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	301: Access to the small ELM regime with a shape control
Name:	Joon-Wook Ahn jahn@pppl.gov	Affiliation:	Oak Ridge National Lab
Research Area:	ELM Control	Presentation time:	Requested
Co-Author(s):	R. Maingi	ITPA Joint Experiment :	No
Description:	This experimental proposal seeks to access the small ELM regime, possibly with wider operation window than the existing small ELM regimes such as type-II and grassy ELMs, by the means of plasma shape control. This is based on the empirical observation of small ELMs at KSTAR in the last two consecutive campaigns; small ELM phases were observed in a rather wide operating windows, i.e. lower densities (ne/nG <0.4) and various configurations (DN, USN, LSN, limited). From the experience at KSTAR, it is expected that the adjustment of three shape parameters could lead to the access of a small ELM regime with good confinement, i.e. higher squareness, non-conventional inner separatrix shape (more straight or even slightly concave), and smaller drsep (|drsep| < 1 - 1.5cm). A stability analysis at DIII-D and experimental results (Leonard, NF 2007) indicate that higher squareness reduces pedestal pressure with smaller ELMs, and is beneficial for maintenance of a steady internal 3/2 TM and good confinement. This result is consistent with the experimental observation at KSTAR that higher squareness led to small ELMs. There is no recent ELM stability analysis regarding the non-conventional inner separatrix shape to the authors' knowledge, but there are early works on the bean/crescent shape in mid-80's which showed that this configuration will offer an easier access to the 2nd stability regime. As for the smaller drsep, recent stability analyses, e.g. Saibene NF 2005 (JET) and Saarelma PPCF 2009 (MAST), demonstrated that this configuration can help, along with high collisionality, improve stability against type-I ELM triggering P-B modes and instead hit the stability boundary set by the high-n ballooning modes, therefore small ELMs. In fact, small ELMs at KSTAR did not require high density and we speculate that this might be due to the combined effect of non-conventional inner separatrix shape. This might also explain why the drsep values for KSATR small ELMs (|drsep| < 1 - 1.5cm) are noticeably larger than the conventional drsep requirement for type-II ELMs (|drsep| < 0.5cm).	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Three shape parameters will be varied; plasma squareness, shape of inner separatrix, and drsep. As the plasma squareness has been already shown to change the ELM size at DIII-D (Leonard, NF 2007), a maximum allowable squareness (to be determined through discussion with the GA staff) will be used in combination with varied inner separatrix shape and drsep. This will save the number of discharges needed for the experiment. Ideally it will be best to change the inner separatrix shape in a single discharge to check its impact on ELMs, e.g. convex --> straight --> concave, so is the drsep (LSN --> DN --> USN). The final goal is to find the optimal combination of these parameters for the most preferred small ELM regime.
Background:	Naturally occurring small/no ELM regimes are a preferred H-mode operation mode for ITER as well as the RMP ELM suppression. However, the operating window has been historically known quite narrow. We propose here to explore the possibility of accessing small ELM regime with a significantly wider parameter window, i.e. density and magnetic configuration, by controlling the shape of plasma.
Resource Requirements:	Major auxiliary heatings (NBI and ECH), EFIT reconstruction
Diagnostic Requirements:	TS, CER, magnetics, ECEI, BES, fast IR camera, edge turbulence diagnostics, divertor spectroscopy, etc.
Analysis Requirements:
Other Requirements: