [3]. A flat spot is observed in the pedestal profiles of wide-
pedestal QH-mode, especially that of the electron temperature. The location of the flat spot
is close to the location of the peaking of the amplitudes of some of these edge modes.
[1] K.H. Burrell et al, Phys. Plasmas 23, 056103 (2016). [2] Xi Chen, et al, Nucl. Fusion
57, 086008 (2017). [3] K. Barada, et al, accepted by Phys. Rev. Lett.
This work was supported in part by the US Department of Energy under DE-FC02-04ER546981,
DE-FG02-08ER549842, DE-FG02-94ER542353, DE-AC02-09CH114664, DE-FG02-08ER549995.
