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Confinement, Transport and Turbulence Properties of NSTX Plasmas

Office of Science. Supported by. Confinement, Transport and Turbulence Properties of NSTX Plasmas. College W&M Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc.

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Confinement, Transport and Turbulence Properties of NSTX Plasmas

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  1. Office of Science Supported by Confinement, Transport and Turbulence Properties of NSTX Plasmas College W&M Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin D. R. Mikkelsen, S.M. Kaye, R.E. Bell, B.P. LeBlanc, H. Park, G. Rewoldt, W. Wang (PPPL*), D. Stutman, K. Tritz (JHU), F. Levinton, H. Yuh (Nova Photonics) *PPPL, Princeton University, Princeton, NJ International Spherical Torus Workshop October 10-12, 2007 Fukuoka, Japan Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAERI Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec 1

  2. Recent Confinement & Transport Topics • Key confinement and transport dependences established (BT, Ip, b, n*, q(r),…) • Dedicated scans have isolated sources of Ip and BT dependences • Theory/simulations have indicated ETG and microtearing modes could be important in controlling electron transport • Localized turbulence characteristics being assessed across wide range of k (upper ITG/TEM to ETG) • Dimensionless parameter scans in bT • Studied effect of plasma shaping on b-degradation of confinement • Momentum and ion heat transport are decoupled in NSTX 2

  3. New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes 12 channel MSE [NOVA Photonics] 51-point CHERS 30-point MPTS LRDFIT Reconstruction Rmag Important for equilibrium and microinstability calculations Tangential microwave scattering measures localized electron-scale turbulence • kr=2 (upper ITG/TEM) to ~24 (ETG) cm-1 • re ~0.01 cm • Dr ~ 6 cm • Dk ~ 1 cm-1 • Can vary location of scattering volume • (near Rmag to near edge) 3

  4. High-Priority ITPA Dimensionless Parameter Scans b-scan at fixed q, BT - b-dependence important to ITER advanced scenarios (Bt98y2~b-0.9) - Factor of 2-2.5 variation in bT - Degradation of tE with b weak on NSTX ne*-scan at fixed q - Factor of >3 variation in ne* -Strong increase of confinement with decreasing collisionality 20% variation in re, ne* k=2.1 d=0.6 4

  5. Power/Beta Scan Was Repeated at Lower k, d to Test Effect of Shape on Confinement Dependence • k=1.8-1.9, d~0.4; ne*, re vary ≤ 20% across scan. • Type III ELM severity increased with heating power. • Strong degradation, beta increases very weakly. • Higher shaping: type V ELMs, little degradation. 5

  6. Confinement Exhibits Strong Degradation with bT in Weakly-Shaped Plasmas • Strong degradation of tE (tE ~ bT-1.0) • Stronger degradation of tE,th than bT,th-0.35 cannot be ruled out 6

  7. Dedicated H-mode Confinement Scaling Experiments Have Isolated the BT and Ip Dependences Scans carried out at constant density, injected power (4 MW) 0.50 s 0.50 s 7

  8. Dedicated H-mode Confinement Scaling ExperimentsHave Revealed Some Surprises Strong dependence of tE on BT Weaker dependence on Ip H98y,2 ~ 0.9 → 1.1 → 1.4 H98y,2 ~ 1.4 → 1.3 → 1.1 4 MW 4 MW tE,98y,2 ~ BT0.15 tE,98y,2 ~ Ip0.93 NSTX tE exhibits strong scaling at fixed q tE~Ip1.3-1.5 at fixed q tE,98y,2~Ip1.1 at fixed q 8

  9. Local Transport Studies Reveal Sources of Energy Confinement Trends Electrons primarily responsible for strong BT scaling in NSTX (tE~BT0.9) Variation in near-neoclassical ion transport primarily responsible for Ip scaling (tE~Ip0.4) Kaye, et al., PRL 98 (2007) 175002 Electrons anomalous Ions near neoclassical Neoclassical Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local) 9

  10. High Rotation (M ~0.5) and Rotational Shear is Common • Low BT (0.35-0.55 T) wExB ~ MHz range • ExB shear values can exceed ITG/TEM growth rates by 5 to 10 X • Opens the door for other -instabilities Steady-state and perturbative momentum confinement studies on NSTX have started 10

