调制磁场清除柱形等离子体发生器中的尘埃颗粒

利用流体模型，计算了调制磁场作用下，圆柱形等离子体发生器中电子、离子及尘埃的运动情况．数值模拟结果表明，尘埃颗粒随着调制磁场频率的改变，产生两种不同的运动方式：pulse运动和除尘运动．解释了pulse运动产生的机理，并提出了柱形等离子体发生器中利用调制磁场清除尘埃颗粒的方法． 关键词： 尘埃颗粒 调制磁场     

绝热电荷扰动对非均匀热尘埃等离子体中三维孤波的影响

尘埃颗粒上不断有电子流和离子流的出入以及二次电离、光电离等因素,所以尘埃颗粒上的带电量不是一个常量而是随着时间和空间变化的,因此尘埃电荷成为尘埃等离子体中的一个新的动力学变量,研究其对等离子体中各类非线性过程的作用成为尘埃等离子体物理中的一个重要课题.当今对于非均匀尘埃等离子中非线性波的研究大多数都集中于一维,对于三维非线性波的研究非常少.基于这种情况,在考虑非均匀性、尘埃颗粒绝热电荷扰动以及外部磁场等物理因素的情况下,运用约化摄动方法得出描述三维孤波的变系数的Korteweg-de Vries(KdV)方程.由结果可以看出,非均匀性、电荷扰动、外部磁场、斜向传播、尘埃温度对三维非线性波的传播有着极大的影响.利用适当的变换,得到了变系数方程的近似解.     

圆柱形磁铁在金属管中的下落时间

用磁荷法计算磁铁产生磁场分布,进而计算圆柱形磁铁在金属管中的下落时间.实验测量圆柱形磁铁在金属管中的下落时间,得到与理论计算相符的结果.     

尘埃颗粒在射频等离子体鞘层中的非线性共振现象的理论研究

一种自洽的理论模型用于研究尘埃颗粒在射频鞘层中垂直方向的非线性共振现象.利用射频鞘层动力学模型，研究了鞘层电场和尘埃颗粒的充电过程.考虑作用在尘埃颗粒上的各种作用力(如重力、电场力、离子拖拽力、中性气体摩擦力及探针的扰动力)，并通过数值求解颗粒运动方程，模拟了该尘埃颗粒在探针扰动下的非线性振荡过程.所得结果不仅很好地再现了最近的实验观察，同时还发现这种非线性振动过程与等离子体参数、施加的射频偏压的功率、探针的扰动电位和位置等有着密切的联系. 关键词： 射频鞘层 尘埃颗粒 尘埃充电 非线性共振 迟滞     

两种带电尘埃颗粒的等离子体鞘层玻姆判据

采用流体方程和尘埃充电自洽模型研究了鞘边含有两种尘埃颗粒的等离子体玻姆判据. 通过拟牛顿法数值模拟了鞘边两种尘埃颗粒的存在对尘埃自身充电以及离子马赫数的影响. 两种尘埃颗粒中含量较少的尘埃颗粒数密度的增加, 导致两种尘埃颗粒表面悬浮势一个降低, 一个升高. 含量较少的尘埃颗粒的数密度越多和半径越小, 都会导致离子马赫数增大. 另外鞘边无论何种尘埃颗粒的速度增加, 鞘边离子马赫数都将减小. 关键词： 等离子体鞘层 尘埃颗粒 玻姆判据     

非热离子对非均匀碰撞热尘埃等离子体中三维孤波的影响

从理论上研究了非热离子、外部磁场、碰撞对非均匀热尘埃等离子体中三维非线性尘埃声孤波的影响。运用约化摄动法得到描述三维非线性尘埃声孤波的非标准的变系数Korteweg-de Vries(KdV)方程。然后把非标准KdV方程变为标准的变系数KdV方程,并且得到了标准的变系数KdV方程的近似解析解。由此解析解可以看出,非热离子的数目、碰撞、非均匀性、波的斜向传播、尘埃颗粒和非热离子的温度对三维非线性尘埃声孤波的振幅和宽度有很大的影响。外部磁场对三维非线性尘埃声孤波的宽度有影响,而对其振幅没有影响。此外,波的相速度与非热离子、波的斜向传播、尘埃颗粒的温度和非均匀性有关。     

