Status of NSTX/DIII-D/MAST aspect ratio core confinement comparison studies M. Peng, for E.J. Synakowski For the ITPA Transport Physics Working Group Kyota, Japan April , 2005

Primary participants: E. Synakowski (PPPL), C. Petty (GA), M. Valovic (MAST). Ran a DIII-D leg of an NSTX/DIII- D/MAST core confinement similarity experiment Starting point: an NSTX LSN H mode with  ~ %, good confinement, high J BS and long pulse. Well- documented w.r.t. transport (excellent ions, lossy electrons). Shape accessible on DIII-D. Focus on two times in the plasma with differing stored energies. A challenge in developing the proposal: what do we match when we run on DIII-D? Single time slice of NSTX target discharge

The NSTX target discharge had fairly constant T e and T i, and time-evolving n e I p = 0.8 MA B T = 0.5 T P NBI = 6 MW E NBI = keV  T = 18% W = 0.25 MJ Density profile broadens during H-mode

Initially, followed a similar (scaled) path in beam timing & I p ramp. Hope for H mode with small/no ELMs, as in NSTX case. Obtained strong ELMs on DIII-D. Their impact to be determined. Density ramp came for free. Matched shape very nicely with  = 2.0,  = 0.4, but slightly smaller minor radius than on NSTX (“shelf” inside DIII-D vessel is an obstruction). The plan on the DIII-D run day was as follows:

3 plasma conditions are being pursued Match  * pol and  p, and a simplifed * definition –I p, n e, T e, T i similar to NSTX values, but with increased B T (1.2 T) to get similar average q. I p ~ 820 kA –Result of first DIII-D day: did well in bracketing NSTX cases of interest. Fix  * pol and  p, and match usual * definition –D’less scaling arguments ==>> B T near 2 T, Ip ~ 1.3 MA. –Together with first condition, yields a matched pair with * varied by the ratio of the aspect ratios –Result for first DIII-D day: got the lower stored energy case matched Fix  * tor and  p, usual *: of highest interest to MAST –D’less scaling arguments ==>> B T near 0.6 T, Ip ~ 0.4 MA. –With first condition, yields a matched pair with  * varied by the ratio of the aspect ratios –Result: Not obtained on first day, got a better match on a recent DIII-D 2 hour run. Preferred case to match by MAST team.

NSTX shape was very well matched DIII-D minor radius slightly smaller than NSTX Scaled NSTX (reduced ~ 5%), actual DIII-D

A surprise: between NSTX and DIII-D, where density and stored energy (properly scaled) were matched, temperatures did not match –Z eff and fast ion confinement possible players. NSTX next-step may include intentially dirtier plasmas, lowered beam energy to reduce fast ion stored energy –Archeology of TRANSPed DIII-D plasmas may reveal decent matches with existing data at some time points –As we went to the NSTX shape, we got different edge stability properties I.e. small ELMs on NSTX, more vigorous ELMs on DIII- D. To be addressed is how to treat these differences. Next step on NSTX is an iteration from the DIII-D run