反磁剪切等离子体中的电阻交换模不稳定性

具有内部输运垒（ITB）的反磁剪切（RS）等离子体位形是在托卡马克中获得高参数的最具前景的途径之一。这种位形不仅改善等离子体约束,而且可以改进象气球模和新经典撕裂模等这类宏观模的稳定性。然而,反磁剪切区域的高压强可以驱动电阻交换模不稳定性,从而破坏中心区的等离子体高参数。为了研究电阻交换模不稳定性的性质,并确定其在RS等离子体中发展的区域,我们利用在HL－2A中使用中性束注入建立的RS位形来分析电阻交换模不稳定性。     

双撕裂模非线性演化过程中有理面上的剪切流

在二维平板几何模型下,利用磁流体力学方程组数值模拟托卡马克装置中双撕裂模非线性演化过程中有理面上剪切流的时间和空间分布.结果表明,双撕裂模非线性演化的早期阶段,有理面上没有形成明显的剪切流.剪切流主要存在于快速磁重联阶段,随着磁重联的结束而逐渐消失,剪切流的强度和空间分布随磁岛的演化而改变.另外,较大的等离子体电阻加速磁重联,但是对剪切流的强度和变化趋势没有直接的影响.     

基于有限体积法数值模拟双撕裂模非线性演化

以磁流体理论为基础,采用基于有限体积法的通量差分分裂格式数值求解具有双曲保守律形式的电阻磁流体方程组。编写C++程序对平板几何位形下的等离子体双撕裂模进行了长时间数值模拟,得到双撕裂模不稳定性的演化图景,捕捉到了双撕裂模非线性发展过程中磁场重联的几个典型阶段,讨论了等离子体电阻和两个有理面之间的距离对双撕裂模不稳定性非线性发展的影响。为研究磁流体动力学提供了一种可行的高精度数值算法。     

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以磁流体理论为基础,采用基于有限体积法的通量差分分裂格式数值求解具有双曲保守律形式的电阻磁流体方程组.编写C++程序对平板几何位形下的等离子体双撕裂模进行了长时间数值模拟,得到双撕裂模不稳定性的演化图景,捕捉到了双撕裂模非线性发展过程中磁场重联的几个典型阶段,讨论了等离子体电阻和两个有理面之间的距离对双撕裂模不稳定性非线性发展的影响.为研究磁流体动力学提供了一种可行的高精度数值算法.     

外部驱动电流抑制双撕裂模的发展

在Hall MHD方程组中引入外部驱动电流项,数值模拟外部驱动电流对双撕裂模不稳定性的影响.结果表明,在x点加入反向外部驱动电流对撕裂模的发展有抑制作用,周期性的外部驱动电流对双撕裂模不稳定性的抑制效果较好,能控制磁岛的尺度在相互驱动的临界尺度以下,避免发生双撕裂模非线性磁场重联的爆发,随着驱动电流周期变大抑制效果变坏,在一定限度内随着驱动电流强度增大抑制效果变好,在一定范围内随着驱动电流宽度变小抑制效果变好.     

用磁探针反演磁岛二维结构的方法及其在HL-2A装置上的应用

介绍了HL-2A装置中利用磁探针数据反演磁岛极向二维结构的新方法,以及在反演基础上建立的撕裂模动态分析方法。在实验中通过磁探针测量确定作为扰动磁场来源的扰动电流。将扰动磁通与由EFIT重建的平衡磁通叠加反演出磁岛的结构,并给出磁岛的宽度。然后,按时间顺序建构二维结构图并依次记录,之后依次展现图像就可以对磁岛进行动态分析。应用此方法进行撕裂模分析,得出了磁岛旋转与电子抗磁漂移方向相同,验证了磁岛宽度与扰动场的关系及ECRH对磁岛的抑制作用。这显示了磁探针反演磁岛结构方法的直观性,对观察并控制MHD不稳定性非常有利。     