强碰撞磁化尘埃等离子体中的漂移波

研究磁化尘埃等离子体的波动机理,对相关实验、工业加工、天体空间探测等具有重要的价值.本文研究了非均匀磁化尘埃等离子体系统的线性和非线性波动特性.对处于均匀外磁场中的非均匀尘埃等离子体系统,存在密度和温度梯度的非均匀环境,考虑尘埃和中性粒子的强碰撞作用,得到了一个二维非线性磁流体动力学方程.通过线性化得到了色散关系,系统既有强碰撞引起的阻尼波,又有粒子漂移产生的谐波;典型的数值参量分析结果表明,量子参量修改了系统的尺度;实频与漂移频率成正比关系,而虚频与尘埃和中子间的碰撞频率有着比较复杂的关系,粒子的碰撞引起了系统的耗散效应.通过函数变换法,获得了漂移的冲击波和爆炸波的解析解.详细讨论了静电势随着主要物理量的变化,研究结果显示,静电冲击波的幅度和爆炸波的宽度都随尘埃密度和外界磁场强度的增大而增大,随着碰撞频率的增大而减小,随漂移速度而变化.当时空相位较小时,静电势变化很快,一旦足够大时,静电势将趋于稳定值,最终达到稳定的状态.最后对系统的稳定性问题进行了分析,发现尘埃电荷量、量子参量、漂移速度都出现在扰动解中.本文所得结果表明,强碰撞效应、量子效应、粒子漂移和外界磁场等主要物理量都对尘埃漂移波的产生、演化和稳定性产生了重要的作用.     

托卡马克等离子体中共振磁扰动场放大效应对离子轨道特性的作用

首先,利用MARS-F程序模拟了HL-2A装置中不同电阻率、环向旋转频率和环向模数条件下等离子体对共振磁扰动(RMP)的线性响应过程,分析了不同情况下共振场放大效应对三维扰动磁场的影响;然后,利用Boris算法追踪了三维场作用下的离子轨道,并详细探究了不同扰动磁场改变离子轨道特性的物理机制.研究发现,考虑等离子体响应后的扰动磁场可增强离子轨道径向展宽,且轨道最大径向展宽随轨道上扰动磁场平均值的增大而增大;同时,离子在通过扰动磁场被强烈放大的区域时轨道径向展宽会显著增加.该物理机制可用于解释RMP缓解边界局域模实验中离子直接损失增加和等离子体径向输运增强现象.     

基于超二次曲面的颗粒材料缓冲性能离散元分析

自然界或工业中普遍是由非球形颗粒组成的复杂体系,与球形颗粒相比,非球形颗粒间的高离散和咬合互锁可使冲击载荷引起的能量有效衰减实现缓冲作用.基于连续函数包络的超二次曲面单元能准确地描述非球形颗粒的几何形态,并可精确地计算单元间的接触碰撞作用.本文采用离散元方法对冲击载荷作用下非球形颗粒物质的缓冲性能进行数值分析,并与圆柱体冲击的理论结果和球体冲击的实验结果进行对比验证.在此基础之上,进一步研究了筒底作用力在不同颗粒层厚度和形状等因素影响下的变化规律.计算结果表明:不同颗粒形状都存在一个临界厚度H_c.当HH_c时,缓冲率随H的增加而增加;当HH_c时,缓冲率的变化不再显著并趋于稳定值.此外,减小颗粒表面尖锐度和增加或减小圆柱形和长方形颗粒的长宽比都会提高颗粒材料的缓冲效果.     

对含有非热力学平衡离子的尘埃等离子体中孤波特性的理论研究

得到了描述由尘埃颗粒电荷变化、非热力学平衡分布的离子和Boltzmann分布的电子组成的未磁化的热尘埃等离子体中的尘埃声波的修正的KdV (mKdV) 方程. 并对诸多的尘埃等离子体参数对尘埃声孤波结构的影响进行了理论研究，结果表明，尘埃等离子体参数决定着尘埃声孤波结构，且只在这些参数特别选定的一些区域，才会出现稳定的孤波. 关键词： 非热力学平衡离子 尘埃颗粒电荷变化 尘埃声孤波     

Cycloid motions of grains in a dust plasma

Hypocycloid and epicycloid motions of irregular grains(pine pollen) are observed for the first time in a dust plasma in a two-dimensional(2D) horizontal plane. These cycloid motions can be regarded as a combination of a primary circle and a secondary circle. An inverse Magnus force originating from the spin of the irregular grain gives rise to the primary circle.Radial confinement resulting from the electrostatic force and the ion drag force, together with inverse Magnus force, plays an important role in the formation of the secondary circle. In addition, the cyclotron radius is seen to change periodically during the cycloid motion. Force analysis and comparison experiments have shown that the cycloid motions are distinctive features of an irregular grain immersed in a plasma.     

Attractive potential around a thermionically emitting microparticle

We present a simulation study of the charging of a dust grain immersed in a plasma, considering the effect of thermionic electron emission from the grain. It is shown that the orbit motion limited theory is no longer reliable when electron emission becomes large: screening can no longer be treated within the Debye-Huckel approach and an attractive potential well can form, leading to the possibility of attractive forces on other grains with the same polarity. We suggest to perform laboratory experiments where emitting dust grains could be used to create nonconventional dust crystals or macromolecules.     

