XUV radiation transfer through high-Z metal foil in a laser-produced plasma

Aluminum and gold foils have been irradiated with 1.05-μm, 100-psec laser pulses at an intensity of 3 × 10¹⁴ W/cm². A spatially resolved spectrum of XUV radiation from a rear-side plasma has been observed in the wavelength range from 10 to 100 Å. The spectral intensity for the Al foil decays exponentially with foil thickness and goes to zero at 3 μm. The intensity for the Au foil also decays exponentially up to 1 μm but remains almost constant from 1 to 6 μm. This result for the Au foil indicates that radiation heat conduction plays an important role in energy transport through high-Z plasmas.     

Cover Picture: Contrib. Plasma Phys. 7/2013

A 250 µm thick high purity beryllium foil is symmetrically irradiated by a total of 20 laser beams at 351 nm wavelength. Each laser carries 480 J in a 1 ns long pulse, the pulses are staggered in time and focussed on a 800 µm diameter focal spot, yielding a flat top intensity profile of 2.4 × 10¹⁴ W/cm² for 3 ns on the target surface. The evolution of the mass density is modelled with the radiation‐hydrodynamics code HELIOS as function of the target depth. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interaction of tea CO2 laser light with plasma

High power double discharge type TEA CO₂ laser was used to study laser plasma interaction on carbon target plasma. The maximum output power was 0.3 GW and full width at half maximum intensity was 100 ns. We measured the reflectivity of the laser light at 10.6 μm in wavelength from the carbon plasma. The reflectivity showed a maximum (≈56%) at the laser power intensity of about 1.1 x 10¹⁰ W/cm², and at above this value, the reflectivity decreased. The absorption was introduced effectively above this intensity. The electron density and temperature were also measured.     

Frequency up-conversion of infrared laser radiation in the human retina

Experimental observation is reported of conversion in a human retina of infrared laser radiation (with a wavelength of 1.06 μm) into the visible upon scattering by a solid target. The wavelength of this visible radiation, as estimated by several independent observers, is around 0.557 μm. Frequency up-conversion is observed down to a peak power on the retina of only 150 W/cm², while, for other laser sources, this threshold is as low as 1 W/cm². It is suggested that the observed conversion is due to the second-harmonic generation in the periodic structure of the retina of a human eye. Deviation of the observed wavelength from that of the second harmonic in vacuum is ascribed to the spectral dependence of the refractive index of rhodopsin molecules within the retina.     

Nonlinear rotation of the polarization of the intense femtosecond laser radiation in BaF<Subscript>2</Subscript>

The dependence of the polarization plane rotation angle in the BaF₂ crystal on the intensity of the femtosecond laser radiation with 0.62 μm wavelength in the ω = ω + ω ? ω process on cubical nonlinearity χ⁽³⁾ is investigated. An anomalous increase in the efficiency of the cross-polarized radiation generation at I > 2 × 10¹² W cm² is observed for the first time. The 10% efficiency of the orthogonal component generation at the ～3 × 10¹² W cm² intensity is obtained.     

VUV-heating of plasma layers and their use as ultrafast switches

We report on new possibilities to generate solid-density plasma at extreme energy density by intense VUV beams. Here we consider 100 fs pulses of 30 eV photons focused to 10¹⁶ and 10¹⁸ W/cm². The temperature evolution in 50 nm thick aluminum foils is discussed on the basis of simulations, performed with the one-dimensional radiation hydrodynamics code MULTI-fs. For 30 eV photons, the foil is shown to switch from transmission to reflection mode on a femto-second time-scale; this is due to the rapid change of the plasma frequency during laser heating which may turn an initially transparent Al-foil into an opaque one. The switching-time depends on the intensity of the laser pulse. Also layered heating structures inside the foil are discussed which occur due to reflection at the rear surface.     

