Calorimetric study of laser-irradiated thin foil targets

Hydrodynmaic efficiency of laser-irradiated thin aluminum and gold-coated aluminum targets was experimentally determined using a specially designed cone calorimeter. Velocity of the accelerated target and ablation pressure were also estimated from the experimental data. The laser irradiance range used in the experiments was between 10¹² and 10¹³ watts/cm². Experiments indicate that the fall in the hydrodynamic efficiency due to gold coating on aluminum target is about 12% at an irradiance of 8 × 10¹² W/cm².     

Hot-electron-induced plasma formation on the rear surface of a foil

The plasma jet formed on the rear surface of a foil in laser–solid interaction is investigated by laser probing. The rear plasma jet, which is in line with the laser, formed a few picoseconds after the incidence of the focused laser, is due to a beam of fast electrons propagating through the target and is collimated by a strong magnetic field in the plasma. Received: 14 January 2003 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531 E-mail: jzhang@aphy.iphy.ac.cn     

Anomalous transmission of an ultrashort ionizing laser pulse through a thin foil

The formation of a highly anisotropic photoelectron velocity distribution as a result of the interaction of a powerful ultrashort laser pulse with a thin foil is found to yield a large skin-layer depth and an anomalous increase of the transmission coefficient. The physical reason for the effect is the influence of the incident wave magnetic field, through the Lorenz force, on the electron kinetics in the skin layer.     

Laser welding of thin foil nickel-titanium shape memory alloy

In this study, two L27 Taguchi experiments were carried out to study the effect of fibre laser welding parameters and their interactions upon the weld bead aspect ratio of nickel-titanium thin foil. The optimum parameters to produce full penetrated weld with the largest aspect ratio and desirable microstructure were successfully obtained by the Taguchi experimental design. The corrosion property of the optimized NiTi weld in Hank’s solution at 37.5 °C was studied and compared with the as-received NiTi. To improve the corrosion properties of the weld, the effect of post-weld-heat-treatments ranging from 573 to 1173 K was investigated. The corrosion properties, surface morphology, microstructure and Ti/Ni ratio of the heat-treated NiTi weld were analysed. It was found that a post-weld heat treatment at 573 K for 1 h provided the best pitting corrosion resistance at the weld zone.     

Generation of fast protons in the interaction of relativistic laser pulses with a thin foil

The interaction of a powerful femtosecond laser pulse with a thin aluminum foil containing a hydrogen admixture is investigated. The calculations in question are performed for actual ion charges and at a solid-state density. The investigation is carried out with the aid of the previously developed theory of interaction of powerful short laser pulses with plasmas. This theory is based on constructing propagators for plasma-particle distributions. The calculated fast-proton distributions are in good agreement with experimental data.     

TEM imaging of an inclined dislocation in an anisotropic thin foil

The elastic distortions nearby the two emerging points of a straight inclined dislocation located in an elastically anisotropic thin foil are expressed with the aid of the integral formalism [D.M. Barnett and J. Lothe, Phys. Norv. 7 (1973) p.13], an approach complementary to that of the Eshelby's “sextic” formalism. They are included in the calculation of the intensities of diffracted beams in transmission electron microscopy to produce theoretical images, a well known procedure when elastic free surface relaxation is ignored. Examples of theoretical images point out some contrast differences between images calculated with the assumptions of isotropic and anisotropic crystals. These calculations can be simplified for a dislocation normal to the surface and a line direction parallel to a two-fold axis.     

Proton acceleration from high-intensity laser interactions with thin foil targets

Measurements of energetic proton production resulting from the interaction of high-intensity laser pulses with foil targets are described. Through the use of layered foil targets and heating of the target material we are able to distinguish three distinct populations of protons. One high energy population is associated with a proton source near the front surface of the target and is observed to be emitted with a characteristic ring structure. A source of typically lower energy, lower divergence protons originates from the rear surface of the target. Finally, a qualitatively separate source of even lower energy protons and ions is observed with a large divergence. Acceleration mechanisms for these separate sources are discussed.     

Heat flux density in the region of droplet contact line on a horizontal surface of a thin heated foil

The evaporating water droplets on a horizontal heated substrate were experimentally studied. The constantan foil 25 μm thick with a size of 42×35 mm² was used as a substrate. The experiments were carried out with a single droplet or with an ensemble of two or three droplets on the foil. The temperature of the lower surface of foil was measured by an IR scanner. To determine the heat flux density at evaporation of liquid near the contact line, the Cauchy problem for the heat conduction equation was solved using the thermographic data. The results of calculations showed that the maximal heat flux density takes place in the region of the contact line and exceeds the average heat flux density from the entire surface of foil. This is explained by the heat inflow from the foil periphery to the droplet due to relatively high value of the coefficient of heat conductivity of the foil material and high evaporation intensity in the contact line region.     

