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Title	81: Active Particle Exhaust of H-mode Plasmas in the Snowflake-minus Divertor Configuration
Name:	Thomas Petrie petrie@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Divertor & SOL Physics	Presentation time:	Not requested
Co-Author(s):	S.L. Allen, Vlad Soukhanovskii	ITPA Joint Experiment :	No
Description:	This experiment is the first attempt at evaluating how efficiently particles in an H-mode discharge can be actively removed in a Snowflake-minus configuration via cryo-pumping. Particle exhaust by the lower divertor cryo-pump is done at several radial locations relative to the entrance to the lower divertor pumping plenum. The pumping exhaust rate of the Snowflake plasma is plotted as a function of distance from the plenum entrance. These values are compared with the pumping exhaust rate of the corresponding standard configuration, for reference. This is done by pumping on the "standard" divertor case during the first part of the discharge until the pumping rate and core density are steady, then pulling the configuration into a Snowflake-minus configuration for the remainder of the shot. Care must be taken to insure that the floor tiles are properly conditioned, since putting the outer strike point(s) on unconditioned tiles will result in outgassing that will significantly complicate the analysis. Measurables: Plot of particle exhaust rate and core line-averaged and pedestal density as a function of Rosp.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	For this experiment, the ion grad-B drift is directed toward the divertor. To condition the divertor tiles, we first do a radial scan of the outer strike point of a standard divertor at high beam power: Rosp = 1.2 m at 1.5s, sweep radially to Rosp = 1.40 m by 3.5 s, then sweep back to 1.2 m by 5.5 s. Then repeat the shot. The "data shots" include the following radial strike point locations: Rosp = 1.20 m, 1.25 m, 1.30 m, 1.335 m, and 1.37 m. Each shot includes both a standard divertor phase and a Snowflake-minus phase. Two shots at each Rosp location are taken, in order to establish that the tiles under the OSP are indeed properly conditioned and not evolving significant deuterium gas during the shot.
Background:	NSTX-U plans to make the Snowflake divertor an important part of their upcoming program. Their planning for operating with SF includes the capability to actively exhaust particles. Modeling has been done with SOLPS to try to simulate a cryo-pump capability similar to the DIII-D approach, but their is no hard data anywhere in the world for benchmarking the modeling. According to V. Soukhanovskii, this issue of adequately being able to pump Snowflakes is very high on the priority list for NSTX-U. DIII-D has the capability (a) to run a Snowflake configuration compatible with pumping and (b) to obtain the relevant pumping data that would be very helpful in NSTX-U planning.
Resource Requirements:	0.5 day experiment, 6 co-beam sources, some shaping preparation for maintaining SF(-) at slightly non-standard Rosp near the plenum entrance.
Diagnostic Requirements:	Asdex gauge in lower divertor plenum, IR camera (preferably the "periscope" system), filterscopes, bolometer, core Thomson scattering, and Langmuir probes (floor).
Analysis Requirements:	SOLPS
Other Requirements:	--