Suppression of electron temperature gradient turbulence via negative magnetic shear in NSTX

Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1?electron gyro-Bohms.     

Experimental demonstration of improved neoclassical transport with quasihelical symmetry

Differences in the electron particle and thermal transport are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The thermal diffusivity is reduced in the QHS configuration, resulting in higher electron temperatures than in the nonsymmetric configuration for a fixed power input. The density profile in QHS plasmas is centrally peaked, and in the nonsymmetric configuration the core density profile is hollow. The hollow profile is due to neoclassical thermodiffusion, which is reduced in the QHS configuration.     

Observation of velocity-independent electron transport in the reversed field pinch

Confinement of runaway electrons has been observed for the first time in a reversed field pinch during improved-confinement plasmas in the Madison Symmetric Torus. Energy-resolved hard-x-ray flux measurements have been used to determine the velocity dependence of the electron diffusion coefficient, utilizing computational solutions of the Fokker-Planck transport equation. With improved-confinement, the fast electron diffusivity drops by 2 orders of magnitude and is independent of velocity. This suggests a change in the transport mechanism away from stochastic magnetic field diffusion.     

Electromagnetic transport from microtearing mode turbulence

This Letter presents nonlinear gyrokinetic simulations of microtearing mode turbulence. The simulations include collisional and electromagnetic effects and use experimental parameters from a high-β discharge in the National Spherical Torus Experiment. The predicted electron thermal transport is comparable to that given by experimental analysis, and it is dominated by the electromagnetic contribution of electrons free-streaming along the resulting stochastic magnetic field line trajectories. Experimental values of flow shear can significantly reduce the predicted transport.     

A Comparative Study of Electron Heat Transport in a Stochastic and a Non‐Stochastic Magnetic Field

We studied the electron heat transport across a non‐local stochastic magnetic field and a non‐local non‐stochastic magnetic field in tokamak plasmas. Analytical results and numerical simulation results were compared with conditions of a stochastic and a non‐stochastic magnetic field respectively in this article. Parameter of stochasticity in perturbed magnetic field was found not tobe a key factor in influencing effective radial heat conductivity in radial direction of tokomak when Chirikov parameter is close to one (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microtearing instabilities and electron transport in the NSTX spherical tokamak

We report a successful quantitative account of the experimentally determined electron thermal conductivity chi(e) in a beam-heated H mode plasma by the magnetic fluctuations from microtearing instabilities. The calculated chi(e) based on existing nonlinear theory agrees with the result from transport analysis of the experimental data. Without using any adjustable parameter, the good agreement spans the entire region where there is a steep electron temperature gradient to drive the instability.     

Quenching of the nonlocal electron heat transport by large external magnetic fields in a laser-produced plasma measured with imaging thomson scattering

We present a direct measurement of the quenching of nonlocal heat transport in a laser-produced plasma by applying large external magnetic fields (>10 T). The temporally resolved Thomson-scattering measurements of the electron temperature profile show that the heat front propagation transverse to a high-power laser beam is slowed resulting in extremely strong local heating. We find agreement with hydrodynamic modeling when including a magnetic field model that self-consistently evolves the fields in the plasma.     

Endogenous magnetic reconnection and associated high energy plasma processes

An endogenous reconnection process involves a driving factor that lays inside the layer where a drastic change of magnetic field topology occurs. A process of this kind is shown to take place when an electron temperature gradient is present in a magnetically confined plasma and the evolving electron temperature fluctuations are anisotropic. The width of the reconnecting layer remains significant even when large macroscopic distances are considered. In view of the fact that there are plasmas in the Universe with considerable electron thermal energy contents this feature can be relied upon in order to produce generation or conversion of magnetic energy, high energy particle populations and momentum and angular momentum transport.     