A method for thin foil thickness determination by transmission electron microscopy

With the intention of determining the local thickness within a crystalline thin foil specimen, by means of transmission electron microscopy (TEM), a method previously proposed by Zuo and Shi [J.M. Zuo, Y.F. Shi, Microsc. Microanal. 7 (Suppl. 2) (2001) 224-225] was applied. Using the convergent beam technique, with the incident beam parallel to a zone axis with low indices, diffraction patterns were obtained for some aluminum alloys with low solute content. These patterns were contrasted with those obtained from simulations based on the dynamic theory with Bloch's waves formalism. The local thickness of the thin foil was then obtained by visually comparing the simulated patterns with the experimental one.Comparison of the proposed method with that based on the analysis of two-beam convergent beam patterns [P.M. Kelly, A. Jostsons, R.G. Blake, J.G. Napier, Phys. Stat. Solidi (a) 31 (1975) 771-780] and with that based on the ratio of intensity of the zero loss peak to the total intensity in an electron energy loss spectrum [R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, second ed., Plenum Press, New York, 1996] was carried out. A very good agreement between thicknesses determined using the different methods was found. The sensitivity of the method of Zuo et al. was found to be about 1 or 2 nm. The advantages and limitations of the different methods are discussed. The method of Zuo et al. can provide fast and reliable results and can be applied in all modern instruments.     

Enhancement of ablation smoothing in laser-irradiated,highZ coated,thin foil targets

An enhanced spatial smoothing of ablative motion of thin plastic foil targets coated with high atomic number ablators such as gold or aluminium, irradiated by a spatially modulated Nd: glass laser beam was observed. Optical shadowgraphy coupled with double foil technique was used to observe the laser-irradiated foil motion. Laser irradiance used for the experiments was in the range of 10¹¹–10¹³ watts/cm². A 60–80% enhancement in the smoothing was observed for a laser beam modulation (width 75–150 μm) at the target surface.     

Hole formation in thin polymer films: a two-stage process

Thin, supported liquid films are known to rupture, creating holes throughout the film, due to defects or to van der Waals interactions. We show that the hole formation process before rupturing occurs in two stages, each characterized by distinct dynamical and morphological features. The time scale for the formation process is orders of magnitude slower than the translational (reptation) relaxation time of the individual chains. This has implications regarding the transition from the formation regime to subsequent hole growth regime on the underlying substrate.     

Energy distribution of electrons ejected from a thin carbon foil by alpha particles and fission products

Secondary electron spectra up to 3.5 keV electron energy arising from the passage of alpha particles and²⁵²Cf fission products through a 2μg/cm² carbon foil are measured for a forward (42°) and a backward (138°) angle of emission. The forward spectra induced by alpha-particles are compared to binary encounter calculations and to a gas target spectrum. Systematic differences in the shapes of the alpha particle and the fission product induced spectra are discussed. Total yields and mean energies of the secondary electrons are derived from the spectra.     

碰撞效应对超强激光与高密度碳薄膜靶相互作用中快电子输运的影响

 在激光驱动粒子加速和快点火背景下,用带碰撞模块的1维粒子模拟程序,研究了高功率亚ps激光与高密度（近固体密度）碳薄膜靶相互作用中的碰撞效应。研究表明,通过和无碰撞粒子模拟结果的比对,在早期瞬态时段,碰撞效应通过慢化提供电阻加热的回流对快电子产生和输运起重要作用。定性上,碰撞效应还减少快电子的产额和最大能,抑制能量输运。然而,在加热后期当往返运动成为主要输运机制时,有碰撞和无碰撞两种情况的差别减小。     

单束激光脉冲俘获及放大机理的理论分析与数值模拟研究

本文提出用固体靶前放置薄膜靶来实现激光场放大的新方案, 研究了针对单束激光脉冲条件的俘获及放大机理. 理论模型与数值模拟均表明入射激光能量可以部分地以驻波形式驻留在靶间区域并得到有效放大. 研究表明在入射激光光压、俘获激光光压和电荷分离场的共同作用下, 薄膜靶电子层压缩和膨胀是能量积累存在反复振荡过程的直接原因, 经过振荡后激光脉冲得到稳定俘获.     

碰撞效应对超强激光与高密度碳薄膜靶相互作用中快电子输运的影响

在激光驱动粒子加速和快点火背景下,用带碰撞模块的1维粒子模拟程序,研究了高功率亚ps激光与高密度（近固体密度）碳薄膜靶相互作用中的碰撞效应。研究表明,通过和无碰撞粒子模拟结果的比对,在早期瞬态时段,碰撞效应通过慢化提供电阻加热的回流对快电子产生和输运起重要作用。定性上,碰撞效应还减少快电子的产额和最大能,抑制能量输运。然而,在加热后期当往返运动成为主要输运机制时,有碰撞和无碰撞两种情况的差别减小。     

Experimental study of the spatial distributions of relativistic electron beams reflected and refracted by a thin foil

The ratio of the cross section for inelastic scattering to the total cross section for scattering of a fast electron by a cluster, depending on the number of atoms in the cluster, is shown to be not a monotonic function. This nonmonotonicity is not related to the well-known nonmonotonic dependences determined, for example, by the magic numbers in the shell model of clusters but is of purely quantum origin: the coherence of elastic electron scattering and the incoherence of inelastic one by a multipartice target.