X光分幅相机在黑腔等离子体填充特性研究中的应用

 在神光Ⅱ装置上2.4 ns长脉冲三倍频激光（激光能量8×300 J）与腔靶耦合实验研究中,X光分幅相机通过激光注入孔观测获得了3种腔尺寸腔内Au等离子体径向运动时空分辨图像。用MATLAB对图像进行了定量处理,结合时间分辨辐射温度测量结果分析表明：在腔内不充气、无低Z衬垫情况下,标准腔（800 μm×1 350 μm）在激光脉冲作用到约1.5 ns时出现明显的Au等离子体堵腔效应;当腔尺寸放大到1.25倍（1 000 μm×1 800 μm）和1.5倍（1 200 μm×2 100 μm）时,腔内等离子体基本不堵腔。给出了3种腔尺寸不同时期腔内Au等离子体径向聚心速度,分析表明：大腔的聚心速度比小腔的慢,后期比初期慢。     

Absorption of iodine laser pulses in potassium and cesium vapors

The absorption of ns iodine laser pulses at a wavelength of 1.315 μm in potassium and cesium vapors has been measured. At elevated temperature there is considerable absorption in both vapors. The absorption was not saturable at intensities up to 1 GW/cm², but increased with increasing intensity.     

Particular features of the transmission of laser radiation with wavelength 0.438 µm and intensity (3–7)·1014 W/cm2 through an undercritical plasma from polymer aerogels

The velocities of energy transport in an undercritical plasma of polymer aerogel with and without copper nanoparticles were measured. Transmission of the laser light through targets of different thicknesses such as submicron three-dimensional polymer networks with densities below the critical value (0.13–0.52 N _cr) for a wavelength of 0.438 μm and intensity of (3–7)·10¹⁴ W/cm² at a half-height pulse duration of 0.32 ns was studied. The transfer of a heating laser radiation was registered on the rear side of the target. It ranged from a level of ∼0.5% for the thickness of a low-density layer of 400 μm and density of 9 mg/cm³ (mass per unit square of 0.36 mg/cm²) up to 50–60% for a thickness of 100 μm and density of 2.25 mg/cm³ (mass per unit square of 0.02 mg/cm²). The time dependences of the optical emission from the rear side of the targets were measured. They appear to be indicative of the plasma dynamics in two-layer targets (polymer foam on Al foil) and enable the estimation of the absorption depth for the laser light in an undercritical plasma. __________ Translated from Preprint No. 8 of the P. N. Lebedev Physical Institute, Moscow (2007).     

Evidence for long-range fission fragments induced by irradiation with electrons

In search of natural superheavy elements, long-range fission fragments penetrating up to 73 μm of mylar were observed, when thick metallic foils of U, Pb and W, covered with six mylar foils were irradiated with 550 MeV or 1300 MeV electron beams. The cross section is of the order of 10⁻³⁰ cm². The effect is produced in natural U, Pb, W and similarly in mass separated ²⁰⁸Pb and ¹⁸⁶W isotopes — not by fission of a superheavy element.     

Thermoelectric line array for spatial specification of focused laser beams

A thin-film line array of thermoelectric detectors is applied for the measurement of intensity distribution of focused laser beams. The damage threshold of these detectors is relatively high. They can tolerate 50 μsec pulses with intensities as high as 3·10⁵ W/cm². High resolutions can be achieved by fabricating line arrays having a large number of thermoelectric junctions with distances as small as 5 μm between centers.     

Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleighben-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts. X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources. By using a time-integrated space-resolved keV spectroscope and pinhole camera, potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength, ratio of pre-pulse intensity to main pulse intensity, and laser intensity (from 7.25 to ～11.3× 10¹⁵ W/cm²⁾. One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested. The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm, and relative x-ray line emission conversion efficiency ξ_x from the incident laser light energy to helium-like titanium K-shell spectrum increases significantly with pre-pulse intensity increasing, increases rapidly with laser wavelength decreasing, and increases moderately with main laser intensity increasing. It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast. It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.     

Bestimmung von Temperatur und Dichte eines Gleitfunkens aus der Bremsstrahlung im Infrarot

The absolute intensity of a vacuum sliding spark (length 2–10 cm, half cycle 0,8 μsec) has been measured in the spectral region between 0,4 and 3μ. From the long wavelength radiation emitted from an optically thick layer, one gets the temperature, from the short wavelength radiation emitted from an optically thin layer, one obtains the density. Using polyethylene as an insulator we reached a temperature of 4·10⁵ °K at electron densities of 8·10¹⁸ cm^?3 and current densities of 1.2·10⁶ A/cm². The temporal development of temperature and density has been determined. The maximum intensity at λ=0.43 μ was found to be 5·10⁴ (7·10¹⁰ W/cm³ ster) as large as that of the positive crater of a properly driven carbon arc. At λ=3 μ this same parameter turned out to be 300 times as large (5·10⁷ W/cm³ ster).     

