Scaling of Electron and Ion Transport in the High-Power Spherical Torus NSTX

Kaye, S. M.; Bell, R. E.; Gates, D.; LeBlanc, B.P.; Levinton, F. M.; Ménard, J.; Mueller, D.; Rewoldt, G.; Sabbagh, S.A.; Wang, W.; Yuh, H.

doi:10.1103/physrevlett.98.175002

Cited by 72 publications

(63 citation statements)

References 9 publications

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“…Ion thermal transport in spherical tokamaks (STs) is routinely down to neoclassical levels in the presence of sufficient E Â B shear, [1][2][3][4] which occurs in strongly beam heated plasmas. However, there remains a great demand to understand the cause of anomalous electron thermal transport and its influence on confinement as it will influence the design of next generation devices.…”

Section: Introductionmentioning

confidence: 99%

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

et al. 2012

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Linear gyrokinetic simulations are performed based on a high collisionality NSTX discharge that is part of dimensionless confinement scaling studies. In this discharge, the microtearing mode is predicted to be unstable over a significant region of the plasma (r=a ¼ 0.5-0.8), motivating comprehensive tests to verify the nature of the mode and how it scales with physical parameters. The mode is found to be destabilized with sufficient electron temperature gradient, collisionality, and beta, consistent with previous findings and simple theoretical expectations. Consistent with early slab theories, growth rates peak at a finite ratio of electron-ion collision frequency over mode frequency, e=i =x $ 1-6. Below this peak, the mode growth rate decreases with reduced collisionality, qualitatively consistent with global confinement observations. Also, in this region, increased effective ionic charge (Z eff ) is found to be destabilizing. Experimental electron beta and temperature gradients are two to three times larger than the inferred linear thresholds. Increasing magnetic shear (s) and decreasing safety factor (q) are both destabilizing for ratios around the experimental values s=q ¼ 0.6-1.3. Both the Z eff and s=q scaling are opposite to those expected for the ETG instability offering an opportunity to experimentally distinguish the two modes. Finally, we note that the kinetic ballooning mode is found to compete with the microtearing mode at outer locations r=a ! 0.8. [

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Section: Introductionmentioning

confidence: 99%

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

et al. 2012

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“…Dedicated dimensionless scaling studies in both NSTX [36][37][38] and Mega-Ampere Spherical Tokamak (MAST) 39,40 find that the normalized energy confinement time exhibits a strong inverse scaling with collisionality, ðXs E Þ $ Ãe Àð0:8À0:95Þ . This scaling predicts improved confinement at smaller values of * envisioned for next generation STs.…”

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confidence: 99%

Simulation of microtearing turbulence in national spherical torus experiment

et al. 2012

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Thermal energy confinement times in National Spherical Torus Experiment (NSTX) dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future spherical tokamak (ST) devices at much lower collisionality. Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport ($98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E Â B flows as experimental values of E Â B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. Refinements in numerical resolution and physics model assumptions are expected to minimize the apparent discrepancy. In cases where the predicted transport is strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the magnetic perturbations associated with the unique structure of microtearing turbulence. V C 2012 American Institute of Physics. [http://dx.

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“…7 implies a confinement time that, for fixed ρ * and β, should scale similar to those deduced from dedicated dimensionless scaling experiments in both NSTX [36][37][38] and MAST [39,40], (Ω i τ E )…”

Section: Transport Parametric Dependenciesmentioning

confidence: 58%

“…The Eulerian gyrokinetic code GYRO [50][51][52][53] is used for both linear and non-linear simulations which are based on a high collisionality NSTX discharge that is part of energy confinement scaling studies [36,37] toroidal beta β tor =19%). Many of the properties of the linear microtearing mode and scaling for this case are described in [11].…”

Section: Experimental Parameters and Linear Analysismentioning

confidence: 99%

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Simulation Of Microtearing Turbulence In NSTX

Guttenfelder¹,

Kaye²,

Nevins³

et al. 2012

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Thermal energy confinement times in NSTX dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future ST devices at much lower collisionality.Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport (~98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E×B flows as experimental values of E×B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. Refinements in numerical resolution and physics model assumptions are expected to minimize the apparent discrepancy. In cases where the predicted transport is strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the magnetic perturbations associated with the unique structure of microtearing turbulence.

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Scaling of Electron and Ion Transport in the High-Power Spherical Torus NSTX

Cited by 72 publications

References 9 publications

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

Simulation of microtearing turbulence in national spherical torus experiment

Simulation Of Microtearing Turbulence In NSTX

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