基于神光Ⅲ原型装置的激光加载条件下准等熵压缩实验研究进展

激光加载条件下的准等熵压缩实验逐渐受到重视.在间接驱动方式下,利用气库靶的方式可以获得准等熵压缩的实验数据.自由面速度和界面速度是两种常用的准等熵压缩诊断对象.气库靶驱动准等熵压缩方式效率较低,但是对打靶激光的强度变化不敏感.长脉冲打靶的直接驱动方式驱动效率较高,但对激光强度变化非常敏感.基于神光Ⅲ原型装置,本文介绍了气库靶驱动的准等熵压缩实验靶型、部分实验结果,并分析了实验中的关键技术,为气库靶驱动准等熵压缩实验技术提供了思路.同时,介绍了长脉冲直接驱动准等熵压缩实验的靶型、典型实验结果,并分析了致盲效应的影响.利用激光直接驱动的方式,获得了三台阶Al/LiF界面的加载过程.该数据是目前国内使用该技术获得的最好数据.通过这些实验结果,证明了在神光Ⅲ原型上开展准等熵压缩实验的可行性.     

激光驱动气库靶对铝的准等熵压缩实验研究

利用激光驱动气库材料形成的等离子体射流对材料进行斜波加载, 可以获得高应变率的准等熵压缩. 在神光III 原型高功率激光装置上开展了激光驱动铝材料的准等熵压缩实验, 成像型速度干涉仪VISAR记录到样品自由面连续、光滑的速度历史, 采用反积分法得到60 GPa以上的峰值压强, 加载上升沿约10 ns,应变率可达10⁸ s ^-1, 并且观察到了压缩波在样品后表面的反射效应.     

激光驱动薄膜产生等离子体射流的条件分析

 以聚乙烯薄膜材料为研究对象,从实验物态方程出发,对强激光驱动薄膜材料时影响激光在薄膜后表面形成等离子体射流的主要因素,包括激光强度、波长、脉宽、气库膜材料及厚度,进行了理论和数值分析。研究表明,到达薄膜后表面的冲击波强度足够高时,能够产生气态或等离子体射流,否则卸载过程仍然为凝聚态;对于聚乙烯材料,形成等离子体射流的基本条件是到达薄膜后表面的冲击波强度达到约80 GPa以上;采用短波长、较高功率密度、较长激光脉宽的激光驱动具有低汽化温度、低电离阈值的薄靶,更容易实现等离子体射流。     

激光驱动准等熵压缩透明窗口LiF的透明性

LiF在激光驱动高压实验中是比较常见的窗口材料, 其在冲击下透射或反射可见诊断光是作为窗口材料的重要特性. 在神光III原型激光装置上开展了带LiF窗口的铝样品准等熵压缩实验, 采用任意反射面速度干涉仪诊断获得准等熵压缩样品(CH/Al/LiF)的反射率. 实验结果表明在准等熵压缩后期反射率诊断出现致盲现象. 为此, 建立了带透明窗口的样品对诊断光的反射率模型, 模型考虑了窗口LiF压缩后透明性变化. 模型计算的CH/Al/LiF样品对可见光的反射率时间演化过程与实验结果符合较好. 研究结果表明: LiF中压缩波追赶逐渐形成强冲击波, 显著降低了LiF的透明性, 并最终发生致盲现象; 第一性原理方法所给出的LiF的能带间隙偏低1–2 eV; 该实验中, LiF的透明性完全消失时, LiF中波头处的温度约为1 eV, 压力为2–3 Mbar.     

基于神光III原型的整形激光直接驱动准等熵压缩实验研究

整形激光直接驱动准等熵压缩实验技术对新材料科学、冲击波物理、地球物理学研究等领域有重要意义. 设计特定的实验条件, 可以将材料压缩到冲击压缩和等熵压缩都无法达到的状态, 从而为材料研究提供新的技术途径. 介绍了在神光III原型装置上开展的直接驱动准等熵压缩实验, 对理论基础、实验靶型、实验结果、关键技术、实验特点和数据都进行了较详细的分析. 通过实验和相应的数据处理程序, 获得了加载面上压力超过400 GPa的数据, 这是目前国际上用激光直接驱动的方式获得的Al材料中最高的压力. 同时, 对经过LiF窗口获得的表观粒子速度与真实粒子速度的关系进行了分析, 获得了一条对表观粒子速度进行动态修正的曲线, 从而使粒子速度的计算更加准确. 对实验的改进方向进行了比较明确的分析. 所提供的数据和分析过程对该领域的研究人员有一定的参考价值.     

