Temperature gradient driven electron transport in NSTX and Tore Supra

Horton, W.; Wong, H. V.; Morrison, Philip; Wurm, Alexander; Kim, Juhyung; Perez, Jean C.; Pratt, J.; Hoang, G. T.; LeBlanc, B.P.; Ball, Rowena

doi:10.1088/0029-5515/45/8/025

Cited by 25 publications

(28 citation statements)

References 24 publications

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“…26 The electrostatic potential in the saturated state is plotted in Fig. 13 for r/a=0.55, and the formation of streamer-like structures with a radial extent of up to 200ρ e (∼ 2 cm) is clearly seen.…”

Section: Theory Calculations and Turbulence Measurementsmentioning

confidence: 97%

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

et al. 2007

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NSTX operates at low aspect ratio (R/a∼1.3) and high beta (up to 40%), allowing tests of global confinement and local transport properties that have been established from higher aspect ratio devices. NSTX plasmas are heated by up to 7 MW of deuterium neutral beams with preferential electron heating as expected for ITER. Confinement scaling studies indicate a strong B T dependence, with a current dependence that is weaker than that observed at higher aspect ratio. Dimensionless scaling experiments indicate a strong increase of confinement with decreasing collisionality and a weak degradation with beta. The increase of confinement with B T is due to reduced transport in the electron channel, while the improvement with plasma current is due to reduced transport in the ion channel related to the decrease in the neoclassical transport level. Improved electron confinement has been observed in plasmas with strong reversed magnetic shear, showing the existence of an electron internal transport barrier (eITB). The development of the eITB may be associated with a reduction in the growth of microtearing modes in the plasma core. Perturbative studies show that while L-mode plasmas with reversed magnetic shear and an eITB exhibit slow changes of L T e across the profile after the pellet injection, H-mode plasmas with a monotonic q-profile and no eITB show no change in this parameter after pellet injection, indicating the existence of a critical gradient that may be related to the q-profile. Both linear and non-linear simulations indicate the potential importance of ETG modes at the lowest B T . Localized measurements of high-k fluctuations exhibit a sharp decrease in signal amplitude levels across the L-H transition, associated with a decrease in both ion and electron transport, and a decrease in calculated linear microinstability growth rates across a wide k-range, from the ITG/TEM regime up to the ETG regime.

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Section: Theory Calculations and Turbulence Measurementsmentioning

confidence: 97%

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

et al. 2007

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“…Understanding electron thermal transport and the causes giving rise to the anomalous levels experimentally observed is a key requirement in order to achieve a predictive capability for the confinement of future fusion devices such as ITER or FNSF (Fusion Nuclear Science Facility). 4 Ample theory and numerical simulations [5][6][7][8][9][10][11][12] have suggested that micro instabilities driven by the electron temperature gradient (ETG) could be responsible for anomalous electron thermal transport levels observed experimentally. The ETG toroidal instability has a characteristic scale length on the order of the electron gyro radius (k ?…”

Section: Introductionmentioning

confidence: 99%

Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment

et al. 2015

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Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k ? q e Շ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed in NSTX with a high-k microwave scattering system [D. R. Smith et al., Rev. Sci. Instrum. 79, 123501 (2008)]. Here we report on stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, which is shown to be consistent with linear stabilization of ETG modes due to the density gradient using the analytical ETG linear threshold in F. Jenko et al. [Phys. Plasmas 8, 4096 (2001)] and linear gyrokinetic simulations with GS2 [M. Kotschenreuther et al., Comput. Phys. Commun. 88, 128 (1995)]. We also found that the observed power of electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient, suggesting density gradient as a nonlinear stabilizing mechanism. Higher density gradients give rise to lower values of the plasma frame frequency, calculated based on the Doppler shift of the measured density fluctuations. Linear gyrokinetic simulations show that higher values of the electron density gradient reduce the value of the real frequency, in agreement with experimental observation. Nonlinear electron-scale gyrokinetic simulations show that high electron density gradient reduces electron heat flux and stiffness, and increases the ETG nonlinear threshold, consistent with experimental observations. V C 2015 AIP Publishing LLC.

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“…In addition, the low toroidal field leads to a large electron gyroradius which facilitates measuring turbulence wavenumber spectrum. Indeed, in a previous study an electromagnetic ETG transport model has been applied to an NSTX RF-heated L mode plasma and good agreement with measurements have been found [12], although there was no measurement of electron-scale turbulence. In order to make direct measurements, a novel 280 GHz microwave scattering system (the high-k scattering system) was implemented on NSTX [13], and this diagnostic has provided first experimental evidence of ETG turbulence in NSTX RF-heated L mode plasmas [14]: It was found that measured electron-scale turbulence is driven by the electron temperature gradient, and the observed critical gradient for linear instability is shown to be consistent with linear gyro-kinetic calculations for the ETG mode.…”

Section: Introductionmentioning

confidence: 93%

Experimental Study of Parametric Dependence of Electron-scale Turbulence in a Spherical Tokamak

Ren¹,

Kaye²,

Mazzucato³

et al. 2012

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Electron-scale turbulence is predicted to drive anomalous electron thermal transport. However, experimental study of its relation with transport is still in its early stage. On the National Spherical Tokamak eXperiment (NSTX), electron-scale density fluctuations are studied with a novel tangential microwave scattering system with high radial resolution of ±2 cm. Here, we report a study of parametric dependence of electron-scale turbulence in NSTX H-mode plasmas. The dependence on density gradient is studied through the observation of a large density gradient variation in the core induced by an ELM event, where we found the first clear experimental evidence of density gradient stabilization of electron-gyro scale turbulence in a fusion plasma. This observation, coupled with linear gyro-kinetic calculations, leads to the identification of the observed instability as toroidal Electron Temperature Gradient (ETG) modes. It is observed that longer wavelength ETG modes, k ⊥ ρ s 10 (ρ s is the ion gyroradius at electron temperature and k ⊥ is the wavenumber perpendicular to local equilibrium magnetic field), are most stabilized by density gradient, and the stabilization is accompanied by about a factor of two decrease in electron thermal diffusivity.Comparisons with nonlinear ETG gyrokinetic simulations shows ETG turbulence may be able to explain the experimental electron heat flux observed before the ELM event. The collisionality dependence of electron-scale turbulence is also studied by systematically varying plasma current and toroidal field, so that electron gyroradius (ρ e ), electron beta (β e ) and safety factor (q 95 ) are kept approximately constant. More than a factor of two change in electron collisionality, ν * e , was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as ν * e is decreased in this collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision frequency, ν e/i . Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG linear growth rate and electron thermal transport more than ν e/i does. Furthermore, electron heat flux predicted by the simulations is found to have an order-of-magnitude spatial variation in the experimental measurement region and is also found to be much smaller than experimental levels except at one radial location we evaluated. The predicted electron heat flux is shown to be strongly anti-correlated with density gradient which varies for a factor of three in the measurement region, which is in agreement with the density gradient dependence study reported in this paper.

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Temperature gradient driven electron transport in NSTX and Tore Supra

Cited by 25 publications

References 24 publications

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

Confinement and local transport in the National Spherical Torus Experiment (NSTX)

Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment

Experimental Study of Parametric Dependence of Electron-scale Turbulence in a Spherical Tokamak

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