强激光与稠密等离子体作用引起的冲击波加速离子的研究

用二维PIC（Particle-in-Cell）程序模拟研究了强激光与稠密等离子体靶作用产生的无碰撞静电冲击波的结构和这种冲击波对离子的加速过程,研究发现由于冲击波前沿附近的双极电场的作用,具有一定初速度的离子能被该双极场俘获并获得加速,最终能够被加速到两倍冲击波速度.冲击波加速可以得到准单能的离子能谱,叠加在通过鞘层加速机理产生的宽度离子能谱上.还对不同激光强度和不同等离子体密度情况下形成的冲击波进行了比较.研究表明,强度相对较低的激光在高密度等离子体中可以产生以一定速度传播的静电孤波结构,后者只能加速 关键词： 强激光 稠密等离子体 无碰撞静电冲击波 离子加速     

Separation of ions on the front of a shock wave in a multicomponent plasma

The structure of a shock wave propagating in a plasma with two types of ions has been studied within the model of multifluid hydrodynamics based on the 13-moment system of Grad’s equations. Although the averaged dynamics of the shock front coincides with the single-component variant of the average-ion model, its structure is different at a noticeable difference between charge-to-mass ratios of different ions, demonstrating their separation on the shock front. For the problem of inertial confinement fusion, the range of parameters for which such a separation is important, as well as physical processes determining the two-component structure of the shock front, has been established.     

高密度平面靶等离子体中激光驱动冲击波加速离子的能谱展宽

本文对超短超强激光脉冲辐照高密度等离子体产生的静电冲击波加速离子的能谱展宽机理进行了数值研究.着重讨论了三种冲击波加速离子的能谱展宽机理:能量沉积到离子中使得冲击波前沿不断减速,被加速离子与背景粒子的碰撞,以及高能离子到达靶背面时受到鞘层场进一步加速.还研究了驱动激光脉冲宽度对冲击波加速离子能谱宽度的影响. 关键词： 激光等离子体 冲击波加速 能谱展宽     

Electron kinetics in collisionless shock waves

We study the kinetic model of the formation of the energy spectrum of nonthermal electrons near the front of a quasilongitudinal, supercritical, collisionless shock wave. Nonresonant interactions of the electrons and the fluctuations generated by kinetic instabilities of the ions in the transition region inside the shock front play the main role in the heating and preacceleration of electrons. We calculate the electron energy spectrum in the vicinity of the shock wave and show that the heating and preacceleration of electrons occur on a scale of the order of several hundred ion inertial lengths in the vicinity of the viscous discontinuity. Although the electron distribution function is significantly nonequilibrium near the shock front, its low-energy part can be approximated by a Maxwellian distribution. The effective electron temperature T _eff ² behind the front, obtained in this manner, increases with the Mach number of the shock wave slower than it would if it followed the Hugoniot adiabat. We determine the condition under which the electron heating is ineffective but the electrons are effectively accelerated to high energies. The high-energy asymptotic behavior of the distribution function is that of a power law, with the exponent determined by the total compression ratio of the plasma, as in the case of acceleration by the first-order Fermi mechanism. The model is used to describe the case (important for applications) of acceleration of electrons by shock waves with large total Mach numbers, with the structure of these waves modified by the nonlinear interaction of nonthermal ions and consisting of an extended prefront with a smooth variation of the macroscopic parameters and a viscous discontinuity in speed with a moderate value of the Mach number. Zh. éksp. Teor. Fiz. 115, 846–864 (March 1999)     

The Self-Similarity Index of the Convergence of Strong Cylindrical Shock Waves in a Gas with a Uniform Density

A model of the convergence of cylindrical shock waves (SWs) in a gas with a uniform density has been considered. The partial differential equations of this model have been reduced to ordinary differential equations, from which the law of convergence of such shock waves and the dependence α = f(γ, γ_eff) of their self-similarity index α on the heat-capacity ratio in front of the shock wave (γ) and behind the shock wave front (γ_eff) of the gas have been found. This dependence for cylindrical shock waves has been shown to agree with the experimental data within the measurement error.     

Ionization effect in the front of a weak shock wave propagating in an inert gas diluted by a small amount of Mo(CO)6

The propagation of shock waves in He and Ar containing 0.01% of molybdenum hexacarbonyl Mo(CO)₆ as a heavy component of the mixture has been investigated with the use of the method of multichannel emission spectroscopy and an electrostatic probe with a spatial resolution of 0.2 mm placed in the core of a flow. The measurements have been carried out in incident shock waves with a high-vacuum shock tube in a Mach number range of 2.5–3.4. The equilibrium parameters behind the shock front are P ₂ = 0.109–1.124 atm and T ₂ = 853–1280 K, the concentration of Mo(CO)₆ is specially controlled, and high-purity He and Ar are used. The experiments are carried out under conditions when collisions between heavy molecules can be disregarded. It has been found that a narrow conduction band with a carrier density of more than 10⁵ cm^?3 appears in the shock front. The carrier density and its time characteristics have been measured. A correlation has been found between the conduction band and peaks of the nonequilibrium radiation in the visible and ultraviolet spectral ranges. This radiation disappears when the equilibrium parameters are reached behind the shock wave. The arrival of the conduction band and radiation band in the shock front at the measurement section advances the arrival of the density gradient of the shock front in most regimes. It has been found that the maximum conduction increases as the square of the Mo(CO)₆ concentration and decreases with increasing pressure. The effective threshold of the appearance of charges in the shock front has been determined as 1.35 ± 0.15 eV. A qualitative mechanism of the effect has been proposed with allowance for possible separation of charges in the shock front and with the inclusion of the “hot” wing of the energy distribution function of pair collisions.     

