DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
Print this page

Print this page
Title	313: Hybrids with co-NBI QH-mode edge
Name:	Clinton (Craig) Petty petty@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Inductive Scenarios	Presentation time:	Not requested
Co-Author(s):	--	ITPA Joint Experiment :	No
Description:	This experiment has both a physics goal and a programatic goal. The physics goal is to determine whether poloidal magnetic flux pumping in hybrids (the phenomena that maintains qmin>1) is due to a coupling between ELMs and the 3/2 mode. If this is the case, a hybrid with QH-mode edge should evolve to qmin<1. We can test whether the 3/2 mode is still modifying the q profile in this new regime by using ECCD aimed at the q=1.5 surface to suppress the 3/2 mode on demand. The programatic goal is to develop a ELM-suppressed hybrid plasma that can be a viable scenario for ITER. A QH-mode edge hybrid would be advantageous over an RMP case as the latter tends to develop locked modes at high beta as the tearing modes interact with the RMP. (On the other hand, I acknowledge that obtaining the required amount of edge co-rotation for QH-mode in ITER is "challenging".)	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The key to obtaining a co-rotation QH-mode is to spin the toroidal rotation to a high value before the L-H transition. To achieve this, we will use the standard hybrid startup with NBI pre-heating, but we will keep the density as low as possible and delay the L-H transition until well after Ip flat top by keeping the plasma shape strongly biased to an upper X-point. The large step-up in NBI power at the start of Ip flat top (which normally triggers the L-H transition) will instead lead to the formation of a strong ITB in and L-mode edge plasma. If allowed to continue for too long, which will eventually lead to a beta limit as the pressure profile is too peaked. However, just before we reach this limit we will trigger the L-H transition by jogging the plasma shape to a lower X-point, which will broaden the pressure profile as the H-mode pedestal forms. As the ITB relaxes to a hybrid-like regime, we expect to form a QH-mode edge owing to the strong co-rotational shear near the plasma edge. Note that after the L-H transition we will return the plasma shape to an upper X-point because that configuration gives the highest toroidal rotation rates. It is important to have q95>4 so that we are in the regime where sawteeth are normally suppressed in hybrid discharges. Since the QH-mode is dependent on q95, we can make some adjustments to give us the type of EHO that we want. The Ip flat top should expend to at least 6 s to allow the current profile time to fully relax. Besides demonstrating a hybrid plasma with a co-rotation QH-mode edge, we want to probe the stability limits by scanning beta_N from a low of 2.5 to above 3 (typically the no-wall limit is around beta_N=3.2). The last phase of the experiment explores the sensitivity of this new hybrid regime to the presence of the 3/2 mode. This will be done by using co-ECCD aimed at the q=1.5 surface to suppress the 3/2 mode on demand. In the normal ELMy hybrid regime the disappearance of the 3/2 mode results in a drop in qmin below 1. We want to see if the q profile responds to the presence of the 3/2 mode in this ELM suppressed regime.
Background:	The hybrid scenario with an ELMy H-mode edge is well known to have an anomalously broad current profile with qmin>1. This is a beneficial feature that helps to avoid triggering the 2/1 mode (owing to the lack of sawteeth and fishbones) and perhaps improves transport as well. Analysis of MSE data indicated that the anomalously broad current profile is due to poloidal magnetic flux pumping that occurs during an interaction between the 3/2 mode and ELMs [CC Petty et al., Phys. Rev. Lett. 102 (2009) 045005]. Thus, if this is correct, we should expect the current profile to behave classically (or neoclassically) in an ELM-suppressed hybrid plasma. Previous DIII-D experiments successfully suppressed ELMs with an n=3 RMP at q95=3.6 in the hybrid scenario at moderate beta values (beta_N<2.5) [CC Petty et al., Nucl. Fusion 50 (2010) 022002]. However, these plasmas were likely to slow down and lock owing to an interaction between the tearing modes and the RMP. Thus, it would be programatically beneficial to create an ELM-suppressed hybrid regime without RMP, which will likely be able to achieve higher beta_N. The QH-mode edge with co-NBI is a candidate for this.
Resource Requirements:	All co-NBI sources are required. Four gyrotrons are required.
Diagnostic Requirements:	--
Analysis Requirements:	--
Other Requirements:	--