Effect of Gas Fueling Location on H-mode Access in NSTX
Authors: R. Maingi, M. Bell, R. Bell, T. Biewer, C. Bush, C.S. Chang, D. Gates, S. Kaye, H. Kugel, B. LeBlanc, R. Maqueda, J. Menard, D. Mueller, R. Raman, S. Sabbagh, V. Soukhanovskii, and the NSTX Team
Date of PPPL Report: October 2003
Date of Revised PPPL Report: April 2004
Presented at: the Ninth IAEA Technical Meeting on H-mode Physics and Transport Barriers, 24-26 September 2003, San Diego, California. Proceedings to be published in a Special Issue of Plasma Physics and Controlled Fusion.
The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in NSTX. We find that gas fueling from the center stack midplane area produces the most reproducible H-mode access with generally the lowest L-H threshold power in lower single-null configuration. The edge toroidal rotation velocity of C2+ is largest (in direction of the plasma current) just before the L-H transition with center stack midplane fueling, and then reverses direction after the L-H transition. Simulation of these results with a guiding center Monte Carlo neoclassical transport code (XGC) is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fueling from either the center stack midplane or center stack top locations, indicating a reduced sensitivity of Hmode access on fueling location in that shape.