Confinement, Transport and Turbulence Properties of NSTX PlasmasD. 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, 2007Fukuoka, JapanCulham Sci CtrU St. AndrewsYork UChubu UFukui UHiroshima UHyogo UKyoto UKyushu UKyushu Tokai UNIFSNiigata UU TokyoJAERIIoffe InstRRC Kurchatov InstTRINITIKBSIKAISTENEA, FrascatiCEA, CadaracheIPP, JlichIPP, GarchingASCR, Czech RepU QuebecCollege W&MColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

Recent Confinement & Transport TopicsKey confinement and transport dependences established (BT, Ip, b, n*, q(r),)Dedicated scans have isolated sources of Ip and BT dependencesTheory/simulations have indicated ETG and microtearing modes could be important in controlling electron transportLocalized turbulence characteristics being assessed across wide range of k (upper ITG/TEM to ETG)

Dimensionless parameter scans in bTStudied effect of plasma shaping on b-degradation of confinement

Momentum and ion heat transport are decoupled in NSTX

New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes12 channel MSE [NOVA Photonics]LRDFITReconstructionRmag51-point CHERS30-point MPTSTangential microwave scattering measureslocalized 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)

Important for equilibrium and microinstability calculations

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 NSTX20% variation in re, ne*ne*-scan at fixed q - Factor of >3 variation in ne* - Strong increase of confinement with decreasing collisionalityk=2.1d=0.6

Power/Beta Scan Was Repeated at Lower k, d to Test Effect of Shape on Confinement Dependencek=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.

Confinement Exhibits Strong Degradation with bT in Weakly-Shaped PlasmasStronger degradation of tE,th than bT,th-0.35 cannot be ruled outStrong degradation of tE (tE ~ bT-1.0)

Dedicated H-mode Confinement Scaling Experiments Have Isolated the BT and Ip DependencesScans carriedout at constantdensity, injectedpower (4 MW)0.50 s0.50 s

Dedicated H-mode Confinement Scaling ExperimentsHave Revealed Some Surprises Strong dependence of tE on BTtE,98y,2 ~ BT0.15H98y,2 ~ 0.9 1.1 1.44 MWtE,98y,2 ~ Ip0.93H98y,2 ~ 1.4 1.3 1.1 Weaker dependence on IptE~Ip1.3-1.5 at fixed q tE,98y,2~Ip1.1 at fixed qNSTX tE exhibits strong scaling at fixed q4 MW

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

High Rotation (M ~0.5) and Rotational Shear is CommonLow 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 -instabilitiesSteady-state and perturbative momentum confinement studies on NSTX have started

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, q0 = 1.2) ce predicted by microtearing mode theory is within a factor of 2 of inferred experimental valuesWong, et al., Phys. Rev. Letters, 99 (2007) 135003ce(m2/s)01020 Both ETG and microtearing may contribute to anomalous electron transport

Theory/Gyrokinetic Calculations Suggest ETG May Play an Important Role in Determining Electron Transport at Low BTNon-linear simulations indicate formation of radial streamers (up to 200re): FLR-modified fluid code [Horton et al., PoP 2005]Good agreement between experimental and theoretical saturated transport level at 0.35 TKim, IFSETG 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 gradient0.35 TGS2TRANSPSaturatedETG levelQe (kW/m2)

Change in High-k Scattering Spectra with r/a and BT Consistent with Variations of ceCore measurement shows increase of fluctuations at higher BT

Also have higher ce at higher BTOutboard edge measurement shows decrease of fluctuations at higher BT

consistent with lower ce at higher BT[see Park et al., EPS 2007, P2.045]

Strongly Reversed Magnetic Shear L-mode Plasmas Achieve Higher Te and Reduced TransportLinear 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)F. Levinton, APS 2006Calculated ETG-driven heat flux outside RS region is consistent with inferred level

ExB shearing rate sets the transport levelETG-ki: reducing ExB shearing rate raises e,potential fluctuation amplitude, /Te, rises,k spectrum downshifts slightly.

ExB shear controls eddy sizeExB 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 rate1/2X1/4X

Steady-State Momentum Transport Determined For BT, Ip ScansNo anomalous pinch necessary to explain steady-state rotation data

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

Momentum Diffusivity NOT Neoclassical Even Though Ion Thermal Diffusivity Is (~)cf,neo

Momentum Confinement Time >> Energy Confinement Time Consistent with local analysis: cf

Perturbative Momentum Transport Studies Using Magnetic Braking Indicate Significant Inward PinchCan determine vpinch only if w, w decoupledAssume cfpert, vpinchpert constant in timeExptl inward pinch generally scales with theoretical estimates based on low-k turbulence-driven pinchvPeeters= cf/R [-4-R/Ln](Coriolis drift)vHahm= cf/R [-3](B, curvature drifts)

cfs-s < cfpert with inward pinchImportant to consider whencomparing cf to ci

SummaryConfinement and transport trends found to differ from those at higher R/aStrong BT, weaker Ip scaling Electron transport variation primarily responsible for BT scalingIons near neoclassical; primarily responsible for Ip scalingUnderstand 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 importantLower k modes important in some cases (micro-tearing & ITG/TEM) No degradation of BtE with bT in strongly-shaped plasmasDegradation is seen in more weakly-shaped plasmasDegradation is invariably tied to change in ELM severityMomentum transport studies have begunSteady-state power balance and perturbative analyses indicate long momentum confinement times (>100 ms), with cf