Results from the Power Threshold Fueling Location and NSTX/MAST Identity Experiments

1. Research Supported by Results from the Power Threshold Fueling Location and NSTX/MAST Identity Experiments Rajesh Maingi Oak Ridge National Laboratory Hendrik Meyer UKAEA Fusion And the NSTX and MAST Teams NSTX Results Review Sept. 20-21, 2004 Princeton, NJ Page 1

2. NSTX and MAST save similar plasma size but MAST vacuum vessel larger and outer walls further away Vpl/VVV ~ 35-40% NSTX Vpl/VVV ~ 6-10% MAST Passive stabilizing plates Page 2

3. Good progress made on first detailed MAST/NSTX identity experiment Goals: • compare L-H power threshold in same CDND (double-null with drsep < 3mm) shape with same Ip, Bt, density, inner-wall midplane fueling location, etc. • determine if PLH lowest with drsep=0 (go to +/- 10 mm) Results: • CDND NBI power very similar near LH transition (~300-350 kW); need to compute PLOSS • LDND (DN with drsep ~ 10 mm, downward) had higher PLH > 1.7 MW • NSTX shape under rtEFIT was a little bigger Page 3

4. drsep or dsep defined as distance between separatrices mapped to outer midplane Courtesy of H. Meyer, EPS 2002 Page 4

5. Dithery H-mode in CDND near PLH in MAST 0.5 0.0 1.0 0.0 0.4 0.0 0.0 Ip [MA] ---- 8842 ---- 8851 ---- 8848 ---- 8852 PNBI [MW] PLH ne [1019 m-3] Type III ELMs Da[au] dithers 0.0 0.1 Time[s] 0.2 0.3 Page 5

6. NSTX dithers not as ‘periodic’ as MAST (CDND near PLH) “firm” L-H “dithers” Page 6

7. NSTX #111378 277msec (NBI 700 kW) MAST #8851 200msec (NBI 580 kW) NSTX #111378 343msec MAST #8851 300msec Z (m) Z (m) R (m) R (m) CDND Shapes were reasonably well matched, although NSTX shape (under rtEFIT) was a little larger Double-null Drsep<1mm (“CDND”) NSTX vac. ves. Double-null Drsep<1mm (“CDND”) NSTX vac. ves. Page 7

8. NSTX ne profile more peaked and Te profile broader early R [m] R [m] R [m] yN yN yN Page 8

9. NSTX ne (Te) lower(higher) later in H-mode phase R [m] R [m] R [m] yN yN yN Page 9

10. Clear H-mode Transition in LDND near PLH in MAST 0.4 0.0 2.0 0.0 0.6 0.0 0.0 ne [1019 m-3] PLH PNBI [MW] Ip [MA] ---- 8856 ---- 8857 ---- 8859 Da[au] L-H (ELM-free) L-H (ELMy) 0.0 0.1 Time[s] 0.2 0.3 0.4 Page 10

11. No transition observed in LDND in NSTX (too low density or MHD at NBI turn-on?) (Reconstructed) (no H-mode) Sawteeth Page 11

12. NSTX #111383 391msec (NBI 1.1 MW) MAST #8859 280 msec (NBI 1.4 MW) NSTX #111381 301msec NSTX #111380 301msec Z(m) Z(m) R(m) R(m) NSTX LDND shape a little larger than MASTMagnetic balance (drsep) scan possible with rtEFIT Single-null Drsep~10mm (“LDND”) NSTX vac. ves. Single-null Drsep~+/- 3mm (“LDND”,”UDND”) NSTX vac. ves. Page 12

13. NSTX never achieved H-mode in LDND, but maybe needed a little more power or higher density? R [m] R [m] R [m] yN yN yN Page 13

14. Summary and Future Work • First joint NSTX/MAST experiment was a good first step • CDND power threshold was remarkably similar in terms of NBI power -> wall proximity does not play a big role in these conditions?! • ne profiles in these low Ip H-modes peaked like ohmic H-modes • Higher power CDND discharge had type IIII ELMs in MAST but largely ELM-free (?) in NSTX • Need more LDND data from NSTX (w/edge rotation data…) Page 14

15. Summary of Power Threshold Dependence on Fueling Location Motivation: • Some gas fueling always required for H-mode access • Test a neoclassical theory formulation which predicted that high-field side (i.e. center stack) gas fueling could lead to a lower power threshold than low-field side (i.e. outer) fueling Results: • Power threshold and Er not markedly different Complications: • Early NBI heating needed to avoid sawteeth but affects discharge evolution and power threshold • Power threshold was slowly increasing during run day (do long L-mode phases de-condition the wall w/small outer gap?) • Early H-mode experiment removed H-mode gas fueling requirement Page 15

16. Backup Page 16