Electrodynamics and dispersion properties of a magnetoplasma containing elongated and rotating dust grains

The electrodynamics and dispersion properties of a magnetized dusty plasma containing elongated and rotating charged dust grains are examined. Starting from an appropriate Lagrangian for dust grains, a kinetic equation for the dust grain and the corresponding equations of motion are derived. Expressions for the dust charge and dust current densities are obtained with the finite size (the dipole moment) of elongated and rotating dust grains taken into account. These charge and current densities are combined with the Maxwell-Vlasov system of equations to derive dispersion relations for the electromagnetic and electrostatic waves in a dusty magnetoplasma. The dispersion relations are analyzed to demonstrate that the dust grain rotation introduces new classes of instabilities involving various low-frequency waves in a dusty magnetoplasma. Examples of various unstable low-frequency waves include the electron whistler, the dust whistler, dust cyclotron waves, AlfvÉn waves, electromagnetic ion-cyclotron waves, as well as lower-hybrid, electrostatic ion cyclotron, modified dust ion-acoustic waves, etc. Also found is a new type of unstable waves whose frequency is close to the dust grain rotation frequency. The present results should be useful in understanding the properties of low-frequency waves in cosmic and laboratory plasmas that are embedded in an external magnetic field and contain elongated and rotating charged dust grains.     

Forces on a small grain in the nonlinear plasma wake of another

The transverse force on a spherical charged grain lying in the plasma wake of another grain is analyzed to assess the importance of ion-drag perturbation, in addition to the wake-potential-gradient. The ion-drag perturbation is intrinsically one order smaller than the wake-potential force in the ratio of grain size (r(p)) to Debye length (λ(De)). So ion-drag perturbation is important only in nonlinear wakes. Rigorous particle-in-cell calculations of the force are performed in the nonlinear regime with two interacting grains. It is found that even for quite large grains, r(p)/λ(De)=0.1, the force is dominated by the wake-potential gradient. The wake-potential structure can then help explain the preferred alignment of floating dust grains.     

Influence of charging current fluctuations on the grain velocity distribution in weakly-ionized plasmas

The distribution function of dust particles immersed in a weakly-ionized plasma is calculated with regard to electron and ion absorption by grains and fluctuations of grain-charging currents. The dust-particle energy is shown to undergo abrupt changes due to specific velocity dependence of the grain friction coefficient and stochastic character of grain charging. Their effect can crucially influence the state of the grain. This model can give some indication concerning the possibility of structure formation and melting of a dusty crystal structure. The hysteresis in the transition from the one-peak distribution function to twin-peak distribution function can be observed with changing of plasma parameters.     

Effect of polarization force on the Jeans instability of self-gravitating dusty plasma

The effect of polarization force acting on massive charged dust grains is investigated analytically on the Jeans instability of self-gravitating dusty plasma. The gravitational force acting on the massive negatively charged interstellar dust grains are considered in presence of both electrical and polarization forces. The basic equations of the problem are formulated and a general dispersion relation is obtained using plane wave approximation in low frequency wave mode. The effect of polarization force in the dispersion relation of the problem, condition of the Jeans instability and expression of the critical Jeans wave number is examined. The unstable growing modes due to self-gravitational force are studied in the situation when polarization force on the dust grain exceeds over the electrical force in magnitude. It is observed that the polarization force increases the growth rate of the system.     

Effect of the Polarization Force on the Dust–Acoustic Soliton Energy

The role the polarization force acting on dust particles may play on the energy carried by the dust‐acoustic (DA) soliton is examined. This force is due to the deformation of the Debye sheath around the dust particulates in the background of non‐uniform plasmas. The energy carried by the DA solitons is derived. This energy decreases with an increase in the effects of plasma‐dust particles polarization interaction. This means that when the magnitude of the polarization force approaches the one of the electrical force, i.e, the net force acting on the grains decreases, the DA soliton energy experiences a depletion. As noticed by Khrapak et al. [S.A. Khrapak et al., Phys. Rev. Lett. 102 , 245004 (2009)], when the polarization force dominates over the electrical one, the plasma can not support DA solitons. This happens whenever the dust grain size exceeds a critical threshold. In this case, the net force acting on the grains is no longer a restoring force. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Superfluidlike motion of an absorbing body in a collisional plasma

Motion of a small charged absorbing body (micrograin) immersed in a stationary weakly ionized high pressure plasma environment is considered. It is shown that the total frictional (drag) force acting on the grain can be directed along its motion, causing the grain acceleration. At some velocity, the forces associated with different plasma components can balance each other, allowing free undamped superfluid motion of the grain. The conditions when such behavior can be realized and the possibility of a superconductive grain current are discussed in the context of complex (dusty) plasmas.     

Small amplitude dust-acoustic soliton energy in dusty plasmas with suprathermal polarization force

The effect of suprathermal polarization force on both linear and weakly nonlinear dust-acoustic solitary structures in a three-component dusty plasma is investigated. For this purpose, a new expression of the polarization force acting on dust particles that include the electronic suprathermal effect is derived. The results are applied to two different experimental dusty plasmas. We have found that the polarization force acting on the dust grains decreases as the electron suprathermality becomes more significant. In addition, we have shown that, for a given value of the spectral index κ , the polarization force magnitude fluctuates from one plasma to another. The changes arising in the propagation of small-amplitude dust-acoustic (DA) solitons due to the presence of this suprathermal polarization force are also analysed. Interestingly, an increase in the magnitude of the polarization force leads to an increase in the amplitude and width of DA soliton and provides more energy to the motion of this soliton.