【作者】 胡强林；

【导师】 刘世炳；

【作者基本信息】 北京工业大学 ， 光学， 2007， 博士

【摘要】 高能量超强超短脉冲激光的出现,迅速地推动了惯性约束核聚变(ICF)研究的发展。ICF“快点火”概念的提出,使超强超短脉冲激光与等离子体的相互作用研究成为世界各强国极度关注的焦点。在以军事武器装备、国防安全为目的的巨资投入、探索创新研究的带动下,形成了学术界强场物理、强场化学、极端条件产生等当今国际上相对独立的三个重大研究前沿。近二十年的研究进展速度惊人,并开始步入应用。物质在强激光场作用下发生电离是上述三大前沿领域在研究中必遇的共性现象,通常将激光电离产生的等离子体视为完全离化的带电粒子,相互作用体系所有的线性与非线性物理过程本质上是由激光电磁场和等离子体静电场支配。但在实际的相互作用中,尤其是在高Z物质靶产生的等离子体中,靶原子却难以完全电离。部分离化的原子(离子)在激光场作用下必然发生极化,从而产生极化场。原子极化场(束缚电子相关)及其与激光电磁场、等离子体静电场(自由电子相关)在相互耦合中的竞争如何修改相互作用研究的已有结果以及将导致什么新的物理现象,是人们关注的热点。另外,等离子体是一种无损伤阈值的特殊物质形态,带电粒子复杂的运动会导致各种形式的电磁辐射,通过超短强激光脉冲对等离子体的激励作用来产生人们极力寻找的高功率电磁辐射源,尤其是高功率太赫兹(THz)辐射源,成为人们关注的焦点。本学位论文正是针对上述两个方面开展理论研究:1.推导出强激光脉冲在部分离化等离子体中传播时的群速度和相速度,在相互作用过程中光子数守恒的假设下,研究了部分剥离原子极化对激光脉冲的自聚焦、光子加速、纵向压缩、以及调制不稳定性等物理现象的非线性作用。结果表明,在部分离化等离子体中,激光脉冲的自聚焦和光子加速比完全离化等离子体中要快得多,同时,原子极化场的存在,会在部分离化等离子体中形成正常色散和反常色散的竞争,从而导致通常的脉冲纵向压缩和调制不稳定性现象发生大的变化;2.从激光在等离子体中传播的色散关系出发,针对原子或部分剥离原子(离子)在强激光场作用下的电离问题,讨论了电离过程对激光频率的调制作用。我们发现,气体原子在激光场中的电离快慢,决定介电函数的响应速率,从而也决定了电离对激光脉冲频率产生调制的快慢,在部分离化等离子体中,激光频率越高,部分离化原子的极化对激光频率蓝移的贡献也越大;3.从麦克斯韦方程组和流体方程出发,采用微扰理论研究了强激光脉冲与部分离化等离子体相互作用产生的准静态磁场,讨论了部分剥离原子极化场对准静态自生磁场的修正。结果显示,在高密度部分离化等离子体中,当激光频率较高时,原子极化场的存在会使回旋自生磁场大大增强,尤其重要的是会导致磁场方向的改变,这将引起超热电子输运的散束现象,对超热电子“快点火”极为不利;4.根据达朗伯方程,探索了超短强激光脉冲在等离子体中诱导THz辐射的可能性,找出了THz辐射产生的条件。研究表明,通过超短激光脉冲驱动的等离子体电流,在激光脉冲群速度大于辐射波相速度的条件下可以产生强THz电磁辐射,辐射频率约为激光脉宽的倒数量级;5.从强激光脉冲作用下等离子体的介电函数出发,通过对激光产生快电子的Cherenkov辐射的研究,发现了THz波发射的另一种产生途径。结果证明,在快电子速度略小于激光脉冲群速度时,能够产生Cherenkov型强THz电磁辐射。本学位论文的主要创新为:1.揭示了部分剥离原子的极化场对激光-等离子体中准静态自生磁场的重要修正作用,尤其是极化场会改变自生磁场方向的结论属首次报道;2.揭示了部分剥离原子的极化在相互作用中对激光脉冲的参量调制规律;3.从理论上证明了强激光脉冲在等离子体中能够诱导高功率THz电磁辐射,并给出了THz辐射产生的条件。更多还原