剖面不变性、反常电子热导及托卡马克装置约束性能分析

本文根据电子温度剖面不变性这一基本实验现象,通过分布量决定的广义对流能流的引入及其与局部量决定的局部反常电子热导关系的分析,在不涉及等离子体整体能量输运物理机制的情况下,给出了托卡马克装置约束性能更可信的描述方式。     

Enhanced heat conductivity in stochastic magnetic field of tokomak affected by the ratio of the parallel to the perpendicular heat diffusivity

Electron heat transport across stochastic magnetic fields is studied numerically in order to find out how the ratio of the parallel to the perpendicular heat diffusivity affects the enhanced heat conductivity and its radial profile in tokomak plasma physics. To find out the details of profile, non-local stochastic magnetic fields, in which the perturbed magnetic islands are separated with each other but very close to in the minor radius of tokomak, are chosen as research objects in our simulation work. Our numerical results indicate that the ratio of the parallel to the perpendicular heat diffusivity is a very important effective factor, which dominate how far the enhanced conductivity contributed by a perturbed magnetic field approach to zero from the rational surface in minor radius. Besides that, a theoretical analysis was provided and compared with the numerical results in this article.     

超热电子在稠密等离子体中输运的混合粒子模拟方法

相对论激光等离子体相互作用以及所产生的带电粒子束在高密度等离子体中的输运行为非常重要.该物理问题的数值模拟研究仍面临技术挑战.本文介绍一种粒子/流体混合模拟方法.该方法中超热电子采用动力学方法描述,背景冷的稠密等离子体采用简化的流体方程描述,适合于超热电子密度远小于背景电子密度,超热电子能量远大于背景电子温度.我们的三维并行混合模拟程序HEETS的模拟结果表明：背景材料的电离和电阻率模型至关重要,将严重影响高能电子输运过程的模拟.     

Instationary Electric Fields,Ion Transport and Strong Density Fluctuations in Tokamaks with Dominant Anomalous Electron Transport

A common explanation is given for ion transport and strong broadband density fluctuations in tokamaks as a result of large anomalous electron transport near dominant magnetic surfaces (resp. in small magnetic islands). The main mechanism is local density flattening connected with an anomalous electron transport induced instationary radial electric field, which forces the ions via polarization drift to follow the electrons. For the density flattening process an exact solution of the time-dependent diffusion equation for a linear initial profile over the island width is used. From this we also derive an expression for a temporal growing radial electric field. This positive field reaches its maximum at the density plateau. Strong viscous diffusion or instability-induced transport between high and low electric field regions may now reverse the density flattening. Therefore relaxation oscillations result which may also explain the observed strong density and potential fluctuations in tokamaks. Several details of recent measurements of impurity ion behaviour and density fluctuations in tokamaks may be better explained with the theory given here.     

Gate-dependent orbital magnetic moments in carbon nanotubes

We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube.     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

Anomalous Hall effect and electron transport in ferromagnetic MnBi films

The electron transport properties of highly c-axis oriented MnBi thin films of various thicknesses have been investigated. Samples are metallic but the low temperature resistivity shows an unusual T(3) dependence. Transverse Hall effect measurements show that both the ordinary and anomalous Hall coefficients decrease with decreasing temperature below 300 K, but the ordinary Hall coefficient (R(0)) undergoes a sign reversal around 105 K, where the magnetic anisotropy also changes sign. Analysis of the Hall data for various samples shows that the anomalous Hall coefficient (R(s)) exhibits a strong ρ(2) dependence, where ρ is the longitudinal resistivity.     

Density-induced interchange of anisotropy axes at half-filled high Landau levels

We observe density-induced 90 degrees rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ( nu = 9/2, 11/2, etc.). Using a field effect transistor, we find the transition density to be 2.9x10(11) cm(-2) at nu = 9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor magnetic field plane in the regime 4.4相似文献   

Stochastic Transport of Magnetic Field Lines in the Symmetric Tokamap

The topological structure and the statistical properties of stochastic magnetic fields are investigated on the basis of the so called tokamap. First, a monotonic safety factor (q‐profile) is assumed. As it is demonstrated, the transition from the continuous model to the discrete mapping in its symmetric form is essential, not only for the symplectic structure, but also for the precise values characterizing the transition to chaos (e.g. the break‐up of the KAM surfaces) in applications. Statistical properties of the symmetric tokamap, such as escape rates and anomalous diffusion properties, are being presented. By a systematic procedure the stable and unstable manifolds of the periodic hyperbolic fixed points and the resulting homoclinic tangles (stochastic layers) are determined. The latter are important for the magnetic field line transport. For a non‐monotonic q‐profile, the differences between the symmetric and non‐symmetric revtokamap become also significant. The symmetric revtokamap represents an open nonlinear dynamical system which is characterized here with the relevant tools. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of internal magnetic fluctuation in the HL-1M tokamak

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Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.94)S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below T(C) = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.