Characteristic investigation of ablative laser propulsion driven by nanosecond laser pulses

The momentum transfer and the specific impulse of the ablative laser propulsion of nanosecond laser irradiation on copper, lead, aluminum and graphite targets are investigated. The effects of the ambient pressure and laser focal spot sizes on the target momentum are measured. The results show that the target momentum strongly relates to the ambient pressure and target property. The highest target momentum about 2.28 g·cm/s is obtained on lead targets under 1 atmospheric pressure. With the increase of the focal spot sizes, the specific impulse decreases. The highest specific impulse in vacuum is about 950 s on copper targets. PACS 52.75.Di; 52.38.Mf; 52.50.Jm     

激光等离子体动量转换效率的实验研究

强激光与固体靶相互作用时，产生的高速喷射的等离子体对靶具有强烈的反冲作用，因此，激光等离子体可以作为一种新型的推进动力源.与传统的化学燃料推动相比，激光等离子体具有较高的比冲和有效载荷比等特点.对纳秒激光脉冲与铝、石墨、铅和碳氢靶相互作用时，等离子体对靶的冲量进行了实验测量，研究了大气与真空环境下的靶动量与激光聚焦面积的关系，并对部分实验结果与理论计算的数值进行了比较.实验结果显示，大气与真空环境下的靶动量有很大的差异，并且真空下的靶动量受材料性质的影响较大，与以往长脉冲激光的实验结果有很大的不同. 关键词： 激光等离子体 动量 动量耦合系数     

激光辐照固体靶产生等离子体反冲研究

提出一种通过诊断等离子体反冲动量来计算激光加载产生冲击压强的方法. 当强激光辐照固体靶表面时, 所产生的高速喷射的等离子体对靶具有反冲作用, 通过诊断等离子体反冲动量的变化可以计算激光辐照固体靶产生的冲击压强变化. 本文利用辐射流体力学软件研究了这种诊断方法, 模拟采用的激光功率密度为5×10¹²-5×10¹³ W/cm², 激光脉宽选取纳秒量级. 模拟结果表明该方法是有效且可行的.     

Formation of linear momentum in a rod during a laser pulse–matter interaction

Recently, we developed an optodynamic experimental technique that makes it possible to measure the linear momentum obtained by a metal target sample in the shape of a rod during a nanosecond laser pulse interaction in the ablative regime. The height of the rod’s rear end axial step-like displacement, caused by the first reflection of the laser-generated ultrasonic wave, is proportional to the linear momentum acquired by the rod. In comparison with commonly used ballistic methods, we can determine the acquired momentum on a much shorter time scale corresponding to the wave transition time, from the front to the rear end of the rod. Using this method we investigated the ambient air pressure dependence on the formation of linear momentum over a laser intensity range, from the ablation threshold to values about ten times higher. Steel rods of various diameters were used to demonstrate the effect of an expanding blast wave which delivers additional momentum to the target, when the laser beam on the target surface is smaller than the target itself. The typical value of the acquired target momentum is on the order of μN s and 10 μN s/J for the momentum coupling coefficient.     

甘油的黏度对激光等离子体推进的影响

以液体甘油为烧蚀靶材,研究了不同环境温度下超短脉冲激光烧蚀后产生的等离子体的推进特性.实验结果表明:甘油产生的动量和烧蚀压力与环境温度密切相关,其原因是甘油的黏度受温度影响较大.     

Contactless Transfer of an Angular Momentum to a Liquid Layer Using a Scanning Laser Beam

The contactless transfer of an angular momentum to a liquid layer using a scanning laser beam has been experimentally investigated. This effect is observed for a glassy carbon target placed in water and irradiated by a pulsed laser beam, scanning the target along a closed trajectory. The target rotates in the same direction as the laser beam if the water layer is thin and in the opposite direction in the case of thick water layer. The possibility of transferring an angular momentum to liquid in the absence of target is demonstrated. The effect observed is interpreted as the result of joint action of convective flows and thermocapillary convection, which are induced in the liquid by laser heating.     