Charged Particle Distribution near the Shock Front in a Glow Discharge

The charged particle distribution near the front of a shock wave propagating in the glow discharge plasma has been investigated. It has been found that the ion concentration before the front varies nonmonotonically. Behind the shock front, the charged particle concentration varies smoothly in contrast to the neutral component density.     

Complex Burgers’ Equation: Evolution of Shock Waves with a Pair of Non-isothermal Ions in an Arbitrarily Charged Dusty Plasma

The evolution characteristics of dust-acoustic shock wave phenomenon in an arbitrarily charged dusty plasma in presence of pair-ion are investigated by deriving modified Complex Burgers’ equation. In this work, the effect of state of both the ions on the shock wave potential is analysed and discussed in detail. It has been observed that the variation of the non-linear coefficient is more consistent with the non-isothermal positive ions moving toward thermal equilibrium than with non-isothermal negative ions. A significant effect on the amplitude of the shock wave potential is observed as the distribution of the non-isothermal positive and negative ions changes from a hump shape through the flat-topped to Boltzmann type. A weighty effect on the height of the shock wave is observed as and when the Mach number of positive and negative ions changes. The shock wave potential shows a greater (lesser) value with negative (positive) ions moving ahead for thermal equilibrium.     

冲击波加热的氩气的发射光谱

为了研究弱冲击条件下氩气的辐射特性,从二级轻气炮发射的89 W飞片以2.00 km/s的速度撞击封装有常态下氩气的LY12铝靶,用六通道瞬态高温计和示波器记录到受一次冲击压缩的氩气的光辐射信号随时间的变化规律:从405 nm到700 nm的5个通道信号为直线段,800 nm通道的信号为指数曲线段,并测量出相应的冲击波速度为4.10±0.09 k m/s.实验表明,冲击压缩的氩气对可见光的吸收系数较小,气体的光学厚度沿长波方向从光学薄向光学厚转变.二次冲击压缩氩气的辐射信号上升前沿的时间分辨光谱用OMA系统记录,并观察到450 nm附近有较强的发射光谱带,但没有观察到明显的线状光谱.通过冲击波后参数和氩气电离度的理论计算,解释了冲击压缩下氩气的发射光谱特性.     

激光驱动的冲击波自生磁场以及外加磁场的冲击波放大研究

冲击波是天体物理观测中常见的现象, 其对粒子的加速被认为是高能宇宙射线的来源. 宇宙中冲击波周围往往存在很强的磁场, 但人们对于此类强磁场的产生放大过程的理解并不充分. 本文利用二维粒子模拟程序研究了激光与磁化或者非磁化等离子体相互作用产生的冲击波现象, 给出了冲击波波前处磁场的产生放大特性. 研究发现, 作用过程中的自生磁场可以储存能量, 从而进一步加速电子; 当存在外加磁场时, 由冲击波加速的电子和离子的能量都比同条件下非磁化等离子体的能量高; 而且外加磁场藉由冲击波放大倍数则与其值有极大关系. 与天文观测中推断的磁场与背景磁场相比放大千倍这一研究结果的比较可以看出, 天体冲击波周围磁场放大主要是由局域内生磁场导致的.     

Structure of a collisionless shock front with relativistically accelerated particles

A nonlinear self-consistent analytic theory is developed to describe the front structure of a strong magnetohydrodynamic (MHD) collisionless shock wave that generates accelerated particles (including ultrarelativistic particles). The theory is used to predict the degree of compression of matter at the plane front of such a wave, which can greatly exceed compression at an ordinary gas-dynamic front, and also the velocity, density, and pressure profiles. The energy spectrum of the accelerated particles, which is produced by the complex velocity profile at the shock transition, is determined self-consistently. New nonlinear effects are predicted that have not been discussed previously in the literature: a strong dependence of the particle acceleration regimes on the rate of injection; the existence of several regimes within a certain range of injected powers with differing spectra of accelerated particles, shapes of the shock transition profile, and magnitudes of compression of the medium; and the possibility of spontaneous jumps between different states of the shock transition. The question of stability of these states is discussed. For the values of the system parameters used here, the nonlinear regimes correspond to extremely low injection rates, of order 10⁻²–10⁻¹⁰ of the plasma flux density advancing into the front, and to exponents of the power-law spectra of accelerated particles between 5 and 3. Zh. éksp. Teor. Fiz. 112, 1584–1602 (November 1997)     

Unlimited relativistic shock surfing acceleration

Nonrelativistic shock surfing acceleration at quasiperpendicular shocks is usually considered to be a preacceleration mechanism for slow pickup ions to initiate diffusive shock acceleration. In shock surfing, the particle accelerates along the shock front under the action of the convective electric field of the plasma flow. However, the particle also gains kinetic energy normal to the shock and eventually escapes downstream. We consider the case when ions are accelerated to relativistic velocities. In this case, the ions are likely to be trapped for infinitely long times, because the energy of bounce oscillations tends to decrease during acceleration. This suggests the possibility of unlimited acceleration by shock surfing.     

