DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	2: Hysteresis and Turbulence Spreading in the H-L Back Transition
Name:	Lothar Schmitz schmitzl@fusion.gat.com	Affiliation:	University of California, Los Angeles
Research Area:	L-H Transition	Presentation time:	Not requested
Co-Author(s):	G.R Tynan, G.R. McKee, Z. Yan, P.H. Diamond, L. Zeng, J.A. Boedo, T.L. Rhodes, E.J. Doyle	ITPA Joint Experiment :	No
Description:	For burning plasma operating near the LH power threshold, understanding the hysteresis and spatio-temporal evolution of the H-L back transition is crucial. In addition, if back transitions can be controlled or the transition sequence slowed down, safe ramp-down of the current and beta-pol without undue stress on the poloidal field system/plasma control may be achieved. The goal of this experiment is to understand the observed hysteresis in the L-H and H-L transition power, and the feedback cycle that potentially controls the back transition. The proposed experiment will investigate the evolution of the pedestal radial profiles (via fast profile reflectometry) and will allow allow mapping of ExB flow velocity and density fluctuation level profile (via DBS/BES), across the LCFS, with high time/spatial resolution during H-L back transitions. This will elucidate the back transition mechanism and allow an assessment of turbulence spreading. The understanding gained may suggest a possible approach for inducing/controlling the back transition sequence in ITER.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Two scenarios will be explored; an ITER-similar shape, and a low triangularity, low P_th shape (#149725) where limit cycle back transitions have been reliably obtained previously and probe insertion is feasible. We will use a) a combination of several de-rated beams to obtain a gradual reduction in P_NBI; b) a shape change - (either USN to LSN, or varying the x-point height) to induce a back transition during ELM-free H-mode. It is important to avoid the customary type-I ELM trigger observed in many H-L back transitions, to decouple ELM/MHD and turbulence dynamics. Turbulence/flow coupling will also be investigated via the reciprocating probe (inserted up to ~ 1 cm inside the LCFS once repetable back transitions are obtained). The experiment will be carried out starting with two different L-mode densities, n. 2x 10^13 cm^-3, and 4-5x10^13 cm^-3. A dedicated experiment is needed to achieve H-L transition not triggered by ELMs and to allow a sufficient number of repeat shots to accommodate the midplane probe and DBS/BES diagnostic mapping.
Background:	The detailed H-L back transition process is not sufficiently understood. The spatial/temporal evolution of the pedestal radial profiles and turbulence characteristics during the back transition can now be addressed on DIII-D with superior diagnostic capabilities, and will likely add substantially to our understanding of forward L-H transitions. Knowing the L-H and H-L transition power hysteresis and its dependence on density is likewise important to ensure reliable H-mode operation. This experiment builds on previous L-H transition experiments focused on forward transitions, in a low triangularity shape favoring extended limit cycle transitions, and in an ITER-similar shape.
Resource Requirements:	Resource Requirements:30,330,150 Beams, possibly ECH
Diagnostic Requirements:	BES large array centered on pedestal region, DBS-5,DBS-8, midplane probe in the low triangularity scenario
Analysis Requirements:	--
Other Requirements:	--