铝靶三倍频激光烧蚀参数实验研究

叙述了采用时间空间积分晶体谱仪和时间分辨晶体谱仪等探测器测量铝平面靶强激光烧蚀参数的方法，给出了三倍频强激光烧蚀铝平面靶的质量烧蚀速率和烧蚀压。实验结果与收集到的国外数据进行了比较，它们在误差范围内一致。     

Experimental investigation of the dynamics of laser-induced gas-plasma flows under femtosecond laser ablation of copper in vacuum

Thermophysical and gas-dynamic characteristics of gas-plasma flows induced by ultrashort laser pulses interacting with a thin-film copper target in vacuum were studied experimentally. Using combined laser interferometry and complex processing of experimental data, we estimated the momentum coupling coefficient and the efficiency of laser-energy conversion to kinetic energy, spatiotemporal distributions of the number density and velocities of particles, pressure, and temperature in the gas-plasma flow. We provide comparative analysis of presented data with those found in the literature, which were obtained by other methods.     

双束探测光测量激光等离子体动量

提出了一种对激光等离子产生的靶速度进行测量的新方法。该方法除了对烧蚀靶的速度进行直接测量外,而且测量结果不受靶宽度及探测光束的影响。与理论值相比,该方法的实验测量误差小于2%。此外,利用该测量方法对激光烧蚀不同温度下液体水的动量进行了测量,结果发现随水温度的降低,产生的动量呈现增加趋势。     

Electron acceleration by a short relativistic laser pulse at the front of solid targets

Acceleration of electrons in a low-density plasma in front of a solid target by a propagating short ultraintense laser pulse is studied. When the laser is reflected at the target surface the accelerated electrons, with energy scaling as the laser intensity, continue to move forward inertially and thus escape from the pulse. Electrons accelerated backwards by the reflected light can attain even higher energies due to their longer acceleration length and their high initial momentum from a relativistic return current.     

Fraunhofer diffraction of atomic matter waves: electron transfer studies with a laser cooled target

We have constructed an apparatus combining the experimental techniques of cold target recoil ion momentum spectroscopy and a laser cooled target. We measure angle differential cross sections in Li(+)+Na-->Li+Na(+) electron transfer collisions in the keV energy regime with a momentum resolution of 0.12 a.u. yielding an order of magnitude better angular resolution than previous measurements. We resolve Fraunhofer-type diffraction patterns in the differential cross sections. Good agreement with predictions of the semiclassical impact parameter method is obtained.     

HeH+体系光缔合反应的动力学研究

采用含时量子波包方法研究基电子态HeH⁺体系优化的光缔合几率,即He+H⁺→HeH⁺随碰撞粒子对初始动量的变化,并研究伴随该过程的多光子跃迁和解离.选取v=6振动能级为目标态,通过优化激光场参数得到碰撞粒子对的最优光缔合几率.结果表明：随着碰撞粒子对初始动量的增加,光缔合所用的最优激光场的持续时间逐渐减少,但光场的强度和失谐逐渐增加.在某些初始碰撞动量附近,由于共振多光子跃迁过程的存在,最优的光场不能严格地将碰撞粒子对缔合到目标态.此外,研究表明随着初始动量的增加,系统的超阈值解离几率近似线性地增加到一个最大值,直至饱和强度.并且,在碰撞粒子对的初始动量较大时,超阈值解离主导解离过程.     

Computer simulation of expansion of a carbon laser plasma after ablation in nitrogen atmosphere

Computer simulation is employed for analyzing the dynamics of plasma expansion in nitrogen under a pressure of 103 and 104 Pa after the action of a nanosecond laser pulse on a graphite plate for obtaining carbonitride nanofilms with the help of laser ablation technology. The binary gas mixture is described using a macroscopic quasi-gasdynamic model taking into account momentum and energy transfer between mixture components. The results of simulation are compared for various initial values of temperature and pressure of the laser jet with the results of direct Monte Carlo simulation and with experimental data. The possibility of controlling the shock wave propagation modes after ablation by varying the initial values of gasdynamic parameters is demonstrated. The effect of nitrogen atmosphere on the propagation of the laser jet is studied.     

激光等离子体推进中靶动量测量方法的改进

总结了激光等离子体推进中靶动量的测量方法,设计了双光束测量方法.该方法能够减小靶宽度和靶的运动速度对测量结果的影响.     

Laser-assisted positron-impact ionization of atomic hydrogen

We study the ionization of atomic hydrogen by a fast positron in the presence of an external linearly polarized laser field. We concentrate on the limit of a small momentum transfer and describe the fast positron's continuum states by Volkov wave functions. The ejected electron is described by a Coulomb-Volkov wave function. We are limited to small laser intensities such that the dressed state of the target is treatable within the time-dependent perturbation theory, even though the laser intensity is still quite high by laboratory standards. Numerical results for the triply differential cross sections and their dependencies on laser-field parameters are discussed and compared with the results of laser-assisted ionization by electron impact.     