电流驱动对磁岛的稳定化

大破裂限定了托卡马克的运行。实验指出，撕裂模或者说磁岛的发展是这种大破裂的主要原因。考虑到无感电流驱动效应，我们重新推导了电阻撕裂模的三维非线性方程组?数值计算表明，跟随磁岛旋转的ＲＦ波电流驱动有效地抑制磁岛的发展。     

旋转圆柱等离子体中撕裂模和Kelvin-Helmholtz不稳定性的激发特性

采用约化的磁流体力学模型,数值研究了柱位形等离子体中q剖面和极向旋转剖面对q=1撕裂模不稳定性和Kelvin-Helmholtz(K-H)不稳定性的影响.随着旋转强度的增加,m/n=1/1模被逐渐抑制,而高阶谐波模式(如m/n=2/2,m/n=3/3等)会经历四个区间:撕裂模失稳区间、撕裂模致稳区间、稳定窗口区间和K-H不稳定性激发区间.更进一步,我们发现,m/n=1/1模的增长率随旋转强度的改变与剪切层所处位置有关,并且剪切层分布在有理面内外的结果基本一致;然而高阶谐波模式却没有此类现象.另外,有理面处磁剪切越小,撕裂模越容易被剪切流抑制,并且越容易激发K-H不稳定性.     

负磁剪切位形中等离子体气球模稳定性研究

根据ＭＨＤ气球模稳定性理论，计算了负磁剪切位形中等离子体气球边缘稳定性区域。结果表明，在同样参数下，负磁剪切区域比正磁剪切区域允许更大的β值。     

负磁剪切等离子体中离子温度梯度驱动不稳定性分析

采用打靶法,数值求解了在负磁剪切托卡马克等离子体中离子温度梯度驱动的复本征模方程.数值计算表明,环向旋转剪切流可以整体上改变剪切板η_i不稳定性.在等离子体极小q磁面附近的芯部区域,环向剪切流有稳定η_i模的作用.     

Structure-driven nonlinear instability as the origin of the explosive reconnection dynamics in resistive double tearing modes

The onset of abrupt magnetic reconnection events, observed in the nonlinear evolution of double tearing modes (DTM), is investigated via reduced resistive magnetohydrodynamic simulations. We have identified the critical threshold for the parameters characterizing the linear DTM stability leading to the bifurcation to the explosive dynamics. A new type of secondary instability is discovered that is excited once the magnetic islands on each rational surface reach a critical structure characterized here by the width and the angle rating their triangularization. This new instability is an island structure-driven nonlinear instability, identified as the trigger of the subsequent nonlinear dynamics which couples flow and flux perturbations. This instability only weakly depends on resistivity.     

The effects of a shear flow on nonlinear evolutions of double tearing mode in Hall magnetohydrodynamics

The effects of a shear flow on nonlinear evolution of double tearing mode in Hall magnetohydrodynamics are investigated. The parallel shear flow displays a suppressing effect on the double tearing mode. Due to the effect of the shear flow, the relative displacements between the magnetic islands are changed, and therefore their mutual interactions become weak. Furthermore, the nonlinear impulsive growth phase of the double tearing mode is delayed with increasing the shear flow velocity. When the magnetic islands are pushing against each other, the width of current sheets decreases drastically, and therefore the Hall effects are dominant and the magnetic reconnection enhances greatly.     

Effects of resonant magnetic perturbation on the instability of single tearing mode with non-shear flow

Non-shear flow can change the O-point position of a magnetic island, and thus it may play an important role in the effects of resonant magnetic perturbation(RMP) on the single tearing mode. We employ the nonlinear magnetohydrodynamics model in a slab geometry to investigate how RMP affects the single tearing mode instability with non-shear flow. It is found that the driving and suppressing effects of RMP on single tearing mode instability will appear alternately. When the flow velocity is small, the suppressing effect plays a major role through the development of the mode, and the tearing mode instability will be suppressed. With the flow velocity increasing, the driving effect will increase, while the suppressing effect will decrease. When the two effects reach equilibrium, the tearing mode will become stable.     

