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Title	43: Density Control and Active Impurity Removal from Double-null H-mode Plasmas
Name:	Thomas Petrie petrie@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Divertor & SOL Physics	Presentation time:	Not requested
Co-Author(s):	--	ITPA Joint Experiment :	No
Description:	This experiment explores the possibility of using changes in the magnetic balance and gas puff program to both control core density and remove impurities from the core of DN and near-DN plasmas. Changing the magnetic balance from dRsep ? 0 to dRsep = + 0.2-0.5 cm (with the ion gradB drift downward) has been shown to reduce pedestal (and line-averaged) density by up to 50%. Previous studies of impurity injection have shown that argon concentration was about a factor of three higher in double-null H-mode plasmas when compared with the dRsep = +0.5 cm cases with the ion gradB drift direction toward the lower divertor. The issue we want to examine here is that do we get preferential loss of core impurities while maintaining core density by making small changes in dRsep. With simultaneous pumping on both outer divertor legs of a magnetically balanced high-triangularity DN, DIII-D IS UNIQUELY CONFIGURED TO MAKE A DEFINITIVE STATEMENT.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	This experiment is straightforward. Start with a DN shape and maintain a constant density throughout the discharge by putting the system in density feedback control starting at t = 2.0 s. The direction of the ion gradB drift is toward the lower divertor. Argon impurities are first injected into the private flux region of both divertors. Wait for steady conditions; this should take the discharge out to about t = 3.5 s. Between t = 3.5 s and t = 3.8 s, change dRsep from 0 to +0.5 cm. Hold dRsep = +0.3 cm from t = 3.8 s to 4.3 s, as argon is expelled from the plasma; density feedback will help maintain core plasma density. Then return dRsep to 0 at t = 4.3 s and finishing up at 4.6 s. Compare argon impurity density before dRsep is changed (t=3.45 s) with the impurity density after dRsep is restored to dRsep = 0 (t = 4.6 s). How long does it take the argon density to return to its original value? ---Repeat with neon.
Background:	The results of previous experiments have suggested the possibility of actively regulating plasma density by altering the magnetic balance of the plasma configuration. We also obtained a very limited set of data that suggested that impurities already in the core plasma can be exhausted by using this same regulating method. Demonstrating that we can (actively) control density and preferentially reduce the impurity content from the core plasma of near-DN plasmas while largely maintaining core density, provides a powerful tool that can significantly improve the prospects of futuristic tokamaks, which may have a serious problem with impurity accumulation in the core, including helium.
Resource Requirements:	Machine time: 0.25 (forward Bt), only the upper outer divertor and lower outer cryo-pumps are at liquid helium temperature, minimum 5 co-beam sources.
Diagnostic Requirements:	Asdex gauges inside both outer baffles, Asdex gauge in the upper PFR, core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, bolometer, core SPRED, and CER.
Analysis Requirements:	SOLPS/UEDGE, ONETWO
Other Requirements:	--