Etude theorique d'un mesureur absolu des flux laser continus base sur les echanges d'impulsion

We present in this paper a theoretical study of an absolute cw laser power meter based on the mechanical effects of the light (momentum transfer from a light beam to a target). We show that an appropriate geometry of the device leads to a result which ignores the target and beam characteristics and depends only on the incident flux value.     

Modeling photothermal and acoustical induced microbubble generation and growth

Previous experimental studies showed that powerful heating of nanoparticles by a laser pulse using energy density greater than 100 mJ/cm², could induce vaporization and generate microbubbles. When ultrasound is introduced at the same time as the laser pulse, much less laser power is required. For therapeutic applications, generation of microbubbles on demand at target locations, e.g. cells or bacteria can be used to induce hyperthermia or to facilitate drug delivery. The objective of this work is to develop a method capable of predicting photothermal and acoustic parameters in terms of laser power and acoustic pressure amplitude that are needed to produce stable microbubbles; and investigate the influence of bubble coalescence on the thresholds when the microbubbles are generated around nanoparticles that appear in clusters.

We develop and solve here a combined problem of momentum, heat and mass transfer which is associated with generation and growth of a microbubble, filled with a mixture of non-vaporized gas (air) and water vapor. The microbubble’s size and gas content vary as a result of three mechanisms: gas expansion or compression, evaporation or condensation on the bubble boundary, and diffusion of dissolved air in the surrounding water. The simulations predict that when ultrasound is applied relatively low threshold values of laser and ultrasound power are required to obtain a stable microbubble from a single nanoparticle. Even lower power is required when microbubbles are formed by coalescence around a cluster of 10 nanoparticles. Laser pulse energy density of 21 mJ/cm² is predicted for instance together with acoustic pressure of 0.1 MPa for a cluster of 10 or 62 mJ/cm² for a single nanoparticle. Those values are well within the safety limits, and as such are most appealing for targeted therapeutic purposes.     

Dense laser-driven electron sheets as relativistic mirrors for coherent production of brilliant X-ray and γ-ray beams

Several techniques exist to obtain brilliant X-ray beams by coherent reflection from relativistic electrons (E _e=γ mc ²) with Doppler frequency upshift of 4γ ². We describe a new approach starting with an ultra-thin solid target. Larger ‘driver’-laser intensities with high contrast are required to produce dense electron sheets. Their acceleration in vacuum results in a transverse momentum component besides the dominant longitudinal momentum component. The counter-propagating ‘production’ laser for optimum Doppler boost in X-ray production by reflection has to be injected opposite to the electron direction and not opposite to the driver laser. Different measures to increase the reflectivity of the electron sheet via laser trapping or free-electron-laser-like micro-bunching are discussed, extending the photon energy into the MeV range. Here, first-order estimates are given.     

Magnetic and orbital dichroism in (e,2e) ionization of sodium

We present the first measurement of (e,2e) ionization cross sections for a laser oriented atomic target by spin polarized electrons. Cross sections are presented as a function of target orientation and polarization direction of the incident electron beam. This study provides insight into mechanisms by which angular momentum is transferred from the valence electron to the two final-state continuum electrons in both singlet and triplet spin channels, by comparing measurement with distorted wave Born approximation and the dynamically screened three Coulomb wave calculations.     

Macroparticle acceleration by a laser-plasma stream

A physical model is proposed for macroparticle acceleration in an expanding laser plasma. Two acceleration mechanisms are considered, based on absolutely inelastic interaction of the laser-plasma ions with the macroparticle surface: acceleration due to the ion momentum and reactive acceleration due to evaporation of the surface layer. The processes are mathematically described with the aid of the gasdynamics equations with thermal conductivity. Approximate analytic expressions are obtained and permit an estimate of the laser-plasma energy as well as of the quasiparticle momentum as functions of the parameters of the laser pulse, of the target, and of the macroparticle. The analytic solutions are compared with numerical computations using the RAPID program; the comparison confirms that the results agree qualitatively and quantitatively.Quantum Radiophysics Division, Lebedev Physics Institute. Translated from Preprint No. 209 of the Lebedev Physics Institute, Academy of Sciences USSR, Moscow, 1990.