Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows,which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic(CH) foils with 1ns-pulsed laser beams of total energy of 1.7 k J in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.     

Studies of collisionless shockwaves using high-power laser pulses in laboratories

The remarkable experimental progress in the studies of collisionless shockwave(CS)in laboratories employing highpower lasers is briefly reviewed.The results show that CS can be generated in laser-produced plasmas due to the microturbulence associated with instabilities.CS is one of the most important astronomical phenomena.It has been found in supernova remnants(SNRs),Sun–Earth space,etc.This paper focuses on CS in ways relevant to SNRs.Laboratory astrophysics(LA),a new interdisciplinary frontier of astrophysics,plasma and laser physics,has developed rapidly in recent years.As an accessory to the astronomical observation,LA experimenters can closely study some astronomical events scaled-down to controllable phenomena.     

对模拟年轻恒星喷流具有潜在应用的由强激光产生的等离子体喷流（英）

利用神光Ⅱ激光装置的两束激光烧蚀半圆柱壳层靶产生了高速等离子体喷流。喷流的参数由光学和X射线诊断测量。喷流是准直的,在真空中传播。一维流体力学模拟被用来间接地计算喷流的速度。喷流的准直可能来源于高Z等离子体的辐射冷却。由于和年轻恒星喷流具有某些几何相似性,实验室喷流对于在实验室中模拟年轻恒星喷流具有潜在应用。     

高反射效率高定向性的热解石墨晶体X射线谱仪

基于高定向热解石墨晶体(highly oriented pyrolitic graphite,HOPG)研制了一种新型反射式X射线谱仪.该谱仪具有高反射效率、较高能谱分辨率及相对较宽的能谱测量范围.根据计算,在相同的入射条件下,该谱仪的效率比一般X射线弯晶谱仪高3个量级;谱仪能谱分辨率理论值最高达350;理论探测范围是6.891keV至9.193 keV.我们将该谱仪应用在高功率密度激光与固体靶相互作用的实验中,发现在普通弯晶谱仪无法采到信号的实验条件下,HOPG谱仪依然采集到清晰的Cu K谱线.分析发现在8.048 keV(Cu的Kα光子能量)附近的能谱分辨能力最高达到40 eV,分辨率大于200.     

Bow shocks formed by a high-speed laser-driven plasma cloud interacting with a cylinder obstacle

A bow shock is formed in the interaction of a high-speed laser-driven plasma cloud with a cylinder obstacle. Its temporal and spatial structures are observed by shadowgraphy and interferometry. The width of the shock transition region is ～ 50 μm, comparable to the ion–ion collision mean free path, which indicates that collision is dominated in the shock probably. The Mach-number of the ablating plasma cloud is ～ 15 at first, and decreases with time resulting in a changing shock structure. A two-dimension hydrodynamics code, USim, is used to simulate the interaction process. The simulated shocks can well reproduce the observed.     

Application of multi-pulse optical imaging to measure evolution of laser-produced counter-streaming flows

A counter-streaming flow system is a test-bed to investigate the astrophysical collisionless shock(CS) formation in the laboratory. Electrostatic/electromagnetic instabilities, competitively growing in the system and exciting the CS formation, are sensitive to the flows parameters. One of the most important parameters is the velocity, determining what kind of instability contributes to the shock formation. Here we successfully measure the evolution of the counter-streaming flows within one shot using a multi-pulses imaging diagnostic technique. With the technique, the average velocity of the high-density-part(ne ≥ 8–9 × 10~(19)cm~(-3)) of the flow is directly measured to be of ～ 10~6cm/s between 7 ns and 17 ns.Meanwhile, the average velocity of the low-density-part(ne ≤ 2 × 10~(19)cm~(-3)) can be estimated as ～ 10~7cm/s. The experimental results show that a collisionless shock is formed during the low-density-part of the flow interacting with each other.     

强激光产生的强磁场及其对弓激波的影响

强激光照射金属线圈后,会在打靶点附近的背景等离子体中诱发冷电子的回流,在金属丝内形成强电流源,从而产生强磁场.本文利用神光II高功率激光器产生的强激光照射金属丝靶,产生了围绕金属丝的环形强磁场.利用B-dot对局域磁感应强度进行了测量,根据测量结果,结合三维模拟程序,反演得到磁场的空间分布.再利用强激光与CH平面靶相互作用产生的超音速等离子体撞击该金属丝,产生了弓激波.通过光学成像手段研究了磁场对冲击波的影响,发现磁场使得弓激波的轮廓变得不明显并且张角变大.同时,通过实验室天体物理定标率,将金属丝表面等离子参数变换到相应的天体参数中,结果证明利用该实验方法可以在实验室中产生类似太阳风的磁化等离子体.     

纳秒激光等离子体相互作用过程中激光强度对微波辐射影响的研究

在强激光与等离子体的相互作用中,通常能够产生时间尺度长达百纳秒量级的微波辐射,形成的复杂电磁环境会干扰或损坏机械电子设备,并给物理过程的准确认识与表征带来困难.然而,目前对于微波辐射的产生机制的研究还不够系统和完善.本文通过系统地改变纳秒激光与等离子体作用过程中入射的激光能量以改变入射激光强度,发现微波辐射强度随激光强度非单调变化.在较低的激光强度下,辐射强度随激光强度的增加先增加后减小,辐射场时间波形呈现连续振荡的特征,辐射频谱包含低于和高于0.3 GHz两部分分量;在较高的激光强度下,辐射强度随激光强度的增加而增加,辐射场时间波形表现为数十纳秒的单极性辐射,辐射频谱主要包括0.3 GHz以下的分量.分析表明,导致微波波形和频谱差别的原因是辐射机制发生了变化.在较低的激光强度下,微波辐射由偶极辐射和靶上电子束向真空出射共同作用产生,其中偶极辐射占主导;在较高的激光强度下,微波辐射主要由靶上电子束向真空出射产生.研究结果对于理解纳秒激光与等离子体相互作用过程中的微波辐射机制具有比较重要的意义,同时也为借助微波辐射诊断激光与等离子体相互作用过程中的逃逸电子、靶面鞘层场等问题提供了参考.     

激光驱动磁重联过程中的喷流演化和电子能谱测量

利用神光Ⅱ激光器和日本大阪大学Gekko激光器构建了激光驱动等离子体磁重联过程. 在垂直于磁重联平面方向发现了高速喷流, 从不同观测方向实验证实了该喷流的存在并测量了喷流的流体力学演化过程, 对其中的电子能谱进行了诊断分析.     

常温下氙气以及氢氙混合气体形成的团簇的特性研究

利用瑞利散射法可以对团簇的尺寸以及团簇形成的演变过程进行研究, 这种方法非常简单易行且对团簇是非破坏性的. 通过对纯氙气以及氢氙混合气体形成的团簇的瑞利散射进行测量, 研究并分析了瑞利散射强度随时间、初始背压 以及气体混合比例的变化, 由此估算了在不同情况下形成团簇的平均尺寸. 通过获得的氢氙混合气体瑞利散射强度与背压的关系I=(1.5 ×10^-5)P^6.47, 发现了混合气体中氢气对氙团簇形成的促进作用, 并从热力学和分子间作用力的角度进行了理论分析, 得到了氢氙混合气体不易于液化这一新现象, 为实现高背压下更大尺寸团簇的产生提供了可能. 所获得的结果为今后基于氙团簇的X射线产生实验, 以及基于氘氙混合气体的中子产生实验研究提供了良好的实验依据.