Energy confinement time and heat transport in initial neutral beam heated plasmas on the large helical device

The confinement characteristics of large net-current-free plasmas heated by neutral-beam injection have been investigated in the Large Helical Device (LHD). A systematic enhancement in energy-confinement times from the scaling derived from the medium-sized heliotron/torsatron experiments have been observed, which is attributed to the edge pedestal. The core confinement is scaled with the Bohm term divided by the square root of the gyro radii. The comparative analysis using a dimensionally similar discharge in the Compact Helical System indicates gyro-Bohm dependence in the core and transport improvement in the edge region of LHD plasmas.     

Flux conversion and evidence of relaxation in a high-beta plasma formed by high-speed injection into a mirror confinement structure

High-beta plasmoids can survive the violent dynamics of supersonic reflection off mirror structures, producing a stable high-beta field-reversed configuration (FRC). This shows both the robustness of FRCs and their tendency to assume a preferred plasma state, possibly conforming to a relaxation principle. The key observations are (1) approximate preservation of the magnetic helicity, (2) substantial conversion from toroidal to poloidal magnetic flux, (3) substantial toroidal flow, and (4) a high-beta quiescent final state. These results are from the Translation, Confinement, and Sustainment experiment where a disorganized plasmoid is injected at super-Alfvenic speed into a confinement chamber. After successive reflections from end mirrors, the plasmoid settled into a near-FRC state with high beta and low toroidal magnetic field. The flux conversion and helicity preservation are inferred by an interpretive model.     

Observations of improved stability and confinement in a high-beta self-organized spherical-torus-like field-reversed configuration

An extremely high-beta (over 85%) self-organized field-reversed configuration (FRC) with a spherical-torus- (ST-)like core is produced in the translation, confinement, and sustainment experiment by highly super-Alfvénic translation of a spheromaklike plasmoid. Substantial flux conversion from toroidal into poloidal occurs during the capture process, resulting in the ST-like core. This plasma state exhibits a remarkable stabilizing property for the ubiquitous centrifugally driven interchange modes present in theta-pinch formed FRCs. This is explained, for the first time, by a simple model taking into account magnetic shear and centrifugal effects. The FRC-ST configuration has up to 4 times improvement in flux confinement times over the scaling of conventional theta-pinch formed FRCs and, thus, a significant improvement in the resistivity and transport.     

Stabilization of interchange modes by rotating magnetic fields

Interchange modes have been a key limiting instability for many magnetic confinement fusion configurations. In previous studies intended to deal with these ubiquitous instabilities, complex, transport enhancing, minimum-B producing coils were added to the otherwise simple linear mirror plasma. Possible solutions for returning to a simple symmetric mirror configuration, such as ponderomotive fields, are weak and difficult to apply. A new method is demonstrated here for the first time, utilizing rotating magnetic fields that are simple to apply and highly effective. A simple and easily comprehensible theory has also been developed to explain the remarkable stabilizing properties. Although this work has been performed on field reversed configurations, it should have a wide application to other confinement schemes, and could become a cornerstone for high-beta plasma stability.     

Analysis of Asymmetric Long-Range Surface-Plasmon Waveguide with High-Confinement Mode

We model and study the asymmetric long-range surface-plasmon waveguides using the finite-element method. We introduce two types of asymmetric structures and discuss their modal properties compared to traditional long-range surface-plasmon waveguides. Although the propagation distance is decreased, the energy-confinement capability is improved for asymmetric long-range waveguiding structures when the geometrical parameters are properly selected. Our simulation result offers guidance for tuning properties of plasmonic waveguides and providing ways for enhancing electromagnetic energy confinement in long-range surface-plasmon waveguides.     

Nonlinear growth of firehose and mirror fluctuations in astrophysical plasmas

In turbulent high-beta astrophysical plasmas (exemplified by the galaxy cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations grow nonlinearly to large amplitudes, deltaB/B approximately 1, on a time scale comparable to the turnover time of the turbulent motions. The principle of their nonlinear evolution is to generate secularly growing small-scale magnetic fluctuations that on average cancel the temporal change in the large-scale magnetic field responsible for the pressure anisotropies. The presence of small-scale magnetic fluctuations may dramatically affect the transport properties and, thereby, the large-scale dynamics of the high-beta astrophysical plasmas.     

Evidence of relaxation and spontaneous transition to a high-confinement state in high-beta steady-state plasmas sustained by rotating magnetic fields

Evidence of relaxation has appeared, for the first time, in the extremely high-beta, steady-state field-reversed configuration plasma states driven by rotating magnetic fields (RMF) in the translation, confinement, and sustainment experiment. The plasma self-organizes into a near-force-free state in the vicinity of the magnetic axis, with significant improvement in confinement. Associated with this change in magnetic topology is the appearance of an axial RMF component; this would, in turn, generate a current drive in the poloidal direction, thus sustaining the magnetic helicity. A newly developed two-dimensional "equilibrium-lite" model is employed to analyze the magnetic properties of the final high-confinement state, and shows a large q and a significant magnetic shear in the core.     

Photon statistics of semiconductor microcavity lasers

We present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.     

