强激光与固体靶作用产生的表面电子加速和辐射研究

利用单电子在固体靶表面准静态电磁场中运动的模型和非线性汤姆孙散射理论,研究了以大角度斜入射的强激光照射在固体靶表面产生的沿靶面方向发射的高能超热电子的运动及其产生的电磁辐射脉冲. 数值模拟表明,靶表面的电子在靶面附近的准静态电磁场和反射的激光场中作振荡. 当电子振荡频率接近激光频率时,电子被有效加速,被加速的电子主要沿靶面方向运动并产生向前的阿秒脉冲辐射. 讨论了电子在加速前的不同初始速度分布对辐射脉冲的时间和空间特性的影响,模拟了不同初始状态的多电子相干辐射脉冲的频谱特性. 关键词： 表面准静态电磁场 超热电子 阿秒脉冲 相干辐射     

相对论效应对大振幅电子等离子体振荡破裂影响的数值模拟

通过数值求解符拉索夫方程和泊松方程，研究了相对论效应和温度效应对等离子体振荡破裂的影响. 不考虑相对论效应情况下，初始扰动幅度较小时，不会发生等离子体振荡破裂，系统具有时间周期性. 此时电子温度的增加，会使得等离子体振荡最大幅度减小. 考虑相对论效应时，即使初始的等离子体电子密度扰动幅度不大，随着时间演化，相对论效应也能导致等离子体振荡破裂，而且初始电子密度扰动越小，产生等离子体振荡破裂所需时间越长. 在初始电子密度扰动较大时，无论考虑和不考虑相对论效应都会出现波破裂，但两者的结果有很大不同. 此外温度效应会降低能发生等离子体波破裂的阈值；等离子体波的相速度越大，能产生的波破裂现象也越明显. 关键词： 等离子体振荡 相对论效应 振荡破裂     

Terahertz emission in tenuous gases irradiated by ultrashort laser pulses

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed.     

等离子体在任意强度的直流电场中产生电流的过程

通过Fokker-Planck模拟,研究了等离子体在任意强度的直流电场中产生电流的过程以及电子分布函数的演变过程.研究发现,不同强度的电场中等离子体的行为存在着明显的差别.在弱电场中,电流与电场满足Spitzer公式,且电流产生的响应时间约等于撤销电场后电流衰减的弛豫时间;在中等强度的电场中,电子分布函数呈现为静止Maxwell分布和漂移Maxwell分布之和,而且在中等强度或者强直流电场中弛豫时间也将远远大于响应时间.根据电子分布函数的演变规律,推导了一组类似于流体力学方程的公式,这组方程像Spitzer公式一样简便地描述了等离子体中电流与电场的关系,并且对电场强度没有限制.数值模拟显示这组方程比Spitzer公式更适用于等离子体的混合粒子模拟中. 关键词： 等离子体电流 电子分布函数 Fokker-Planck模拟 Spitzer公式     

Terahertz emission in tenuous gases irradiated by ultrashort laser pulses

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed.     

Collisional effects on the current-filamentation instability in a dense plasma

The collisional current-filamentation instability (CFI) is studied for a nonrelativistic electron beam penetrating an infinite uniform plasma. It is analytically shown that the CFI is driven by the drift-anisotropy rather than the classical anisotropy of the beam and the background plasma. Therefore, collisional effects can either attenuate or enhance the CFI depending on the drift-anisotropy of the beam-plasma system. Numerical results are given for some typical parameters, which show that collisional effects cannot stabilize but enhance the CFI in a dense plasma. Thus, the CFI may play a dominant role in the fast electron transport and deposition relevant to the fast ignition scenario (FIS).     

Ultrafast Electron Diffraction with Spatiotemporal Resolution of Atomic Motion

Ultrafast electron diffraction （UED） is a rapidly advancing technique capable of recording the atomic-detail structural dynamics in real time. We report the establishment of the first UED system in China. Employing this UED apparatus, both the coherent and the concurrent thermal lattice motions in an aluminium thin-film, trigged by ultrafast laser heating, have been observed. These results demonstrate its ability to directly measure a sub-milli-angstrom lattice spacing change on a sub-picosecond time scale.     

Electron Dynamics and Characteristics of Attosecond Electromagnetic Emissions in Relativistic Laser-Plasma Interactions

Generation of attosecond electromagnetic(EM) pulses and the associated electron dynamics are studied using particle-in-cell simulations of relativistic laser pulses interacting with over-dense plasma foil targets. The interaction process is found to be so complicated even in the situation of utilizing driving laser pulses of only one cycle. Two electron bunches closely involved in the laser-driven wavebreaking process contribute to attosecond EM pulses through the coherent synchrotron emission process whose spectra are found to follow an exponential decay rule. Detailed investigations of electron dynamics indicate that the early part of the reflected EM emission is the high-harmonics produced through the relativistic oscillating mirror mechanism. High harmonics are also found to be generated through the Bremsstrahlung radiation by one electron bunch that participates in the wavebreaking process and decelerates when it experiences the local wavebreaking-generated high electrostatic field in the moving direction.     

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.     

面向激光等离子体尾波加速的毛细管放电实验研究

具有合适径向密度分布的等离子体通道可以用于超短超强激光导引,这使得等离子体通道在激光尾波加速中有着重要的应用.本文介绍了在上海交通大学激光等离子体实验室开展的毛细管放电和光导引实验.通过光谱展宽法测量了充氦气的放电毛细管中的等离子体密度分布,在长度为3 cm、内径为300μm的毛细管中实现了轴向均匀,径向呈抛物线型的等离子体密度分布.通过改变放电延时和喷气时长,确定和优化了产生等离子体通道的参数区间,得到的最大通道深度为28μm,与实验中使用的激光焦斑半径匹配.在此基础之上,开展了不同能量的激光脉冲在放电等离子体通道中的导引研究,结果发现当通道深度与焦斑半径匹配时,激光可以不散焦地在通道中传输,实现激光导引.这项研究为未来的激光尾波级联加速和锁相加速等研究奠定了基础.