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	255: Super H-Mode
Name:	Philip Snyder snyderpb@ornl.gov	Affiliation:	Oak Ridge National Lab
Research Area:	Torkil Jensen Award	Presentation time:	Requested
Co-Author(s):	K. Burrell, R. Groebner, T. Osborne	ITPA Joint Experiment :	No
Description:	Background: Global tokamak fusion performance is strongly tied to the H-mode pedestal height. Because fusion power scales with pressure^2, while instabilities are driven by gradients, it is possible to consider tokamak optimization via moving much of the gradient region as far radially outward as possible. This also allows optimal plasma shaping in the gradient region as well as strong wall stabilization. As our understanding of both pedestal physics and core transport & stability continue to improve, it is of interest to try to make best use of this understanding (even if tentative) to explore methodologies to qualitatively improve potential fusion performance. Because extensive optimization studies have already been done, we can expect that new, qualitatively improved regimes will be characterized by difficult access. However, in some cases, theory can provide guidance into possible approaches to accessing such regime, and provide motivation via the substantial predicted benefits of the regime itself. One such predicted regime is what is sometimes referred to as "Super H-Mode". The existence of this regime is predicted by pedestal stability studies and by the EPED pedestal model. Theory predicts that, in strongly shaped discharges, it should be possible to access very high pedestal pressure at high density. However, starting at high density results in a high collisionality which suppresses bootstrap current and prevents access to high pressure (resulting in a relatively low pedestal pressure and ELMs). But there is a predicted parameter trajectory, starting at low density and later increasing density, that should allow access to this Super H-mode regime. With very strong shaping this can lead to markedly higher pedestal pressure. Access to this regime may be optimal via starting in QH-mode, but it is also possible to consider access via an ELM-ing AT regime. The EPED model predicts that access to a Super H-Mode edge should be possible in both DIII-D and ITER. Initial studies in DIII-D have suggested that it is possible to go at least partway into this regime, but that wall conditions and impurity concentration are very important issues for going further.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Approach: In a clean machine, ideally shortly after boronization, attain very strongly shaped plasmas, in a configuration optimized via EPED calculations, but with a LSN in order to minimize impurity accumulation. We'd like to consider two approaches: 1) Start in counter rotating QH-mode at low density, and increase density by reducing torque and core pellet fueling. After deuterium density increases have reached their limit, explore puffing of Neon (or other low-Z gas) to raise Zeff and increase collisionality at fixed density. Adjust density and Zeff together to optimize access to Super H regime. 2) Start in a low density co-rotating AT-like plasma - after reaching an initial ELMing steady state, steadily increase density via core pellet fueling. Then explore increases of Zeff using low-Z impurities to optimize approach to Super H-mode. In both cases, optimize shape and wall coupling to achieve very high pedestal pressure. Employ EC to stabilize core tearing modes and attempt to achieve very high global betaN.
Background:	--
Resource Requirements:	core pellet fuelling, low-Z impurity source (eg Neon)
Diagnostic Requirements:	Thomson, CER, fluctuation diagnostics across pedestal
Analysis Requirements:	EPED runs prior to expt
Other Requirements:	--