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Confinement and Local Transport in the National Spherical Torus Experiment (NSTX)

This presentation focuses on the confinement and transport trends in the National Spherical Torus Experiment (NSTX) and their underlying processes. It discusses major accomplishments, key dependences, dimensionless parameter scans, and the role of electron and ion transport. Experimental and theoretical studies are used to understand these processes.

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Confinement and Local Transport in the National Spherical Torus Experiment (NSTX)

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  1. Office of Science Supported by Confinement and Local Transport in the National Spherical Torus Experiment (NSTX) College W&M Colorado Sch Mines 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 Stanley M. Kaye1, M.G. Bell1, R.E. Bell1, C. W. Domier2, W. Horton3, J. Kim3, B.P. LeBlanc1, F. Levinton4,N.C. Luhmann2, R. Maingi5, E. Mazzucato1, J.E. Menard1, D.R. Mikkelsen1, H. Park1, G. Rewoldt1, S.A. Sabbagh6, D. Smith1, D. Stutman7, K. Tritz7, W. Wang1, H. Yuh4 21st IAEA/Fusion 2006 Meeting Oct 16 – 21, 2006 Chengdu, China 1PPPL, Princeton University, Princeton, NJ, USA 08543 2 University of California, Davis, CA, USA 95616 3IFS, University of Texas, Austin, TX, USA 78712 4Nova Photonics Inc., Princeton, NJ, USA, 08540 5ORNL, Oak Ridge, TN, USA 37831 6Dept. of Applied Physics, Columbia University, NYC, NY, USA 10027 7The Johns Hopkins University, Baltimore MD 21218 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 Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec

  2. NSTX Addresses Transport & Turbulence Issues Critical to Both Basic Toroidal Confinement and Future Devices • NSTX offers a novel view into plasma T&T properties • NSTX operates in a unique part of dimensionless parameter space: R/a, bT, (r*,n*) • Dominant electron heating with NBI: relevant to a-heating in ITER • Excellent laboratory in which to study electron transport: electron transport anomalous, ions close to neoclassical • Large range of bT spanning e-s to e-m turbulence regimes • Strong rotational shear that can influence transport • Localized electron-scale turbulence measurable (re ~ 0.1 mm) Major Radius R0 0.85 m Aspect Ratio A 1.3 Elongation k 2.8 Triangularity d 0.8 Plasma Current Ip 1.5 MA Toroidal Field BT 0.55 T Pulse Length 1.5 s NB Heating (100 keV) 7 MW bT,tot up to 40% 2

  3. This Presentation Will Focus on Confinement & Transport Trends in NSTX and Their Underlying Processes • Major accomplishments • Key confinement and transport dependences established (BT, Ip, b, n*, q(r),…) • High priority ITPA tasks have been addressed • Dimensionless parameter scans in bT, ne* • Established more accurate e (=a/R) scaling with NSTX (& MAST) data included in the ITPA database t98y2~Ip0.93 BT0.15 ne0.41 P-0.69 R1.97 e0.58… tnew~Ip0.73 BT0.36 ne0.39 P-0.62 R2.14 e1.03(Kaye et al., PPCF 48 [2006] A429) • Localized turbulence characteristics being assessed across wide range of k (upper ITG/TEM to ETG) • Theory/simulations have indicated ETG modes could be important in controlling electron transport 3

  4. Dimensionless Parameter Scans Have Addressed High-Priority ITPA Issues 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. 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 5

  6. 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 6

  7. Variation of Electron Transport Primarily Responsible for BT Scaling Broadening of Te & reduction in ce outside r/a=0.5 with increasing BT Ions near neoclassical Neoclassical 7

  8. 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 • Good agreement between experimental and theoretical saturated transport level at 0.35 T • Experimental ce profile consistent with that predicted by e-m ETG theory [Horton et al., NF 2004] at 0.35 T • Not at higher BT 8

  9. Ion Transport Primarily Governs Ip Scaling- Ions Near Neoclassical Level - GTC-Neo neoclassical: includes finite banana width effects (non-local) ci,GTC-NEO (r/a=0.5-0.8) 9

  10. Microwave scattering system measures reduced fluctuations (n/n) in both upper ITG/TEM and ETG ranges during H-mode ~ ELMs Turbulence Measurements + Gyrokinetic Calculations Have Helped Identify Possible Sources of Transport Ion and electron transport change going from L- to H-modes Electron transport reduced, but remains anomalous Ion transport during H-phase is neoclassical - Localized measurement (axis to edge) - Excellent radial resolution (6 cm) 10

  11. Theory/Gyrokinetic Calculations Indicate Both ITG/TEM and ETG are Possible Candidates for Electron Transport GS2 calculations indicate lower linear growth rates at all wavenumbers during H- than during L-phase:ETG unstable Non-linear GTC results indicate ITG modes stable during H-phase; ci ~ neoclassical Experimental ce profile consistent with that predicted by e-s ETG theory (Horton et al, Phys. Plasmas [2004]) 11

  12. ~ • n/n decreases from L- to H-phase for kr=2 to 24 cm-1 (upper ITG/TEM to ETG range) • – Associated with reduction in transport • Linear and non-linear theory have indicated ETG modes could be important • –Need also to consider lower-k modes • (microtearing, ITG/TEM) NSTX Plays a Key Role in Multi-Scale Transport & Turbulence Research • 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?) • Data provided to ITPA H-mode database for R/a and bT scalings • No degradation of BtE with bT 12

  13. Backup Vugraphs 13

  14. New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes 12 channel MSE [NOVA Photonics] 51-point CHERS 20-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) 14

  15. LRDFIT Reconstruction New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes Tangential micorwave scattering measures localized electron-scale turbulence 12 channel MSE[NOVA Photonics] • 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) Important for equilibrium and microinstability calculations 15

  16. Dimensionless Variable Scans Have Addressed HighPriority ITPA Physics Issues (e, b - scaling) b-scan at fixed re, ne* - b-dependence important to ITER advanced scenarios (Bt98y2~b-0.9) - Degradation of tE with b weak on NSTX ITER98PB(y,2) scaling does not represent low R/a data well 20% variation in re, ne* NSTX data used in conjunction with ITPA data to establish e(=a/R) scaling with more confidence t98y2~Ip0.93BT0.15ne0.41P-0.69R1.97e0.58… tnew~Ip0.73BT0.36ne0.39P-0.62R2.14e1.03 (Kaye et al., PPCF 48 [2006] A429) 2-2.5 variation in bT 16

  17. Stronger Reversed Magnetic Shear Is Associated with Reduced Transport Weakvs Reverse-Shear L-mode Global non-linear GTC and GYRO simulations show that a pure ITG mode is unstable without ExB flow shear included TEM & ETG calculations underway 17

  18. Pellet Perturbations Are Being Used to Probe Relation of Critical Gradient Physics to q-Profile Soft X-ray array diagnoses fast DTe R/LTe t=440→444 ms H-mode with monotonic q-profile exhibits stiff profile behavior → Te close to marginal stability R/LTe t=297→301 ms Reversed magnetic shear L-mode responds to pellet perturbation over several ms Stutman, JHU 18

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