Formation of highly organised,periodic microstructures on steel surfaces upon pulsed laser irradiation

We present results on the growth of highly organised, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz, beam quality factor of M ²∼1.5) laser irradiation in standard atmospheric environment (room temperature and normal pressure) with laser spot diameter of the target being ∼50 μm. The target surface was irradiated at laser fluence of ∼2.2 J/cm² and intensity of ∼0.31×10⁹ W/cm², resulting in the controllable generation of arrays of microstructures with average periods ranging from ∼30 to ∼70 μm, depending on the hatching overlap between the consecutive scans. The received tips of the structures were either below or at the level of the original substrate surface, depending on the experimental conditions. The peculiarity of our work is on the utilised approach for scanning the laser beam over the surface. A possible mechanism for the formation of the structures is proposed.     

靶的厚度对激光产生的x射线输运到靶后能谱的影响

利用一维辐射流体动力学程序MULTI数值模拟研究了功率为10¹⁴W/cm²、脉冲宽度为1ns、波长为0.35μm的短脉冲强激光辐照不同厚度的平面Au靶时，靶厚度对靶背面x射线能谱结构和辐射强度的影响. 关键词： 激光等离子体 辐射流体力学 x射线转换     

Hot electrons produced from long scale-length laser-produced droplet plasmas

Microplasmas produced from 15 μm methanol droplets irradiated by 100 fs laser pulses in the intensity range 10¹⁴–10¹⁶ W cm^?2 are investigated via measurements of the hot electron temperature and x-ray yields under different conditions of intensity, polarization state, and plasma scale-length. The scale length of the drop-let plasma is increased with an intentional prepulse that is 10 ns ahead of the main pulse. Hot electron temperatures up to 48 keV have been measured at intensities of 2.5 × 10¹⁵W cm^?2 and the scaling of temperature as a function of intensity is determined for a long scale-length droplet plasma. The polarization and ellipticity dependence of the hard x-ray yield from the microdroplet plasmas are used to probe the shape of the droplet after irradiation by a prepulse.     

基于差频中红外激光的痕量气体高分辨光谱检测研究

研究了基于差频光源的高分辨中红外激光光谱检测系统,差频中红外光源使用两台近红外半导体激光器作为种子光源,采用PPLN晶体作为非线性混频器件,结合准相位匹配技术实现了3.2~3.7 μm中红外相干光源输出,最大差频输出功率约为1 μW.以CH₄为例检验了系统的高分辨红外光谱检测特性,选择CH₄分子3 028.751 cm^-1 v₃基频吸收线作为分析谱线,10 cm光程的检测限为0.8 ppm.光谱数据分析表明,系统检测限主要受到标准具光学噪音的限制.     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

超薄靶激光质子加速实验研究

在超短超强飞秒SILEX-Ⅰ激光装置上,开展了薄膜靶激光质子加速的实验研究。实验发现激光预脉冲、靶厚度对质子加速有很大的影响。在激光强度3×1018~3×1019W/cm2条件下,采用前表面厚度为3μm铜、后表面镀4μm厚CH靶,质子的最大能量达到3.15 MeV。而对190 nm厚CH膜靶,质子的最大能量为0.54 MeV。初步研究了激光偏振对质子加速的影响,相同激光功率条件下,圆偏振激光加速产生的质子最大能量略低于P偏振打靶。这些结果与靶后鞘层加速机制相一致。     

Fiber laser micro-cutting of stainless steel sheets

The authors report on laser micro-cutting results for stainless steel foils with the aid of a 100 W fiber laser. This novel laser source combines a high output power in relation to conventional laser sources for micro-processing applications with an excellent beam quality (M²=1.1). Different material thicknesses were evaluated (100 μm to 300 μm). Processing was carried out with cw operation of the laser source, and with nitrogen and oxygen as assisting gases. Besides the high processing rate of oxygen assisted cutting, a better cutting performance in terms of a lower kerf width was obtained. PACS 42.82.Cr; 42.62.Cf; 81.05.Bx