【Abstract】 The advent of high-energy ultra-short and -intense laser pulse has greatly promoted the progress in the research of inertial confinement fusion (ICF). The proposal of the“fast ignition”concept in ICF has the investigation of ultra-short and -intense laser pulse interaction with plasmas becomes a focus, for which the world power has invested substantive fund. The development fleetly in the field for the purpose of military equipment and national security has boosted the research of the elements science greatly and formed three span-new domains, intense field physics, intense field chemistry, and extreme condition creation. The fast progress for twenty years allows us coming into turn ours steps to application in the above three domains. In the interaction, the atoms in matter are inevitably ionized in such an intense laser field and the plasma is instantly formed on the matter surface. In theoretical, the plasma produced by the intense laser pulse is generally treated as fully ionized charged particle, in which interaction system the linear and nonlinear physic processes are essentially dominated by the laser electromagnetic field and the plasma electrostatic field. In the practical applications, however, the target-atom especially the high-Z atoms usually cannot be fully stripped by the pulse laser field. The partially stripped atoms (ions) have to be polarized and consequentially to form a polarization field under the intense laser field. The coupling of the polarization field associated with bound electrons, plasma electrostatic field associated with free electrons, and electromagnetic field associated with laser pulse how to modify the well-known physic results and what new phenomenons can be induced in the competition of the interaction have been attracting researchers’attention. On the other hand, plasma, as a special matter state without damage threshold, is a preferred medium for the generation of high power electromagnetic radiation by the excitation of the intense ultrashort laser pulse as well as the complex diversiform movements of the charged particles, which is one of the aims researchers seeked, especially for the terahertz (THz) source with high-power. This thesis just focused on the above interests and procured the progresses as follows:1. The group and phase velocities of an intense laser pulse propagating in the partially stripped plasma are derived. In the assumption of the photon-number conservation, the photon acceleration, self-focusing, longitudinal bunching, and the modulation instability of the laser pulse are investigated in the presence of the partially stripped atom polarization. The results show that, the self-focusing and photon acceleration of the laser pulse in the partially stripped plasma are much faster than in the fully stripped plasma, the existence of the atom polarization field results in the competition between normal and abnormal dispersions as well as the significant change in the longitudinal bunching and modulation instability of the laser pulse in the partially stripped plasma.2. Based on the dispersion relation of intense laser pulse propagating in the partially stripped plasma, the laser frequency modulation generated by the atomic ionizing process is investigated. We find that the ionizing speed of gas atom in the laser field determines the responding speed of the dielectric function as well as the modulating speed of the pulse frequency, in which the result shows the higher the laser frequency, the greater the blue-shift of the laser frequency due to the partially stripped atomic polarization.3. Starting from the Maxwell equations and hydrodynamic equations, the quasistatic magnetic field resulted from an intense laser pulse propagating through the partially stripped plasma is examined and the modification induced by the atomic polarization field to the self-generated magnetic field is discussed based on the perturbation theory. The results reveal that, for the higher plasma densities and higher laser frequencies, the atomic polarization field can greatly enhance the self-generated magnetic field especially can reverse its direction which can inevitably induce the un-bunch of the hot electron beam as well as be of disadvantage to the fast ignition of hot electron in the ICF.4. According to the d’Alembert equation, the possibility of THz radiation excited by an ultrashort laser pulse driven the plasma current is proved and the generation condition of the THz radiation is found in theory. The result indicates that the higher power THz radiation can occur when the group velocity of the laser pulse is greater than the phase velocity of the radiated wave and the frequency of the radiated wave approximates the reciprocal of the pulse duration.5. Based on the dielectric function of plasma in an intense laser pulse, we foretell another generation mechanism of THz radiation in theory, which results from the Cherenkov effect induced by the laser generated fast electron. The result shows that the stronger Cherenkov type THz radiation can emitted if the velocity of the fast electron slightly less than the group velocity of the laser pulse.The innovative progress in this thesis shows as follows:1. The significant modification of the partially stripped atomic polarization field to the self-generated quasistatic magnetic field is revealed and the direction of the magnetic field can be changed by the atomic polarization field, which is firstly exposed.2. The contribution of the atomic polarization field to modulate the parameters of laser pulse is uncovered.3. High power THz electromagnetic radiation by intense laser pulse interaction with plasma is predicted in theory and the condition that THz radiation occurs is presented.更多还原