Reaching high poloidal beta at Greenwald density with internal transport barrier close to full noninductive current drive

In the ASDEX Upgrade tokamak, high poloidal beta up to beta(pol) = 3 at the Greenwald density with H-mode confinement has been reached. Because of the high beta, the plasma current is driven almost fully noninductively, consisting of 51% bootstrap and 43% neutral beam driven current. To reach these conditions the discharge is operated at low plasma current ( I(P) = 400 kA) and high neutral beam heating power ( P(NBI) = 10 MW). The discharge combines an edge (H mode) and internal transport barrier at high densities without confinement-limiting MHD activities. The extrapolation to higher plasma currents may offer a promising way for an advanced scenario based fusion reactor.     

高比压环流器等离子体位形的稳定性

本文用解析方法表示了高比压环流器的平衡位形,进而计算了局部理想磁流体判据,并着重讨论了低q运行的可能性。结果发现存在一大类具有这些特性的环流器平衡位形,有的位形其中心约束区域有很高的稳定性余度。对p′(ψ)剖面不同的位形以及不同位移函数的位形的稳定性做比较,发现大的位移及平坦的p′(ψ)剖面有更好的稳定性。 关键词：     

Numerical investigation of non-local electron transport in laser-produced plasmas

Non-local electron transport in laser-produced plasmas under inertial confinement fusion (ICF) conditions is studied based on Fokker-Planck (FP) and hydrodynamic simulations. A comparison between the classical Spitzer--H\"arm (SH) transport model and non-local transport models has been made. The result shows that among those non-local models the Epperlein and Short (ES) model of heat flux is in reasonable agreement with the FP simulation in overdense region. However, the non-local models are invalid in the hot underdense plasmas. Hydrodynamic simulation is performed with the flux limiting model and the non-local model, separately. The simulation results show that in the underdense region of the laser-produced plasmas the temperature given by the flux limiting model is significantly higher than that given with the non-local model.     

Progress in anomalous transport research in toroidal magneticconfinement devices

Transport is the outstanding physics issue in the quest for fusion by magnetic confinement. In spite of the intrinsic difficulty, a great deal of progress has been made in the past 25 years. Experiments have gone from being dominated by high anomalous losses, of the order of Bohm diffusion losses, to operation with no anomalous transport. This success is due to a combination of improved experimental infrastructure and the high degree of knowledge on how to control plasma discharges, Both have made it possible to access enhanced confinement regimes and to unravel new effects in confinement physics. Although there is not yet a complete understanding of the dynamical mechanisms underlying the anomalous transport process, there is some understanding of important components such as the ion transport loss mechanism at the plasma core and of the main mechanism for turbulence suppression in the enhanced confinement regimes     

利用低杂波改善约束的实验研究

介绍了在HT-6M托卡马克上,利用双低杂波的组合,成功地实现了准稳态的高约束模式运行. 能量约束时间提高两倍以上,粒子的约束在较高密度下依然改善3倍以上.通过应用波扩散及 电流径向扩散方程计算低杂波电流传播的方法,对一组典型的数据进行数值模拟.计算表明 ,在HT-6M低杂波实验中,由于纵场较低,密度较高,低杂波的能量沉积在离磁轴较远的位 置,使等离子体电流密度分布成为反剪切位形,内部输运垒地形成,大大提高了等离子体的 约束状况.实验数据也给出了反剪切的证据. 关键词： 托卡马克 低杂波 约束改善     

Confinement physics for thermal, neutral, high-charge-state plasmas in nested-well solenoidal traps

A theoretical study is presented which indicates that it is possible to confine a neutral plasma using static electric and solenoidal magnetic fields. The plasma consists of equal temperature electrons and highly stripped ions. The solenoidal magnetic field provides radial confinement, while the electric field, which produces an axial nested-well potential profile, provides axial confinement. A self-consistent, multidimensional numerical solution for the electric potential is obtained, and a fully kinetic theoretical treatment on axial transport is used to determine an axial confinement time scale. The effect on confinement of the presence of a radial electric field is explored with the use of ion trajectory calculations. A thermal, neutral, high-charge-state plasma confined in a nested-well trap opens new possibilities for fundamental studies on plasma recombination and cross-field transport processes under highly controlled conditions.     

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通过求解二维热输运方程,数值分析了横越单磁岛的热输运现象。仅有背景加热时,当磁岛宽度小于临界磁岛宽度时,磁岛对能量约束影响不明显;当磁岛宽度大于临界磁岛宽度时,磁岛降低的能量损失随着磁岛宽度呈线性增大,有效径向热导系数在磁岛区域呈高斯分布且最大值在有理面处。当等离子体背景加热与ECRH共同加热时,ECRH对磁岛区域的温度分布及能量约束均有别于背景热源,这为进一步研究ECRH抑制撕裂模的问题提供了基础。     

TOKAMAK BALLOONING MODE STUDIES-(I)

In this paper, the first part deals with the structure of the second stability regime of the ideal MHD ballooning modes in circular tokamaks. It is shown that in high-beta regime,the poloidal magnetic field destabilizes strongly the plasma and can eventually lead to the appearance of a new instability regime at modest beta-value for a flat q-profile. In the second part, th a various kinetic effects on ballooning modes or high-beta tokamaks are calculated. Results show that a new weak ballooning mode branch can exist in the second MHD stability regime in addition to the MHD-like strony ballooning mode branch in the first instability regime with its growth rate decreased by kinetic stabilizations.