  11. Low-k microtearing modes may be important in driving electron transport in some NSTX plasmas • ce predicted by microtearing mode theory is within a factor of 2 of inferred experimental values • Linear GS2 calculations indicate microtearing modes are unstable in NSTX “hybrid” discharge (monotonic q, q0 = 1.2) 20 ce (m2/s) 10 0 Wong, et al., Phys. Rev. Letters, 99 (2007) 135003 • Both ETG and microtearing may contribute to anomalous electron transport 11

  12. Theory/Gyrokinetic Calculations Suggest ETG May Play an Important Role in Determining Electron Transport at Low BT ETG linearly unstable only at lowest BT - 0.35 T: R/LTe 20% above critical gradient - 0.45, 0.55 T: R/LTe 20-30% below critical gradient Non-linear simulations indicate formation of radial streamers (up to 200re):FLR-modified fluid code [Horton et al., PoP 2005] GS2 0.35 T Kim, IFS Saturated ETG level TRANSP Good agreement between experimental and theoretical saturated transport level at 0.35 T Qe (kW/m2) 12

  13. Change in High-k Scattering Spectra with r/a and BT Consistent with Variations of ce Outboard edge measurement shows decreaseof fluctuations at higher BT consistent withlowerce at higher BT Core measurement shows increaseof fluctuations at higher BT Also havehigherceat higher BT [see Park et al., EPS 2007, P2.045] 13

  14. Strongly Reversed Magnetic Shear L-mode Plasmas Achieve Higher Te and Reduced Transport Linear GS2 calculations indicate reduced region of mtearing instability for RS plasma (r/a=0.3) ETG stable in this region in both plasmas (Wong, EPS ‘07) Calculated ETG-driven heat flux outside RS region is consistent with inferred level F. Levinton, APS 2006 14

  15. ExB shearing rate sets the transport level ETG-ki: reducing ExB shearing rate raises e, potential fluctuation amplitude, /Te, rises, k spectrum downshifts slightly. 15

  16. ExB shear controls eddy size ExB shearing rate varied from 2X to 1/4 experimental rate. Eddies grow longer (and wider) as shearing rate is reduced. Extent of radial domain is ~400 e. 2X 1/2X 1/4X experimental ExB rate 16

  17. Steady-State Momentum Transport Determined For BT, Ip Scans No anomalous pinch necessary to explain steady-state rotation data 17

  18. Core Momentum Diffusivities are Up to An Order of Magnitude Lower Than Thermal Diffusivities cf is not related to ci, so does cf scale with ce? cf,neo is too small to matter) 18

  19. Momentum Diffusivity NOT Neoclassical Even Though Ion Thermal Diffusivity Is (~) cf,neo << cf cf, neo can be negative! 19

  20. Momentum Confinement Time >> Energy Confinement Time Consistent with local analysis: cf<<ci • Use dL/dt = T – L/tf relation to determine instantaneous tf • Model post-braking spin-up to determine perturbative tf using L(t) = tf* [T – (T-L0/tf) * exp(-t/tf)], where L = Angular momentum T = Torque (NB torque only) L0 = Angular momentum at time of nRMP turn-off Steady-state tE ~ 50 ms 20

  21. Perturbative Momentum Transport Studies Using Magnetic Braking Indicate Significant Inward Pinch • Can determine vpinch only if w, w decoupled • Assume cfpert, vpinchpert constant in time • Expt’l inward pinch generally scales with theoretical estimates based on low-k turbulence-driven pinch vPeeters= cf/R [-4-R/Ln] (Coriolis drift) vHahm= cf/R [-3] (B, curvature drifts) cfs-s < cfpert with inward pinch Important to consider when comparing cf to ci 21

  22. Summary • Confinement and transport trends found to differ from those at higher R/a • Strong BT, weaker Ip scaling • Electron transport variation primarily responsible for BT scaling • Ions near neoclassical; primarily responsible for Ip scaling • Understand the source of the difference in confinement trends at different R/a (low vs high-k turbulence dominant at different R/a, BT?) • Theory indicates that high-k ETG modes could be important • Lower k modes important in some cases (micro-tearing & ITG/TEM) • No degradation of BtE with bT in strongly-shaped plasmas • Degradation is seen in more weakly-shaped plasmas • Degradation is invariably tied to change in ELM severity • Momentum transport studies have begun • Steady-state power balance and perturbative analyses indicate long momentum confinement times (>100 ms), with cf<<ci • Momentum diffusivity does not scale with ion thermal diffusivity • Inferred vpinch cf, magnitude is not inconsistent with theory predictions 22

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