Confinement and Local Transport in the National Spherical Torus Experiment (NSTX) Stanley M. Kaye 1, M.G. Bell 1, R.E. Bell 1, C. W. Domier 2, W. Horton 3, J. Kim 3, B.P. LeBlanc 1, F. Levinton 4,N.C. Luhmann 2, R. Maingi 5, E. Mazzucato 1, J.E. Menard 1, D.R. Mikkelsen 1, H. Park 1, G. Rewoldt 1, S.A. Sabbagh 6, D. Smith 1, D. Stutman 7, K. Tritz 7, W. Wang 1, H. Yuh 4 21 st IAEA/Fusion 2006 Meeting Oct 16 – 21, 2006 Chengdu, China 1 PPPL, Princeton University, Princeton, NJ, USA University of California, Davis, CA, USA IFS, University of Texas, Austin, TX, USA Nova Photonics Inc., Princeton, NJ, USA, ORNL, Oak Ridge, TN, USA Dept. of Applied Physics, Columbia University, NYC, NY, USA The Johns Hopkins University, Baltimore MD Supported by Office of Science 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 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

SMK – 21 st IAEA 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,  T, (  ,   –Dominant electron heating with NBI: relevant to  -heating in ITER Major Radius R m Aspect Ratio A1.3 Elongation  2.8 Triangularity  0.8 Plasma Current I p 1.5 MA Toroidal Field B T 0.55 T Pulse Length1.5 s NB Heating (100 keV)7 MW  T,tot up to 40% –Excellent laboratory in which to study electron transport: electron transport anomalous, ions close to neoclassical –Large range of  T spanning e-s to e-m turbulence regimes –Strong rotational shear that can influence transport –Localized electron-scale turbulence measurable (  e ~ 0.1 mm)

SMK – 21 st IAEA 3 This Presentation Will Focus on Confinement & Transport Trends in NSTX and Their Underlying Processes Major accomplishments –Key confinement and transport dependences established (B T, I p, , , q(r),…) –High priority ITPA tasks have been addressed Dimensionless parameter scans in  T  e  Established more accurate  (=a/R) scaling with NSTX (& MAST) data included in the ITPA database  98y2 ~I p 0.93 B T 0.15 n e 0.41 P R 1.97  0.58 …  new ~I p 0.73 B T 0.36 n e 0.39 P R 2.14  1.03 (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

SMK – 21 st IAEA 4 Dimensionless Parameter Scans Have Addressed High-Priority ITPA Issues  -scan at fixed q, B T -  -dependence important to ITER advanced scenarios (B  98y2 ~  -0.9 ) - Factor of variation in  T - Degradation of  E with  weak on NSTX 20% variation in  e, e * e  -scan at fixed q - Factor of >3 variation in e * - Strong increase of confinement with decreasing collisionality  =2.1  =0.6

SMK – 21 st IAEA 5 Dedicated H-mode Confinement Scaling Experiments Have Isolated the B T and I p Dependences Scans carried out at constant density, injected power (4 MW) 0.50 s

SMK – 21 st IAEA 6 Dedicated H-mode Confinement Scaling Experiments Have Revealed Some Surprises Strong dependence of  E on B T  E,98y,2 ~ B T 0.15 H 98y,2 ~ 0.9 → 1.1 → MW  E,98y,2 ~ I p 0.93 H 98y,2 ~ 1.4 → 1.3 → 1.1 Weaker dependence on I p  E ~I p at fixed q  E,98y,2 ~I p 1.1 at fixed q NSTX  E exhibits strong scaling at fixed q 4 MW

SMK – 21 st IAEA 7 Variation of Electron Transport Primarily Responsible for B T Scaling Broadening of T e & reduction in  e outside r/a=0.5 with increasing B T Ions near neoclassical Neoclassical

