113 research outputs found
An Enhanced Nonlinear Critical Gradient for Electron Turbulent Transport due to Reversed Magnetic Shear
The first nonlinear gyrokinetic simulations of electron internal transport
barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed
magnetic shear can suppress thermal transport by increasing the nonlinear
critical gradient for electron-temperature-gradient-driven turbulence to three
times its linear critical value. An interesting feature of this turbulence is
nonlinearly driven off-midplane radial streamers. This work reinforces the
experimental observation that magnetic shear is likely an effective way of
triggering and sustaining e-ITBs in magnetic fusion devices.Comment: 4 pages, 5 figure
An Enhanced Nonlinear Critical Gradient for Electron Turbulent Transport due to Reversed Magnetic Shear
The first nonlinear gyrokinetic simulations of electron internal transport
barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed
magnetic shear can suppress thermal transport by increasing the nonlinear
critical gradient for electron-temperature-gradient-driven turbulence to three
times its linear critical value. An interesting feature of this turbulence is
nonlinearly driven off-midplane radial streamers. This work reinforces the
experimental observation that magnetic shear is likely an effective way of
triggering and sustaining e-ITBs in magnetic fusion devices.Comment: 4 pages, 5 figure
Simulating Gyrokinetic Microinstabilities in Stellarator Geometry with GS2
The nonlinear gyrokinetic code GS2 has been extended to treat
non-axisymmetric stellarator geometry. Electromagnetic perturbations and
multiple trapped particle regions are allowed. Here, linear, collisionless,
electrostatic simulations of the quasi-axisymmetric, three-field period
National Compact Stellarator Experiment (NCSX) design QAS3-C82 have been
successfully benchmarked against the eigenvalue code FULL. Quantitatively, the
linear stability calculations of GS2 and FULL agree to within ~10%.Comment: Submitted to Physics of Plasmas. 9 pages, 14 figure
Microtearing instabilities and electron thermal transport in low and high collisionality NSTX discharges
Microtearing mode (MTM) real frequency, growth rate, magnetic fluctuation amplitude, and resulting electron thermal transport are studied insystematic NSTX scans of relevant plasma parameters. The dependency of the MTM real frequency and growth rate on plasma parameters,suitable for low and high collision NSTX discharges, is obtained by using the reduced MTM transport model [T. Rafiq et al., Phys. Plasmas 23,062507 (2016)]. The plasma parameter dependencies are compared and found to be consistent with the results obtained from MTM using thegyrokinetic GYRO code. The scaling trend of collision frequency and plasma beta is found to be consistent with the global energy confinementtrend observed in the NSTX experiment. The strength of the magnetic fluctuation is found to be consistent with the gyrokinetic estimate. In earlierstudies, it was found that the version of the multi-mode (MM) anomalous transport model, which did not contain the effect of MTMs, providedan appropriate description of the electron temperature profiles in standard tokamak discharges and not in spherical tokamaks. When the MMmodel, which involves transport associated with MTMs, is incorporated in the TRANSP code and is used in the study of electron thermal transportin NSTX discharges, it is observed that the agreement with the experimental electron temperature profile is substantially improved
Micro-tearing modes in spherical and conventional tokamaks
The onset and characteristics of Micro-Tearing Modes (MTM) in the core of spherical (NSTX) and conventional tokamaks (ASDEX Upgrade and JET) are studied through local linear gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Report GA-A26818 In all these plasmas, finite collisionality is needed for MTMs to become unstable and the electron temperature gradient is found to be the fundamental drive. However, a significant difference is observed in the dependence of linear growth rate of MTMs on electron temperature gradient. While it varies weakly and non-monotonically in JET and ASDEX Upgrade plasmas, in NSTX it increases with the electron temperature gradient
Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment
Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, kâ„Ïe âČ 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.United States. Department of Energy (ontract No. DE-AC02- 09CH11466)United States. Department of Energy. Office of Science (Contract No. DE-AC02-05CH11231
Collisionality and safety factor scalings of H-mode energy transport in the MAST spherical tokamak
A factor of 4 dimensionless collisionality scan of H-mode plasmas in MAST shows that the thermal energy confinement time scales as BÏE,th â Îœ-0.82±0.1*e. Local heat transport is dominated by electrons and is consistent with the global scaling. The n
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