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 byOffice ofScience

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAERIIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

College W&MColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

DRM ISTW ‘07 2

Recent Confinement & Transport Topics

• Key confinement and transport dependences established (BT, Ip, , , q(r),…)

– Dedicated scans have isolated sources of 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 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]

LRDFITReconstruction

Rmag

51-point CHERS 30-point MPTS

Tangential microwave scattering measureslocalized electron-scale turbulence

• kr=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 Rmag to near edge)

Important for equilibrium and microinstability calculations

DRM ISTW ‘07 4

High-Priority ITPA Dimensionless Parameter Scans

-scan at fixed q, BT

- -dependence important to ITER advanced scenarios (B98y2~-0.9) - Factor of 2-2.5 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

=1.8-1.9, ~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

-0.35 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 BT and Ip Dependences

Scans carriedout at constantdensity, injectedpower (4 MW)

0.50 s 0.50 s

DRM ISTW ‘07 8

Dedicated H-mode Confinement Scaling ExperimentsHave Revealed Some Surprises

Strong dependence of E on BT

E,98y,2 ~ BT0.15

H98y,2 ~ 0.9 → 1.1 → 1.4

4 MW

E,98y,2 ~ Ip0.93

H98y,2 ~ 1.4 → 1.3 → 1.1

Weaker dependence on Ip

E~Ip1.3-1.5 at fixed q E,98y,2~Ip

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 forstrong BT scaling in NSTX (E~BT

0.9)

Electrons anomalous Ions near neoclassical

Variation in near-neoclassical ion transport primarily responsible for Ip scaling (E~Ip

0.4)

Neoclassical

Neoclassical levels determined from GTC-Neo: includes finite banana width effects (non-local)

Kaye, et al., PRL 98 (2007) 175002

DRM ISTW ‘07 10

High Rotation (M ~0.5) and Rotational Shear is Common

• Low BT (0.35-0.55 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, q0 = 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) 135003

e

(m2/s)

0

10

20

• 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 BT

Non-linear simulations indicate formation of radial streamers (up to 200e): 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 BT

- 0.35 T: R/LTe 20% above critical gradient - 0.45, 0.55 T: R/LTe 20-30% below critical gradient

0.35 T

GS2

TRANSPTRANSP

SaturatedSaturatedETG levelETG level

Qe (

kW/m

2)

DRM ISTW ‘07 13

Change in High-k Scattering Spectra with r/a and BT Consistent with Variations of e

Core measurement shows increase of fluctuations at higher BT

Also have higher e at higher BT

Outboard edge measurement shows decrease of fluctuations at higher BT

consistent with lower e at higher BT

[see Park et al., EPS 2007, P2.045]

DRM ISTW ‘07 14

Strongly Reversed Magnetic Shear L-mode Plasmas Achieve Higher Te 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, /Te, 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 BT, Ip 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-L0/) * exp(-t/)], whereL = Angular momentumT = Torque (NB torque only)

L0 = 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 vpinch 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

vPeeters= /R [-4-R/Ln](Coriolis drift)

vHahm= /R [-3](B, curvature drifts)

s-s <

pert with inward pinchImportant 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 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• Lower k modes important in some cases (micro-tearing & ITG/TEM)

• No degradation of BE 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 vpinch , magnitude is not inconsistent with theory predictions