飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射。实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子。在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生。     

飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射.实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子.在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生.     

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为了研究靶材料对快电子能量分布的影响,采用电子谱仪测量了飞秒激光与Cu和CH靶相互作用中在靶前和靶后产生的快电子能谱。结果显示,在靶前Cu和CH靶的快电子能谱相似,反应了快电子发射对靶材料的依赖性较弱;在靶后Cu和CH靶的快电子能谱具有明显的差异,说明电子的输运过程与靶材料密切相关。冷电子环流以及自生磁场是导致Cu靶快电子能谱"软化"的原因,而对于CH靶麦克斯韦分布的快电子能谱主要由碰撞机制决定。     

激光等离子体相互作用中Weibel不稳定性

利用粒子模拟法对超强激光与等离子体相互作用中产生的Weibel不稳定性及其产生机制进行了详细的研究。给出不稳定性的线性色散关系和饱和磁场与各向异性参数之间的函数关系,发现Weibel不稳定性的存在使超强激光在等离子体中激发的自生磁场饱和,饱和自生磁场的存在使粒子速度分布在激光传播方向上表现出各向异性。讨论了Weibel不稳定性的线性和非线性饱和过程,对更好地理解快点火物理中自生磁场的产生、快电子输运等过程有重要意义。     

超强激光脉冲下等离子体块的整体加速研究

为了克服激光加速中强流离子束空间电荷效应对粒子输运的影响,提出一种利用两块不同密度的固体靶先后和一束强度约为1022 W/cm2、脉冲长度为5T(T为激光周期)的超强脉冲激光相互作用的方案,实现了中性等离子体块的加速。通过一维PIC数值模拟研究发现,在合适的参数下,加速后的电子与质子几乎以相同的速度共同飞行长达60λ(λ为激光波长)的距离,其中质子与电子的能量分别为GeV和100MeV量级。     

利用渡越辐射研究超热电子输运

自快点火概念提出以来，与快电火概念相关的物理研究已成为激光惯性约束聚变研究的前沿热点问题。对于惯性约束聚快点火方案，超热电子的产生和输运及能量沉积研究是目前最受关注的课题之一。因为超短超强激光产生的超热电子电流很大（有时远远超过Alfven极限），持续时间短，涉及高密度，在输运中伴随着强烈的电场和磁场。再加之Wibel不稳定性的成丝效应，都将严重影响超热电子的输运和能量沉积效率，使得快点火涉及到的超热电子输运和能量沉积的物理过程变得异常复杂。超热电子产生、输运涉及到固体密度等离子，因此高密度区的输运过程实验探测就显得非常重要。     

无碰撞等离子体中电子束流不稳定性的时空演化研究

应用二、三维相对论电磁粒子模拟程序研究双电子束流在无碰撞等离子体中传播引起的横向 电磁(Weibel类型)不稳定性和纵向静电不稳定性的发展演化过程.讨论了纯粹Weibel不稳定 性的发生和非线性饱和过程，观察到电流束合并、磁场重联等引起的电子横向加热现象.研 究了电流束传播方向激发的静电场对快电子束传播的影响，观察到其导致的束的横向调制、 磁场通道破坏现象.对这些过程的细致研究对更好的理解快点火物理中自生磁场的产生、快 电子输运等过程有重要意义.     

超强激光与等离子体作用中的能量输运

应用多光子非线性Compton散射模型和数值计算方法,研究了Compton散射对超强激光与等离子体作用中能量输运的影响,提出了将Compton散射光和入射超强光作为电子能量输运的新机制,给出了电子热传导新模型和能量输运数值计算结果。结果表明:散射使等离子体中Weibel不稳定性和自生磁场增强效应导致耦合光传输方向的电子密度显著减小,更多激光能量以热流形式分布在横向方向。散射使电子吸收能量的时间缩短和自生磁场线性阶段最大增长率增大效应导致等离子体表面处沿耦合激光横向方向的热流几乎被完全限制,电子在激光传输方向的能量显著增加。     

次稠密等离子体对激光与锥形靶相互作用的影响

利用三维粒子模拟程序模拟了强激光在锥形靶内的传播情况.发现锥内次稠密等离子体的存在使激光在锥顶部的最大聚焦强度有所降低，产生的相对论电子的最大能量和数目增加.激光在锥壁激发起强的电流和磁场，次稠密的存在还使锥内产生强的准静态磁场，磁场的存在使相对论电子速度分布在垂直激光传播方向上表现出各向同性. 关键词： 超强激光脉冲 锥形靶 快点火 粒子模拟     

快点火中相对论电子束能量沉积的动理学研究

针对相对论快电子束在高密度压缩芯区等离子体中的能量沉积过程开展物理建模、程序研制和数值模拟研究。从等离子体粒子碰撞的基本物理出发,综合考虑了高能电子与背景等离子体之间的短程两体碰撞过程和长程集体效应,建立了相对论Fokker-Planck动理学模型,通过采用球谐展开的方法,推导得到了适于数值求解的方程形式并根据方程特点开展相应的数值算法研究及程序研制并完成了物理考核,对快点火能量沉积的典型物理算例进行了模拟研究,并针对即将在神光Ⅱ升级装置上开展的快点火物理实验进行了初步的物理分析。     

Cascade cross modulation due to the nonlinear interaction of elastic waves in samples with cracks

The phenomenon under study consists in that, in an inhomogeneous material with nonlinearity caused by the presence of soft defects (the so-called “nonclassical” nonlinearity), cascade nonlinear effects are fairly strong and may even become comparable to the first-order effects. Similar cascade effects in media with a common nonlinearity of the crystal lattice are much weaker. This difference can be used as an important diagnostic indicator in nondestructive testing. Experimental data obtained for samples with cracks, which exhibit both ordinary modulation and cross-modulation effects, as well as a cascade cross modulation, are presented. The origin of the enhanced level of cascade effects is explained by modeling with the use of a simple model of nonlinearity of an inhomogeneous material containing soft Hertzian contacts.     

