# Presentation: 536539

## Confinement, Transport and Turbulence Properties of NSTX Plasmas

**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

***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, J******ü******lich***
***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***

## 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

## New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes

*New Diagnostic Capabilities Have Facilitated Progress in Understanding Transport Processes*

- 12 channel MSE [NOVA Photonics]

- LRDFIT
- Reconstruction

- Rmag

- 51-point CHERS

- 30-point MPTS

- Tangential microwave scattering measures
- localized 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

## High-Priority ITPA Dimensionless Parameter Scans 

***High-Priority ITPA Dimensionless Parameter Scans ***

- -scan at fixed q, BT
-   - -dependence important to ITER
-     advanced scenarios (B98y2~-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

## 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.

## 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)

## 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

## Dedicated H-mode Confinement Scaling ExperimentsHave Revealed Some Surprises

***Dedicated H-mode Confinement Scaling Experiments******Have Revealed Some Surprises***

-      ***Strong dependence of ************E****** on B******T***

- 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 I******p***

- E~Ip1.3-1.5 at fixed q                  E,98y,2~Ip1.1 at fixed q

- NSTX E exhibits strong scaling at fixed q

- 4 MW

## Local Transport Studies Reveal Sources of Energy Confinement Trends

***Local Transport Studies Reveal Sources of Energy Confinement Trends***

- Electrons primarily responsible for
- strong BT scaling in NSTX (E~BT0.9)

- Electrons anomalous

- Ions near neoclassical

- Variation in near-neoclassical ion
- transport primarily responsible
- for _I__p_ scaling (E~Ip0.4)

- Neoclassical

- Neoclassical 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 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***

## 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**
**(m****2****/s)**

- 0

- 10

- 20

* **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 BT

***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 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

- TRANSP

- Saturated
- ETG level

- Qe (kW/m2)

## 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]

## 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

## _x0005_ExB shearing rate sets the transport level

- ETG-ki: reducing ExB shearing rate raises e,
- potential fluctuation amplitude, /Te, rises,
- k spectrum downshifts slightly.

## 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

## Steady-State Momentum Transport Determined For BT, Ip Scans

***No anomalous pinch necessary to explain steady-state rotation data***

## 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)

## Momentum Diffusivity NOT Neoclassical Even Though Ion Thermal Diffusivity Is (~)

- ,neo << 

- , neo can be negative!

## 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/)], where
- 	L = Angular momentum
- 	T = Torque (NB torque only)
- 	L0 = Angular momentum at time of nRMP turn-off

- Steady-state

- E ~ 50 ms

## Perturbative Momentum Transport Studies Using Magnetic Braking Indicate Significant Inward Pinch

- Can determine vpinch only if ,  decoupled
- Assume pert, pinchpert 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 pinch
*Important to consider when*
*comparing ****** to ****i*

## 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, 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 vpinch  , magnitude is not inconsistent with theory predictions