Confinement, Transport and Turbulence Properties of NSTX Plasmas

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 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 S.M. Kaye*, R.E. Bell, B.P. LeBlanc, D. Mikkelsen, H. Park, G. Rewoldt, W. Wang (PPPL), D. Stutman, K. Tritz (JHU), F. Levinton, H. Yuh (Nova Photonics) *PPPL, Princeton University, Princeton, NJ EPS Meeting July 2-6, 2007 Warsaw, Poland 1

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 Poster Will Focus on Confinement & Transport Trends in NSTX and Their Underlying Processes • Key confinement and transport dependences established (BT, Ip, b, n*, q(r),…) • Dedicated scans have isolated 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 transport studies just beginning • Momentum and ion transport decoupled in NSTX 3

4. 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 4

5. 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 (3 cm) 5

6. 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 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) Electrons anomalous Ions near neoclassical Neoclassical Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local) 9

10. 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) 10

11. Change in High-k Scattering Spectra with r/a and BT Consistent with Variations of ce • Outboard edge measurement showsdecreaseof fluctuations at higher BT (consistent withlowerce at higher BT) • Core measurement showsincreaseof fluctuations at higher BT (consistent withhigherceat higher BT) [see Park et al., P2.045] 11

12. 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 (see Wong, this conf) Calculated high-k-driven turbulent transport heat fluxes outside RS region consistent with inferred level (D. Mikkelsen) F. Levinton, APS 2006 12

13. Dimensionless Parameter Scans Have Addressed High-Priority ITPA Issues – Beta Dependence of Confinement 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 13

14. 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 • ELM severity increased with increasing power • Weak increase of b with power • Type III ELMs severely degrade confinement 14

15. 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 15

16. Momentum Confinement Studies • Low BT (0.35-0.55 T) operation leads to values of wExB up to the MHz range • These ExB shear values can exceed ITG/TEM growth rates by factors of 5 to 10 • Statistical studies indicate cf does not scale with ci in NSTX Due to suppression of ITG modes? 16

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

18. Core Momentum Diffusivities Up to An Order of Magnitude Lower Than Thermal Diffusivities Does cf scale with ce? What is cf,neo? (0.1-0.4 m2/s by very simple estimate) 18

19. R~1.32 m R~1.15 m Dedicated Perturbative Momentum Confinement Experiments Recently Carried Out • Use non-resonant n=3 magnetic perturbations to damp plasma rotation; turn nRMP off for plasma spin-up • Use plasma spin-up dynamics to determine tf, cf, vpinch 19

20. Instantaneous and Perturbative Determination of tf During Spin-up Agree • Use dL/dt = T – L/tf relation to determine instantaneous tf • Model spin-up to determine perturbative tf using L(t) = tf* [T – (T-L0/tf) * exp(-t/tf)], where L = Angular momentum T = Torque L0 = Angular momentum at time of nRMP turn-off 20

21. Detailed Energy and Momentum Confinement Studies in NSTX Are Underway • 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 • Need also to consider lower-k modes in some cases (ITG/TEM) • No degradation of BtE with bT in strongly-shaped plasmas • Degradation seen in more weakly-shaped plasmas • Degradation invariably tied to change in ELM severity • Momentum transport studies are just beginning • 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 • Possibly due to suppression of ITG modes in NSTX, and neoclassical transport controlling both ion heat and momentum diffusion 21