DRM ISTW ‘07 1 Confinement, Transport and Turbulence Properties of NSTX Plasmas 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 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 Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec 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

DRM ISTW ‘07 2 Recent Confinement & Transport Topics Key confinement and transport dependences established (B T, I p, , , q(r),…) –Dedicated scans have isolated sources of I p and B T 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  T –Studied effect of plasma shaping on  -degradation of confinement Momentum and ion heat transport are decoupled in NSTX

DRM ISTW ‘07 3 New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes 12 channel MSE [NOVA Photonics] LRDFIT Reconstruction R mag 51-point CHERS30-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

DRM ISTW ‘07 4 High-Priority ITPA Dimensionless Parameter Scans  -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

DRM ISTW ‘07 5 Power/Beta Scan Was Repeated at Lower  to Test Effect of Shape on Confinement Dependence  = ,  ~0.4; e ,  e 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.

DRM ISTW ‘07 6 Confinement Exhibits Strong Degradation with  T in Weakly-Shaped Plasmas Stronger degradation of  E,th than  T,th cannot be ruled out Strong degradation of  E (  E ~  T -1.0 )

DRM ISTW ‘07 7 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

DRM ISTW ‘07 8 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

DRM ISTW ‘07 9 Local Transport Studies Reveal Sources of Energy Confinement Trends Electrons primarily responsible for strong B T scaling in NSTX (  E ~B T 0.9 ) Electrons anomalousIons near neoclassical Variation in near-neoclassical ion transport primarily responsible for I p scaling (  E ~I p 0.4 ) Neoclassical Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local) Kaye, et al., PRL 98 (2007)

DRM ISTW ‘07 10 High Rotation (M ~0.5) and Rotational Shear is Common Low B T ( T)  ExB ~ 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

DRM ISTW ‘07 11 Low-k microtearing modes may be important in driving electron transport in some NSTX plasmas Linear GS2 calculations indicate microtearing modes are unstable in NSTX “hybrid” discharge (monotonic q, q 0 = 1.2)  e predicted by microtearing mode theory is within a factor of 2 of inferred experimental values Wong, et al., Phys. Rev. Letters, 99 (2007)  e (m 2 /s) Both ETG and microtearing may contribute to anomalous electron transport

DRM ISTW ‘07 12 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 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 TRANSP Saturated ETG level Q e (kW/m 2 )

DRM ISTW ‘07 13 Change in High-k Scattering Spectra with r/a and B T Consistent with Variations of  e Core measurement shows increase of fluctuations at higher B T Also have higher  e at higher B T Outboard edge measurement shows decrease of fluctuations at higher B T consistent with lower  e at higher B T [see Park et al., EPS 2007, P2.045]

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

DRM ISTW ‘07 15 ExB shearing rate sets the transport level ETG-ki: reducing ExB shearing rate raises  e, potential fluctuation amplitude,  /T e, rises, k  spectrum downshifts slightly.

DRM ISTW ‘07 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 experimental ExB rate 1/2X 1/4X

DRM ISTW ‘07 17 Steady-State Momentum Transport Determined For B T, I p Scans No anomalous pinch necessary to explain steady-state rotation data

DRM ISTW ‘07 18 Core Momentum Diffusivities are Up to An Order of Magnitude Lower Than Thermal Diffusivities   is not related to  i, so does   scale with  e ?  ,neo is too small to matter)

DRM ISTW ‘07 19 Momentum Diffusivity NOT Neoclassical Even Though Ion Thermal Diffusivity Is (~)  ,neo <<    , neo can be negative!

DRM ISTW ‘07 20 Momentum Confinement Time >> Energy Confinement Time Consistent with local analysis:   <<  i Use dL/dt = T – L/   relation to determine instantaneous   Model post-braking spin-up to determine perturbative   using L(t) =   * [T – (T-L 0 /   ) * exp(-t/   )], where L = Angular momentum T = Torque (NB torque only) L 0 = Angular momentum at time of nRMP turn-off Steady-state  E ~ 50 ms

DRM ISTW ‘07 21 Perturbative Momentum Transport Studies Using Magnetic Braking Indicate Significant Inward Pinch Can determine v pinch only if ,  decoupled Assume   pert,  pinch pert constant in time Expt’l inward pinch generally scales with theoretical estimates based on low-k turbulence-driven pinch v Peeters =   /R [-4-R/L n ] (Coriolis drift) v Hahm =   /R [-3] (  B, curvature drifts)   s-s <   pert with inward pinch Important to consider when comparing   to  i

DRM ISTW ‘07 22 Summary 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 ?) –Theory indicates that high-k ETG modes could be important Lower k modes important in some cases (micro-tearing & ITG/TEM) No degradation of B  E with  T 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   <<  i –Momentum diffusivity does not scale with ion thermal diffusivity –Inferred v pinch   , magnitude is not inconsistent with theory predictions