1 Edge Characterization Experiment in High Performance (highly shaped) Plasmas R. J. Maqueda (Nova Photonics) R. Maingi (ORNL) V. Soukhanovskii (LLNL) J.-W. Ahn (UCSD) NSTX Edge Team PPPL, March 25 th 2008 Focus on NHTX-like shape with “lowest” heat flux (i.e., high flux expansion). Study divertor heat flux and radiation (detachment) as function of: - injected power - density - divertor flux expansion Use discharges to study blob generation mechanism. Best possible diagnostic coverage essential

2 LSN instead High flux expansion is key to management of plasma wall interactions in NHTX NHTX R. Maingi, NSTX 2008 Research Forum Modeling of NHTX by J. Canik shows that variations in geometry strongly affect heat flux and divertor parameters (n e, T e, etc). Goal: perform detailed edge characterization of boundary of high performance, highly shaped plasmas (“NHTX”), e.g. - Does divertor footprint correlate one-to-one with flux expansion? - Does divertor radiated power correlate with input power? Proposal: Group-wide XP for detailed characterization of high performance boundary plasma. - Effort similar to XP-434 (2004) at  ~2 and  ~0.45. Lots of new diagnostics: 30 point Thomson, better IR camera, midplane and divertor imaging, divertor bolometer, more 1-D CCD cameras, etc.

3 Generation of blobs in outboard midplane during H-mode In NSTX the edge turbulence and blob activity increases with pedestal height. I SOL above 1-  (a.u) Pedestal n e (10 13 cm -3 ) GPI diagnostic -H-mode turbulence and blobs present a continuum from a turbulence level just above that measurable to that approaching L- mode level. -The level of intermittent edge activity (blobs) shown by SOL D  light increases with pedestal n e (and P e ). Hot topic: two invited talks at 2007 APS-DPP (experimental by Furno, theoretical by Krasheninnikov). NOTE:Piggy-back discharges, many things changed in addition to pedestal characteristics. Perform power scan in LSN H-modes, from Ohmic to 6 MW of NBI. Keep other parameters “fixed”, avoid ELMs and other modes BONUS: Correlate midplane GPI with divertor turbulence/blobs.

V. A. Soukhanovskii, XP 816 Review, 25 March 2008, Princeton, NJ 4 Divertor poloidal flux expansion scan will be accomplished using PF1AL and PF1B coils  Divertor poloidal flux expansion: f m =(B  /B tot ) MP / (B  /B tot ) OSP  Use divertor coils PF1A and PB1B to change f m  Used ISOLVER (courtesy of D. Gates, J. Menard) to model equilibria for various PF1A and PF1B current trends  Obtained prescription: Baseline shot , PF1AL only, flux expansion f m = (22 at s) Keep PF1AU same (11.3 kA) or decrease Decrease PF1AL by 2-4 kA, add 2-4 kA to PF1B Configuration retains same X-point height, triangularity and elongation, but reduces f m by up to (see example on next page) PF1A PB1B

V. A. Soukhanovskii, XP 816 Review, 25 March 2008, Princeton, NJ 5 Divertor poloidal flux expansion scan will be accomplished using PF1AL anf PF1B coils no PB1BI PB1B = 3 kA

6 Experiment plan – 1 day H-mode discharges w/base case: LSN, 0.9 MA, 5.0 kG OSP at 40 – 45m  ~2.2,  ~0.7 f exp ~22 (model ) 1)Fine power scan (  P NBI =1 MW): from 0 MW (Ohmic) to 6 MW (repeat high power shots – GPI timing) 12 shots Decision point: if H-mode access use 2 MW preheat, othewise: #NBI (MW)ObjectiveABC 12.0Power scan (2 MW preheat) #NBI (MW)ObjectiveABC 1’1’ 4.0Power scan (4 MW preheat) Time P NBI (MW) L-H transition 40 ms

7 Second case is then either: Continue with power scan: Notes: - Maximum 2 attempts for Ohmic H-mode. 2)Fuelling/density scan at fixed power (4 MW): “increased” and “decreased” relative to base 4 shots #NBI (MW)ObjectiveABC 24.0Power scan (if 2 MW preheat) ’2’ 2.0Power scan (if 4 MW preheat) #NBI (MW)ObjectiveABC 36.0Power scan 2.0 4Ohmic Power scan (no repeat) (condition src. C down to 1 MW) Power scan (no repeat) Power scan Power scan #NBI (MW)ObjectiveABC 84.0 Density scan: reduced density (lower HFS puff flow) Density scan: increased density (increased HFS puff) Experiment plan – 1 day (cont.)

8 3)Scan flux expansion at lower divertor at fixed power (4 MW) by raising the x-point: 2 lower values compared to base 4 shots 4)Time permitting: run more repetition shots and more flux expansion values. Total:20 shots Experiment plan – 1 day (cont.) #NBI (MW)ObjectiveABC Flux expansion scan: reduced by increasing PF1B and decreasing PF1AL (2-3 kA each) Flux expansion scan: reduced further by another increase/decrease in PF1B/PF1AL #NBI (MW)ObjectiveABC Flux expansion scan: increased, use PF1B only (X-point limiter) ** Obtain more shot repetitions for shots above ***