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Suppressing electron turbulence and triggering internal transport barriers with reversed magnetic shear in the National Spherical Torus Experimenta)
a)Paper YI3 3, Bull. Am. Phys. Soc. 56, 279 (2011).
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10.1063/1.4718456
/content/aip/journal/pop/19/5/10.1063/1.4718456
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/5/10.1063/1.4718456
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Profiles of temperature, safety factor, and magnetic shear for an RF-heated NSTX e-ITB, shot 129354 at 232 ms (solid). The electron temperature, safety factor, and magnetic shear for a non e-ITB RF NSTX plasma, shot 124948 at 300 ms, are shown for comparison (dashed).

Image of FIG. 2.
FIG. 2.

Linear critical electron temperature gradients, for the parameters of NSTX 129354 during an e-ITB, testing the effects of and ion model. ETG-ki uses gyrokinetic ions, and ETG-ai uses the adiabatic ion approximation. The boxes represent possible and mostly likely experimental values of and within the e-ITB, where .

Image of FIG. 3.
FIG. 3.

Radial profiles of experimental and ETG linear critical gradients, NSTX 129354, using Eq. (3) to estimate the linear critical gradients. For reference, the critical gradient formula for positive shear large aspect ratio tokamaks,4 Eq. (1), is also plotted where . The lower bounds on the critical gradients represent , the dashed profiles , and the upper bounds .

Image of FIG. 4.
FIG. 4.

Electron heat flux as a function of driving gradient for and . The experimental temperature gradient lengths and electron heat fluxes are, respectively, given by and .

Image of FIG. 5.
FIG. 5.

Electron heat flux as a function of driving gradient and magnetic shear for .

Image of FIG. 6.
FIG. 6.

Critical gradients as a function of magnetic shear, . ETG becomes linearly unstable at gradients above (solid with square). Above (dashed with circles), turbulent thermal diffusivities exceed . The dotted line with diamonds represents the nonlinear upshift of the critical gradient, . For comparison, the original cyclone ITG test case found an upshift in the critical gradient for transport that extended by 2, from to at .24

Image of FIG. 7.
FIG. 7.

Poloidal cross sections of saturated density fluctuations below and above for . The flux-surface widths have been enhanced by a factor of four for visual clarity.

Image of FIG. 8.
FIG. 8.

Density fluctuations as a function of poloidal angle and , showing off-midplane peaking at .

Image of FIG. 9.
FIG. 9.

Time averaged heat flux spectra for different values of z = 21.8, and the spectrum of the fastest growing linear mode.

Image of FIG. 10.
FIG. 10.

Plasma parameters and simulation domain used in the NSTX 129354 global simulations.

Image of FIG. 11.
FIG. 11.

Cross-section of electron density fluctuations, 129354 global simulation, showing an electron internal transport barrier. . Although peak amplitudes of exist, for clarity only are shown.

Image of FIG. 12.
FIG. 12.

Time evolution of radial heat flux profile, 129354 global simulation, . The white line marks the location of minimum . Heat fluxes as calculated by transp increase as one moves radially from to .

Image of FIG. 13.
FIG. 13.

Time-averaged heat flux profiles for global simulations with different values of . For reference, experimentally inferred levels range from to .

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2012-05-17
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Suppressing electron turbulence and triggering internal transport barriers with reversed magnetic shear in the National Spherical Torus Experimenta)
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/5/10.1063/1.4718456
10.1063/1.4718456
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