Steady state ion acceleration by a circularly polarized laser pulse

The steady state ion acceleration at the front of a cold solid target by a circularly polarized flat-top laser pulse is studied with one-dimensional particle-in-cell (PIC) simulation. A model that ions are reflected by a steady laser-driven piston is used by comparing with the electrostatic shock acceleration. A stable profile with a double-flat-top structure in phase space forms after ions enter the undisturbed region of the target with a constant velocity.     

On the origin of nonequilibrium radiation from iodine molecules at the shock wave front

The direct simulation Monte Carlo (DSMC) method is used for modeling the problem on the shock wave front in the 0.7%I₂-99.3%He mixture for a shock wave Mach number of 4.85. The choice of this system is due to the fact that intense radiation peaks have been observed experimentally precisely in such systems and it has been convincingly proven that this effect is induced by high-energy collisions between I₂ molecules. The results of simulation provide additional sound arguments in favor of the conclusion that the translational nonequilibrium state at the shock wave front in a light gas with a small admixture of heavy nonreacting molecules may cause the experimentally observed nonequilibrium radiation peaks.     

冲击压缩下非均质脆性固体的弛豫破坏研究

通过平板冲击实验研究了富含微缺陷的非均质脆性固体的冲击压缩响应特性.选取“强角闪石化橄榄二辉岩”作为样品材料，利用激光速度干涉仪测量样品后自由面的速度历史，在冲击加载应力远低于样品材料Hugoniot弹性极限的条件下，观测到了表征破坏波出现的再加载信号，并且该破坏波的速度远大于玻璃中破坏波的速度，以接近于冲击波的速度在样品内向前传播，其形成机理与玻璃样品中的破坏机理不同，称之为“就位扩展机理”.采用同一冲击加载应力(～3.9GPa)作用于不同厚度的样品,获得了破坏波穿过样品的运动过程，确定出样品中破坏波的轨迹线近似为一条不过原点的直线，相应的产生此破坏波的弛豫时间约为0.5 μs.     

超高压下冲击波速度直接测量技术

针对超高压下透明材料的高压离化机理,分析了透明材料中冲击波直接诊断技术的基本方法. 利用Drude-自由电子气模型,分析了不同冲击压力下冲击波阵面反射率的变化. 从理论上比较了不同探针光波长反射率的区别,发现探针光波长为660 nm时比探针光波长为532 nm时获得的冲击波阵面反射率要高. 对探测器"致盲"问题也进行了研究. 通过分析反射信号的时间顺序和强度大小,发现"致盲"效应是由X光对透明窗口离化引起的. 同时,发现方波驱动脉冲平台的前沿到达时刻和X光离化效应出现的时刻相同,冲击波信号到达时刻晚于X光离化时刻. 通过实验结果,得到蓝宝石中冲击波速度为35 km/s时,其波阵面的反射率约为40%. 通过理论分析和实验数据比对的方法,验证了蓝宝石中的减速曲线. 给出了加蓝宝石窗口后的测速公式. 经过和实验对比,确认了测速公式的正确性. 关键词： 冲击波 光学诊断 成像 干涉仪     

The possibility of negative gravitational energy

The status and physical importance of the question “Can the energy of gravitational fields be negative?” is discussed. To study this question further a particular model of a gravitational shock wave has been developed. The energy corresponding to this model is a scalar functional of the two-geometry of the shock front. Values of the energy for special choices of the shock front have been calculated. These cases all give rise to positive energy, the energy becoming more positive as the shock front becomes more curved. However, no general proof is known to show that the energy is positive for all choices of two-geometries.     

Cylindrical and Spherical Ion-Acoustic Shock Waves in a Relativistic Degenerate Multi-Ion Plasma

A rigorous theoretical investigation has been made to study the existence and basic features of the ion-acoustic (IA) shock structures in an unmagnetized, collisionless multi-ion plasma system (containing degenerate electron fluids, inertial positively as well as negatively charged ions, and arbitrarily charged static heavy ions). This investigation is valid for both non-relativistic and ultra-relativistic limits. The reductive perturbation technique has been employed to derive the modified Burgers equation. The solution of this equation has been numerically examined to study the basic properties of shock structures. The basic features (speed, amplitude, width, etc.) of these electrostatic shock structures have been briefly discussed. The basic properties of the IA shock waves are found to be significantly modified by the effects of arbitrarily charged static heavy ions and the plasma particle number densities. The implications of our results in space and interstellar compact objects like white dwarfs, neutron stars, black holes, and so on have been briefly discussed.