24A07
Recent Progress in Transport and Turbulence Research at NSTX
Walter Guttenfelder1, Yang Ren1, Howard Yuh2, Stanley Kaye1, J. Luc Peterson1, David Smith3,and the NSTX Team1

Princeton Plasma Physics Laboratory, USA2
Nova Photonics, Inc., USA
3
University of Wisconsin, USAWe discuss recent findings in the understanding of anomalous electron thermal transport: first nonlinear
gyrokinetic simulations of micro-tearing turbulence which predict experimentally relevant level of electron
thermal transport; the parametric dependence of high-k turbulence measured by a microwave scatteringdiagnostic and its implication on electron thermal transport; suppression of Electron Temperature Gradient(ETG) driven turbulence correlated with the electron internal transport barrier (eITB) formation in reversed
shear L-modes. In addition, first low-k turbulence measurements from a newly implemented BeamEmission Spectroscopy (BES) diagnostic will be presented.

1. Introduction
The National Spherical Torus eXperiment(NSTX) provides a unique laboratory for studying
plasma turbulence and its relation to transport due toits high plasma beta, low aspect ratio and large E×Bflow shear. A comprehensive understanding of thisrelation requires not only measurements of bothlarge and small wavenumbers, but also detailed
comparisons with nonlinear gyrokinetic simulations.
Recent developments in diagnostics and numericalsimulations at NSTX enable us to gain further
insight to understanding turbulent plasma transport.
In this overview, we present recent progress intransport and turbulence research at NSTX,
focusing on thermal-gradient driven core transport.
2. Nonlinear microtearing simulationsFirst non-linear gyrokinetic simulations ofmicrotearing mode turbulence have recently beenreported [1] for a high-collisionality NSTX H-mode
discharge [2]. Using the Eulerian gyrokinetic code
GYRO [3] with finite collisionality (νei=1.26 cs/a,Zeff=2.9) and electromagnetic perturbations (δA||,βe=8πneTe/B02= 8.8%), it was shown for NSTX at
r/a=0.6 (where only microtearing modes are linearlyunstable) that the magnetic “flutter” from the strong
δBr fluctuations is responsible for ~98% of the total
electron thermal transport, χe,sim≈1.2 ρscs2/a=6 m2/s,
which is in the experimental range χe,exp=5-8 m2/s.
Estimates using the spectrum of rms saturated
amplitudes (〈|δBr/B|2〉1/2~0.15%) predict that island
widths should be larger than the minimum
separation in rational surfaces wisland/δrrat ≤8, andPoincare plots confirm global stochasticity
throughout the entire simulation domain [4]. Most
remarkably, Fig. 1 shows that the predictedtransport increases with collisionality, χe,sim~νe1.1,
roughly consistent with global energy confinement
scaling in both NSTX and MAST [5], ΩiτE~ν∗-(0.820.95). This gives strong evidence for the importanceof microtearing modes in determining confinement
scaling in this high-beta NSTX plasma, as no othermicro-instability is predicted to be unstable forthese parameters.
However, including theexperimental value of E×B shear, γE,exp≈0.17 cs/a(comparable to the maximum linear growth rate)
reduces the predicted transport considerably.
Additional simulations are underway to test theinfluence of numerical and physical parametervariations.

Fig. 1. Predicted χe vs. νei from microtearing simulations.

3. ETG stabilization with density gradientWe have recently demonstrated the stabilizationof electron-scale turbulence [6] due to a large
density gradient increase induced by an ELM event.
Fig. 2 shows the normalized k⊥ spectra measured byhigh-k microwave scattering [7] before (498, 515ms) and after (532 ms) a fivefold increase in local

24A07density gradient in the vicinity of r/a=0.6. It is clearthat after the ELM event (532 ms) the fluctuation
power for k⊥ρs≤15 is reduced, most significantly at
smallest wave numbers, k⊥ρs≤10.
10

S/ne2 (arb.)

101010−7t=515 ms
t=498 ms

−8−9Upper bound−10t=532 ms
10−110510k ρ

15electron temperature gradient and magnitude ofnegative magnitude shear for a collection ofdischarges [9].
High-k microwave scattering
measurements are largest in intensity for theweakest shear (s>-0.4) and become smaller orbursty in nature for the strongest negative shear
values (-2.5<s<-0.4), suggesting it is correlated with
reduced transport and corresponding stronger
temperature gradients. GYRO simulations verify
that the ETG instability is suppressed at strong
negative magnetic shear, resulting from both linear
and nonlinear stabilizing effects. This suppression
occurs in the absence of strong E×B shear
confirming that negative magnetic shear alone issufficient for ETG suppression in these e-ITB
discharges.

20⊥ sFig. 2. Measured high-k scattering intensity vs. k⊥ρs,before (498, 515 ms) and after (532 ms) an ELM eventthat causes a large increase in density gradient.

Before the ELM event (498, 515 ms) themeasured Te gradients from Thomson Scattering arelarger than the threshold for the Electron
Temperature Gradient (ETG) instability (determinedfrom GS2 simulations) over most of the high-k
measurement region. However, after the ELM event(532 ms), the threshold Te gradients are much larger
due to the large density gradient (as expected fromtheoretical considerations [8]).
The resulting
stabilization of ETG modes is consistent with thereduced fluctuation intensity in Fig. 2 and thecorresponding reduction in effective thermal
diffusivity [6]. Non-linear ETG simulations appear
to predict this trend at some of the measurement
locations. While the microtearing mode is stable inthis plasma, the trapped electron mode (TEM) isdriven unstable by the large density gradient.
However, the growth rates are weaker than the E×Bshear rates. Additional nonlinear simulations will
test whether TEM turbulence can survive the strong
flow shear and contribute to the electron thermaltransport in this plasma.
4. ETG stabilization with reverse shear
In RF-heated NSTX L-mode plasmas, negative
magnetic shear is found to suppress electron
turbulence and improve electron thermal transport.
Fig. 3 (left) shows an example of a strongly peaked
Te profile indicative of an electron internal transportbarrier (e-ITB). The steep gradient region in the eITB coincides with the region of most negative
magnetic shear (s≈-2, Fig. 3, right). A correlation
has been found between the maximum observed

Fig. 3. (left) Measured Te profiles at different times in areverse shear plasma. (right) Safety factor and magnetic
shear profiles for a reverse shear plasma.

5. First BES measurements
First Beam Emission Spectroscopy (BES)
measurements have recently become available on
NSTX [10]. These measurements have shown
reduction of ion scale turbulence during the L-H
transition from edge to core, and large edge poloidal
correlation lengths.
Acknowledgements
This work was supported by DOE contract DEAC02-09CH11466.
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