无氧铜的准等熵压缩性

 利用递变冲击阻抗材料叠合而成的组合飞片，在二级轻气炮上对无氧铜进行了准等熵压缩性测量，加载时间约达1 μs。用激光速度干涉仪连续记录了不同厚度处无氧铜样品自由面速度随时间的变化过程，并通过拉格朗日波分析技术计算得到40 GPa下的准等熵的应力-应变曲线。结果表明：在低应力区，无氧铜的准等熵压缩线位于冲击绝热线之上；到32 GPa以上，准等熵压缩线才回落到冲击绝热线之下。这个现象与Barker、Chhabildas等对铝与钨的实测现象是一致的，它表明：在低应力区，材料的冲击强化效应与加载速率密切相关。     

基于神光III原型装置开展的激光直接驱动准等熵压缩研究进展

整形激光驱动准等熵压缩是高效的物质动态压缩方式,是开展材料高压压缩特性研究的重要手段.本文系统介绍了近年来基于神光Ⅲ原型激光装置开展的整形激光直接驱动准等熵压缩研究工作：创立了凝聚态物质的准等熵压缩理论模型,改进了实验设计方法和数据处理方法,掌握了制靶工艺,开展了激光加载实验并在多种靶型中实现了数百GPa的准等熵压缩.实验获取的参数范围超过传统加载方式,数据质量达到国际先进水平.     

脉冲激光-铜靶等离子体产生及其演化过程的瞬态光谱研究

应用时间分辨光谱技术, 研究了高能量纳秒脉冲激光作用下铜靶表面等离子体产生及演化的物理过程. 实验中相互作用区固-气-液三相对激光能量的吸收明显地反映于激光反射强度随时间的演化中, 使得靶表面物质形态的改变导致激光反射强度随时间呈双峰分布.同时, 随着激光峰值功率密度的增加, 靶面等离子体(气)-固-液相变发生的时间相应提前.因此, 这些瞬态性质是诊断激光-固体靶相互作用中靶面物质相变时间的有效方法.     

基于神光Ⅲ原型装置开展的激光直接驱动准等熵压缩研究进展

整形激光驱动准等熵压缩是高效的物质动态压缩方式,是开展材料高压压缩特性研究的重要手段.本文系统介绍了近年来基于神光III原型激光装置开展的整形激光直接驱动准等熵压缩研究工作:创立了凝聚态物质的准等熵压缩理论模型,改进了实验设计方法和数据处理方法,掌握了制靶工艺,开展了激光加载实验并在多种靶型中实现了数百GPa的准等熵压缩.实验获取的参数范围超过传统加载方式,数据质量达到国际先进水平.     

多次冲击波压缩DT冰理论研究

 冲击波压缩DT是ICF内爆过程中的主要熵增因素,如何用有限数目的时间整形冲击波实现近等熵压缩过程是优化点火靶设计的关键。利用理想气体和实际DT状态方程,计算系列冲击波压缩后的熵增和密度变化,分析表明,当第一个冲击波压强不大于0.1 TPa,后续再用3个冲击波达到10 TPa,总熵增不影响点火靶DT压缩性能,增加冲击波个数对DT熵增和压缩度改善效果很小。     

激光诱导铝等离子体中原子和离子组分膨胀特性

为了了解等离子体中原子与离子组分的膨胀特性及背景气体存在状态下其运动状态的改变规律,设计了一系列实验,并进行了深入探究.采用波长为532 nm的纳秒激光剥蚀铝样品形成等离子体,并使用配有em ICCD检测器的C-T型三光栅单色仪对等离子体进行时序采集,同时使用2400 g·mm~(-1)的光栅替代窄带滤光片进行不同组分成像诊断,得到铝等离子体中Al Ⅰ (396.1 nm), Al Ⅱ (466.3 nm), Al Ⅲ (447.9 nm)的光谱分辨图像.在不同背景气压下采集了等离子体各组分光谱图像,探究背景气体对等离子体演化的影响.结果表明,在等离子体形成过程中,离子组分相对于原子组分分布在羽流前端,且角度分布较小.原子与离子组分的真空膨胀速度均处于10~4m·s~(-1)量级.等离子体中离子组分的运动速度较高,且其运动速度随着离子价态的增加而增大,但在本实验使用的能量密度范围下,随激光能量的变化波动不大.中性原子的运动速度较慢,但随能量的增加而增大.随着膨胀过程的进行,各组分羽流沿样品表面法线方向推进且发射强度逐渐降低,对应的羽流密度和温度也相应降低.环境气压逐渐增大时,各研究组分运动状态与在高真空度下时有明显区别.在气压大于1 Pa后,等离子体与环境气体发生相互渗透,膨胀前端出现的晕影,产生扰动,发生束缚缓速.且等离子羽因气压增大而收缩、与背景气体的碰撞概率增加,使得羽流发射强度加强,等离子体的寿命随之延长.提出的新颖诊断方法与实验所得结果可为等离子体组分动力学过程的研究提供参考.     

Phenomenological model of the unstable stage of a vacuum spark discharge

A model of the unstable stage of a spark discharge in vacuum is proposed, which describes all typical manifestations of this stage, including current spikes in the diode, an increase in the potential at the cathode flame front, collective acceleration of ions in vacuum and plasma diodes, change in the cathode erosion mechanism, and the emergence of electron microbeams with a high current density at the anode. It is shown that these processes are associated with the formation of a charged electron layer of a spatially inhomogeneous plasma at the cathode flame boundary at the unstable stage of the spark discharge in vacuum. The emergence of this layer is associated with a limited emissive ability of the plasma at the cathode flame front during its expansion in vacuum. This leads to disruption of the plasma (field-induced emission of electron from the boundary region of the flame) and the formation of a short-lived charged plasma, viz., high-density ion cluster at the cathode flame boundary. The estimates obtained using this model are in good agreement with the experimental data on physical processes at the unstable stage of a vacuum spark discharge.     