SMK – 21 st IAEA 8 Theory/Gyrokinetic Calculations Suggest ETG May Play an Important Role in Determining Electron Transport at Low B T Non-linear simulations indicate formation of radial streamers (up to 200  e ): FLR-modified fluid code [Horton et al., PoP 2005] Good agreement between experimental and theoretical saturated transport level at 0.35 T Experimental  e profile consistent with that predicted by e-m ETG theory [Horton et al., NF 2004] at 0.35 T Not at higher B T Kim, IFS ETG linearly unstable only at lowest B T T: R/L Te 20% above critical gradient , 0.55 T: R/L Te 20-30% below critical gradient 0.35 T GS2

SMK – 21 st IAEA 9 Ion Transport Primarily Governs I p Scaling - Ions Near Neoclassical Level -  i,GTC-NEO (r/a= ) GTC-Neo neoclassical: includes finite banana width effects (non- local)

SMK – 21 st IAEA 10 Turbulence Measurements + Gyrokinetic Calculations Have Helped Identify Possible Sources of Transport Ion transport during H-phase is neoclassical Microwave scattering system measures reduced fluctuations (n/n) in both upper ITG/TEM and ETG ranges during H-mode ~ Electron transport reduced, but remains anomalous Ion and electron transport change going from L- to H-modes - Localized measurement (axis to edge) - Excellent radial resolution (6 cm) ELMs

SMK – 21 st IAEA 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 Experimental  e profile consistent with that predicted by e-s ETG theory (Horton et al, Phys. Plasmas [2004]) Non-linear GTC results indicate ITG modes stable during H-phase;  i ~ neoclassical

SMK – 21 st IAEA 12 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 B T, weaker I p scaling Electron transport variation primarily responsible for B T scaling Ions near neoclassical; primarily responsible for I p scaling Understand the source of the difference in confinement trends at different R/a (low vs high-k turbulence dominant at different R/a, B T ?) Data provided to ITPA H-mode database for R/a and  T scalings –No degradation of B  E with  T n/n decreases from L- to H-phase for k r =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) ~

SMK – 21 st IAEA 13 Backup Vugraphs

SMK – 21 st IAEA 14 New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes 12 channel MSE [NOVA Photonics] LRDFIT Reconstruction R mag 51-point CHERS20-point MPTS Tangential microwave scattering measures localized electron-scale turbulence k r =2 (upper ITG/TEM) to ~24 (ETG) cm -1  e ~0.01 cm  r ~ 6 cm  k ~ 1 cm -1 Can vary location of scattering volume (near R mag to near edge) Important for equilibrium and microinstability calculations

SMK – 21 st IAEA 15 New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes Tangential micorwave scattering measures localized electron-scale turbulence 12 channel MSE [NOVA Photonics] LRDFIT Reconstruction Important for equilibrium and microinstability calculations k r =2 (upper ITG/TEM) to ~24 (ETG) cm -1  e ~0.01 cm  r ~ 6 cm  k ~ 1 cm -1 Can vary location of scattering volume (near R mag to near edge)

SMK – 21 st IAEA 16 Dimensionless Variable Scans Have Addressed High Priority ITPA Physics Issues (  - scaling)  -scan at fixed  e, e * -  -dependence important to ITER advanced scenarios (B  98y2 ~  -0.9 ) - Degradation of  E with  weak on NSTX NSTX data used in conjunction with ITPA data to establish  (=a/R  scaling with more confidence ITER98PB(y,2) scaling does not represent low R/a data well  98y2 ~ I p 0.93 B T 0.15 n e 0.41 P R 1.97  0.58 …  new ~I p 0.73 B T 0.36 n e 0.39 P R 2.14  1.03 (Kaye et al., PPCF 48 [2006] A429) variation in  T 20% variation in  e, e *

SMK – 21 st IAEA 17 Stronger Reversed Magnetic Shear Is Associated with Reduced Transport Weak vs 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

SMK – 21 st IAEA 18 Pellet Perturbations Are Being Used to Probe Relation of Critical Gradient Physics to q-Profile H-mode with monotonic q-profile exhibits stiff profile behavior → T e close to marginal stability Reversed magnetic shear L-mode responds to pellet perturbation over several ms Soft X-ray array diagnoses fast  T e R/L Te t=297→301 ms R/L Te t=440→444 ms Stutman, JHU