Effets produits par l'irradiation avec des neutrons et avec des rayons gammas sur la série jaune et la série verte du spectre d'absorption excitonique de la cuprite

The effects of irradiations in a reactor and in a Cobalt 60 source on the green and yellow series of the excitonic absorption spectrum of cuprous oxide are studied. In the reactor irradiation experiments, the effects of fast neutrons are separately studied from those of thermal neutrons and of gamma rays. It is shown that the perturbations of the absorption spectra of Cu₂O by fast neutrons are very similar to those produced by an external electric field. They are also similar to those produced by quenching which creates probably internal strains. Charged defects occur probably in the crystal during bombardment and these charged defects produce an internal electric field and strains. It is not possible to study separately the effects of the electric field and the effects of strains. The electric field can be evaluated by comparison of its effects on the excitonic spectrum with those of an external field. An evaluation of the rate of creation of defects by fast neutrons in the crystal is deduced. It is also shown that the effects of gamma-rays are very different from those of fast neutrons. It is possible with gamma-ray irradiations to decrease residual internal fields which may exist in a crystal.     

Physics of non-adiabatic transport and field-domain effect in quantum-well infrared photodetectors

A previous theory for studying the distribution of non-uniform fields in multiple-quantum-well photodetectors under an ac voltage is generalized by including non-adiabatic space-charge-field effects. Numerical calculations indicate that field-domain effects are only important at high temperatures or high voltages when both injection and sequential-tunneling currents are significant. On the other hand, it is found that the non-adiabatic effects included in this generalized theory become significant at low temperatures and low voltages when field-domain effects are negligible. In order to explain the non-adiabatic charge-density fluctuations quantum-statistically, a non-adiabatic differential equation is derived based on the self-consistent Hartree model by using a shifted Fermi–Dirac model for the local fluctuation of electron distributions. The non-adiabatic effect is found to cause an “equilibrium” state variation with time under an ac voltage.     

Aerodynamic sound generation caused by viscous processes

With the aim of resolving some questions arising from previous studies by Morfey, Kempton, Hardin, and Kambe and Minota, the effects of viscosity on aerodynamic sound generation by unheated, low Mach number flow are investigated theoretically. First, an analytical solution in terms of a multipole expansion is derived by means of the method of matched asymptotic expansions. This solution is especially well suited to compare the results obtained by the various authors mentioned above. Second, an estimation of the importance of viscous effects on aerodynamic sound generation is given by which one can show that viscous effects have been overestimated in the past.     

Electron-phonon interaction in boron-doped silicon nanocrystals: Effect of Fano interference on the Raman spectrum

Raman spectroscopy is employed for studying silicon nanocrystal arrays in boron-doped amorphous silicon films. The nanocrystals were formed in the initial amorphous films by the pulsed impact of an excimer laser. The electron-phonon interaction effects are observed experimentally in the heterostructure formed by a silicon nanocrystal and an amorphous matrix. These effects can be described in the framework of the familiar Fano interference model.     

Isospin effects of the Skyrme potential and the momentum dependent interaction at intermediate energy heavy ion collisions

We improve the isospin dependent quantum molecular dynamical model by including isospin effects in the Skyrme potential and the momentum dependent interaction to obtain an isospin dependent Skyrme potential and an isospin dependent momentum interaction. We investigate the isospin effects of Skyrme potential and momentum dependent interaction on the isospin fractionation ratio and the dynamical mechanism in intermediate energy heavy ion collisions. It is found that the isospin dependent Skyrme potential and the isospin dependent momentum interaction produce some important isospin effects in the isospin fractionation ratio.     

Isospin effects of the Skyrme potential and the momentum dependent interaction at intermediate energy heavy ion collisions

We improve the isospin dependent quantum molecular dynamical model by including isospin effects in the Skyrme potential and the momentum dependent interaction to obtain an isospin dependent Skyrme potential and an isospin dependent momentum interaction. We investigate the isospin effects of Skyrme potential and momentum dependent interaction on the isospin fractionation ratio and the dynamical mechanism in intermediate energy heavy ion collisions. It is found that the isospin dependent Skyrme potential and the isospin dependent momentum interaction produce some important isospin effects in the isospin fractionation ratio.     

Rayleigh-type nondegenerate four-wave mixing: ultrafast measurement and field correlation

We report on an ultrafast longitudinal time measurement by a nonresonant Rayleigh-type nondegenerate four-wave mixing (NFWM). We investigated the field-correlation effects on Rayleigh-type NFWM by examining the time-delayed dependence of the NFWM spectra. Based on the field-correlation effects, a time-delayed method is proposed to suppress the thermal effect, and ultrafast relaxation time can be measured even in an absorbing medium.     

Excitonic effects and optical spectra of single-walled carbon nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an ab initio many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.