Flow Characteristics in the Exhaust of a Pulsed Megawatt Gas Fed Arc

The transient flow generated by a pulsed, megawatt-level, gas-fed arc with an applied magnetic nozzle has been examined with a new design piezoelectric pressure transducer. Sensor thermal conduction and accelerations have been examined and eliminated in the 500?sec period of plasma flow. Existence of a large magnitude cold gas pressure front of 20?sec duration has been reconfirmed and its relationship to the following plasma flow of about 200?sec duration has been examined for the first time. At a point 30 cm from the arc source, initially near vacuum conditions (typically with an arc current of 11.2 kA and 1 tesla applied magnetic field), a pressure pulse of unionized gas with a magnitude of 104 N/m2 is followed by plasma flows with nearly constant impact pressure of 103 N/m2. Pressure and number density in this plasma region are seen to decrease with applied magnetic field strength. With electron density derived from Thomson scattering measurements (1020 m-3) plasma flow velocities on the order of 5 × 104 m/sec are calculated.     

Plasma Blow-Off from Electrode Surfaces in the Presence of Vacuum-Ultraviolet Radiation

Experiments have been performed using a low-energy pulser (100 J electrical) to simulate the PITHON wire initiation process by exploding two stainless steel wires and thus providing a UV radiation source. The plasma boundary and density profie near the surface of a sample (electrode material) placed near the radiating wires was observed by taking both a shodowgraph and a hologram. The total energy radiated per wire ranged from 2.0 to 9.5 J/wire for peak currents of 13.5 and 25 kA/wire, respectively. The plasma density profile shows ne = 1.0 × 1018/cm3 near the surface of the sample and ne = 2 × 1016/cm3 near the plasma vacuum interface (plasma front). The final plasma front position has a linear dependence on the total incident energy density with the constant being 1.43 mm/(J/cm2), thus the average expansion velocity dependence on energy density is 1.02 cm/?s/ -J/cm2. Upon varying the sample material, the sample temperature, magnetic field, and the surface roughness, only the magnetic field and the rough electrode surface reduced the plasma front position by approximately a factor of two.     

Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turns out that the injection of electron bunches longer than the plasma wavelength can also generate accelerated femtosecond bunches with a relatively low energy spread. This is of great interest because such injecting bunches can be provided, e.g., by photo cathode rf linacs. Here we show that when an e-bunch is injected into the wakefield, it is important to take into account the interaction of the injected bunch with the laser pulse in the vacuum region located in front of the plasma. We show that at low energies of the injected bunch, this leads to ponderomotive scattering of the bunch and results in a significant drop of the collection efficiency. For certain injection energies the ponderomotive scattering may result in a smaller energy spread in the accelerated bunch. It is found that the injection position in the laser wakefield plays an important role. Higher collection efficiency can be obtained for certain injection energies, when the bunch is injected in plasma at some distance from the laser pulse; the energy spread, however, is typically larger in this case. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch. PACS 52.38.Kd; 41.75.Jv; 41.85.Ar     

Hydrocarbon film formed on the surface of a semiconductor irradiated by an electron beam

The rate of formation of a hydrocarbon film on the surface of a sample subjected to the action of an electron beam is studied at room temperature and in cooling a sample to the liquid nitrogen temperature. The thickness and the optical radiation transmission of such films are measured as functions of the electron beam radiation time, the sample temperature, and the level of vacuum. The film thickness is measured with atomic force microscopy. The absorption of the films is determined by comparing the cathodoluminescence intensities from a pure sample surface and from the surface covered with a film. The experimental results can be used to estimate the film formation rate as a function of the sample temperature and the vacuum and to determine the optical radiation absorption at a wavelength of 300, 360, 550, and 665 nm.     

Vacuum discharge instability at laser initiation of a cathode spot

The dynamics of fast (a current rise time of ≤10¹¹ A/s) laser-induced vacuum discharges with moderate amplitudes of the current (≤10 kA) and voltage (≤20 kV), as well as medium storage energy (20 J), is studied. It is shown experimentally that the initial conditions specified by the energy and duration of laser radiation are a decisive factor governing the discharge dynamics. Two types of beam-plasma instabilities separated in space and time are discovered, and their occurrence conditions are analyzed. The first type of instability, observed early in the discharge, is associated with pinch structures at the front of the cathode jet expanding into a vacuum. The second type arises either at the peak or at the trailing edge of the current pulse and is accompanied by generation of hard (with an energy of ≥100 keV) bremsstrahlung from the anode region. The increase of the hard component energy over the current source potential is attributed to breaking due to plasma erosion.     

Dynamics of electric fields driving the laser acceleration of multi-MeV protons

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (approximately 10(18) W/cm2) and short (approximately 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.