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Title	103: Probing and controlling the L->H and H->L transitions by small pellet injection
Name:	Sang-hee Hahn hahn76@kfe.re.kr	Affiliation:	Korea Institute of Fusion Energy
Research Area:	L-H Transition	Presentation time:	Requested
Co-Author(s):	P. Gohil, G. Tynan, L. Baylor, P. H. Diamond, D. Battaglia, Kazuhiro Miki, Weiwen Xiao, L. Schmitz, Z. Yan, G. McKee, M. Xu	ITPA Joint Experiment :	No
Description:	We propose studies of stimulated L-->H and H -->L transitions by small/shallow pellet injections. The motivations for this study are both pragmatic and scientific. For pragmatic motivations, we have i) ITER will operate at marginal Pth. Control of back transitions is of great interest as well as L-->H in a practical view. Previous work done at D3D indicated possibilities of reduction of the transition heating power by 30% in the pellet-stimulated transition. Since the pellets are very fast actuators, the work may address an effective, minimum invasive method for controlling enhanced/controlled hysteresis. ii) From the previous work done at various machines/apparatus, the key of the phenomena seems to be edge radial electric field (Er) development. That would evoke questions on the critical deposition depth & size of the particle injections which may develop stiff density profiles to produce enough Er. iii) The pellet-stimulated transition has a lifetime per injection, hence it would provide a way to study the back transitions without changing major parameters (current / heating / shape �¢?�¦). For physics/scientific motivations, we note: i)Pellet-induced profile perturbations are techniques for explorations of the mean flow-zonal flow-turbulence system which controls the L-->H transition. The response of the system to localized gradient perturbations and cooling fronts can be probed with modern fluctuation diagnostics. ii)Explore the physics of the �¢??stimulated transition�¢?, as opposed to usual spontaneous transitions �¢?? Are they at different transition pathways? What role does the ZF, seemingly critical to the spontaneous transition, play in this stimulated transition? iii)Such perturbative experiments are ideal for critical tests for any L-->H models. A 1D transport model has been used to predict the existence of novel states, such as driven sustained H-mode with subcritical heat power.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	H-mode D2 discharge with EC power is desired for usage of LFS probe arrays, in order to identify MF shear 1) Do a D2 LSN discharge with a EC power scan, from (0.5, 0.6, 0.7, 0.8, 0.9,1.0, 1.1, 1.2) x Pthr in a single discharge. Use EC modulations for the power change knob. 2) Inject a pellet per each power step. Observe if there's L-H-L transition under Pthr. 3) Choose the power level with L-H-L, make a scan of period. If period of L-H-L is tau, period is (0.3 0.6 0.9 1.5)x tau. 4) If possible, change pellet length to bigger/smaller size and repeat 2). Find L-H-L period. repeat 3) with the power level found.
Background:	Experimental observations of invoking L-H transitions by strong gas injections have a long history: A time-limited transition triggered by rapid gas puff / LiD pellet at Tuman III showed limited evidences the mode was triggered by edge radial E changes (Askinazi et al, 1993). After a decade, the DIII-D experiments by LFS pellets found and measured that the pellet triggered immediate edge Er change, which reduced the Pthr by ~30 % (P. Gohil et al, PPCF, 2003). No fluctuation data was available at that time to relate the observation with the mechanism of turbulence suppressions. Those previous works exclusively focused on L-> H dynamics and did not try to deal with H-> L control issue. Recent experiments done at HL-2A (Duan, NF 2010) and KSTAR, using a supersonic molecule beam injection (SMBI), gave chances to revisit the dynamics including the back transition. The SMBI-stimulated transition has shallower depositions of the particles on the edge, and has a lifetime depending on the amount of dosage and base pressure, which can be used as a very handy technique to control the L-H-L cycle. The SMBI has very similar characteristics as LFS small pellets used in DIII-D for ELM mitigation studies (L. Baylor, IAEA 2012).
Resource Requirements:	6-12 shots of total run might be desired, can be divided by 2 sessions if pellet length change is allowed: 1.3 mm x (0.9 - 1.8mm) D2 pellet injection system at LFS (midplane is ideal) EC heating & modulations enough to get an spontaneous H-mode (3-6 Gyrotrons)
Diagnostic Requirements:	Bolometers & Zeff Filterscopes (D-alpha) BES large array focused on the edge DBS Microwave reflectometry CER at edge : Er, Ti and rotations Edge Thomson for pedestal Te / ne profile LFS probes for MF shear measurement
Analysis Requirements:	N/A
Other Requirements:	N/A