Does the Center Stack Shoulder Gas Injector Allow H-mode Access with Improved Density Control?

1. Does the Center Stack Shoulder Gas Injector Allow H-mode Access with Improved Density Control? R. Maingi, D. Mueller ISD Group Meeting Dec. 5, 2003 Princeton, NJ

2. ne [1019m-3] #108728 0.710 sec 0.460 sec 0.260 sec SNBI Sgas dNe/dt 0.227 sec Uncontrolled (non-disruptive) density rise in long pulse H-modes • Edge density builds and diffuses in • Time dependence of density rise similar to center stack midplane injector decay time #108728 Ip [MA] PNBI/10 [MW] ne [1019 m-3] Da [au] [torr-l/s] WMHD*10 [MJ] H98pby2

3. Gas Injectors Graphite tiles ParametersDesignAchieved Major Radius 0.85m Minor Radius 0.67m Plasma Current 1MA1.5MA Toroidal Field 0.6T0.6T Heating and Current Drive NBI (100keV) 5MW7 MW RF (30MHz) 6MW6 MW Wall Conditioning: ~350 deg. bakeout of graphite tiles Regular boronization (~3 weeks) Helium Glow between discharges Center stack gas injection Passive stabilizing plates NSTX Explores Low Aspect Ratio (A=R/a) physics regime }A ≥ 1.27

4. Summary of Center Stack Top and Lower Dome Observations • Center stack injector has a smaller initial dump and shorter e-folding decay time of flow rate • Center stack top injector enabled H-mode access in double-null but not lower single-null (gas trapped in DN?) • Lower dome dumps gas in very quickly • Lower dome did not enable H-mode access, but prevented locked/tearing mode reconnection in L-mode discharge when timing was ‘optimized’ (lucky)

5. Load-and-Dump Gas Injectors Have Different Flow Characteristics and Delay Time

6. CS Mid LFS CS Top L-H Center-stack Midplane Gas Injector Fueling Has Lower PL-H Than LFS and CS Top in LSN shape • Experiment run • when L-H • power threshold • was ~ 1-2 NBI src • (start of run 2003) • Note LFS shot and • CS tophad no L-H • -> higher PL-H R~1.48m

7. CS mid, LSN 109866@0.2s LFS top, LSN 109875@0.201s CS top, LSN 109869@0.2s X-point+LFS top, LSN 109884@0.2s CS top, USN 109877@0.2s CS top, DN 109885@0.2s Unfiltered Recycling Light Pattern Changes with Gas Injector Location and Plasma Shape LSN=lower single null; USN=upper single null; DN=double null

8. CS Mid LFS CS Top CS shoulder injector enabled H-mode access and reasonable performance in double-null configuration • LH time delayed with CS shoulder (wall conditions different or plasma physics?)

9. Proposed Run Plan (1 day) Goal: direct comparison of H-mode access and density rise in DN configuration for shoulder and midplane fueling • Establish H-mode in DND with CS fueling midplane; scan Pplenum from 600-1500 torr in 3-4 steps (start with #109828) • Switch to CS shoulder fueling; scan Pplenum from 300-800 torr and optimize timing of gas start time Goal: direct comparison of efficacy of shoulder injector in LSN and USN vs. DN configurations • Start with DN above and lowest fueling for H-mode and perform drsep scan (distance between lower and upper X-points mapped to outer midplane), from -2 to +2 cm in 0.5-1 cm increments • Test at lower fueling rate • Repeat at higher fueling rate 1/2 day dividing point

10. Backup

11. L-H Lower dome + LFS Gas Injectors Can Match CS Midplane Flow Rate But Do Not Allow H-mode Access Ldome Ldome LFS CS Mid Ldome+LFS Ldome+LFS • Experiment run • when L-H • power threshold • was ~ 1-2 NBI src • (start of run 2003) R~1.48m