Magnetic flux generation and wave emissions during coalescence of magnetic islands in pair plasmas

It is shown by using a two-dimensional fully relativistic electromagnetic particle-in-cell code that the tearing instability in a current sheet of pair plasmas is caused by Landau resonances of both electrons and positrons. Strong magnetic flux can be generated during coalescence of magnetic islands in the nonlinear phase of the tearing instability. The magnetic flux produced in an O-type magnetic island is caused by the counterstreaming instability found by Kazimura et al. [Astrophys. J. Lett. 498, L183 (1998); J. Phys. Soc. Jpn. 67, 1079 (1998)]. It is also shown that charge separation with a quadrupole-like structure is generated from the localized strong magnetic flux. During the decay of the quadrupole-like charge structure as well as the magnetic flux, there appear wave emissions with high-frequency electromagnetic waves and Alfvén waves as well as Langmuir waves.     

A study of shear flows induced by nonlinear evolution of double tearing modes in Hall magnetohydrodynamics

Shear flows induced by the nonlinear evolution of double tearing modes are studied numerically using Hall magnetohydrodynamics simulations in a slab geometry. The Hall effect is shown to plays an important role when the thickness of current sheets decrease and falls in the range of ion inertia length d _i . Effective shear flows in the magnetic islands are generated during the process of magnetic reconnection and disappear finally. The induced toroidal velocity are shown to be more sensitive than the induced poloidal velocities in the Hall dominant systems. Moreover, the temporal evolution of the shear flows in the Tokamak resonant surface is studied and discussed.     

Fast resistive reconnection regime in the nonlinear evolution of double tearing modes

Phases of nonlinear double tearing modes are studied numerically. The first two phases lead to the formation and growth of magnetic islands and are followed by a fast reconnection phase to complete the process, driven by a process of neighboring magnetic separatrices merging and magnetic islands coupling. The fast growth can be understood as a result of the island interaction equivalent to a steadily inward flux boundary driven. Resistivity dependences for various phases are studied and shown by scaling analysis for the first time. It is found that after an early Sweet-Parker phase with a eta(1/2)-scale, a slow nonlinear phase in a Rutherford regime with a eta(1)-scale is followed by the fast reconnection phase with a eta(1/5)-scale.     

Compressibility effects on double tearing mode interlocking in differentially rotating plasmas

Effects of compressibility on the double tearing modes (DTMs) in rotating plasmas are numerically investigated by using a compressible magnetohydrodynamics (MHD) model. It is found that due to the compressibility effects, the threshold of the interlocking magnetic island width in the slow and intermediate rotation regimes is larger than the counterpart in the incompressible plasmas. In the fast rotation regime, the compressible effect makes the DTM islands interlock more easily and faster. Moreover, in the very fast rotation regime, the plasma rotation can more effectively suppress the DTM islands. The scalings of the interlocking threshold in the different rotation regimes are obtained. Effects of plasma viscosity and beta on the DTM interlocking in the compressible plasmas are also discussed.     

流动等离子体对托卡马克中撕裂模的影响

本文分析了托卡马克中的等离子体具有一个整体流动速度时,撕裂模的活动情况。分析与数值计算指出,等离子体的整体流动(等价于具有一个旋转频率)不影响撕裂模的活动,只是使撕裂模具有一个和等离子体相同的旋转频率。外加螺旋场在撕裂层能有效地抑制住等离子体的旋转。螺旋场对撕裂模的稳定作用不受等离子体旋转的影响。 关键词：     

反常电子粘滞性引起的线性双撕裂模研究

用圆柱位形的磁流体动力学方程对由反常电子粘滞性引起的双撕裂模的线性行为做了数值研究.分析了两个有理面的间距、粘滞率大小和极向模数对双撕裂模模式结构的影响,给出了各种情况下双撕裂模线性增长率跟表征反常电子粘滞率大小的电子雷诺数的定标关系.