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	378: Study of the connection between I-mode and low density Type III ELMs and optimization of I-mode
Name:	Thomas Osborne osborne@fusion.gat.com	Affiliation:	General Atomics
Research Area:	ELM Control	Presentation time:	Not requested
Co-Author(s):	R. Maingi	ITPA Joint Experiment :	No
Description:	Both the predator-prey oscillations near the L-H transition (Schmitz, Boedo) and the recent tentative I-mode discharge on DIII-D have characteristics similar to low density Type III ELM regime studies in the mid-1990s. In the proposed experiment build a better understanding of the connection between these regimes and exploit our understanding of the Type III regime and of pedestal optimization to improve the performance of I-mode discharges. ELM like relaxation oscillations are observed on D-alpha signals in all these regimes, being very high frequency in the I-mode discharge. Broadband density fluctuations seen on the reflectometers were observed to shutoff at the transition from the low density Type III ELM regime to the ELM free regime. In recent Type III discharges these modes have also been associated with a magnetic perturbation in the 50kHz range with n~-8 (electron drift direction). ECE measurements localize these modes to the region near the top of the pedestal in a zone where the Te gradient is still large but the density gradient is relatively low suggesting etae may be an important parameter. The pedestal pressure gradient in this regime is well below the (EPED1 predicted) KBM limit. The pedestal width is observed to rapidly narrow in the transition from Type III to the Type I ELM regime with the pressure gradient quickly coming into agreement with the EPED1 predicted KBM limit. This fact suggests the possibility of obtaining high pedestal pressure in this regime while avoiding Type I ELMs if sufficient pedestal width were obtained. The power required for transition from Type III to type I scaled as P=Ip**2.4/ne**2 which allowed operation continuously in the Type III regime at heating power well above the L-H transition power (6xPLH was achieved) at high enough plasma current and low enough density and discharges with continuous low density type III ELMs have been obtained at sufficiently low power or density. The threshold conditions for transition from Type III to I regimes in terms of local parameters was shown to be associated with a critical alpha parameter, suggestive of the Rogers-Drake and Pogutse-Igitkhanov models for the scaling of the low density branch of the L-H transition. During the type III studies it was found that the pedestal pressure and H factor increases as the threshold to transition to standard ELM free H-mode (followed by Type I ELMs) is approached (H93H=1 was achieved). If the recent I-mode discharges are indeed related to the Type III discharges, these scalings could act as a guide to obtaining high performance ELM free I-mode discharges.	ITER IO Urgent Research Task :	No
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