Hot ablative implosions driven by 0.53 μm laser radiation

A six beam 0.53 μm laser symmetrically irradiated thin walled gas filled spherical glass shells at 4 × 10¹⁴ W cm^-2, the upper limit for ablatively driven implosions. The high ablation pressure induced sufficient X-ray and thermonuclear product emission for diagnosis of the 200 eV temperature and 3 g cm^-3 density in the implosions. X-ray preheat, Rayleigh Taylor instability and non uniform irradiation are identified as serious limits to performance of the implosions.     

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     

基于D3He反应产生的单能质子对ICF内爆过程的照相模拟研究

 为实现单能质子对惯性约束聚变(ICF)内爆过程的诊断,使用FLUKA蒙特卡罗程序模拟分析了影响质子对内爆过程诊断的因素。通过设置不同初始质子数目,来分析质子产额对小球图像质量的影响,结果发现,要得到清晰图像,需要10⁹的质子数。参照实验中常用的靶参数,模拟了直接驱动和间接驱动条件下的照相情况,分析了间接驱动情况下,金属腔对质子束的散射会造成内爆小球图像模糊,说明质子照相不适合间接驱动内爆。给出了一种时间分辨的质子照相方法,通过调整质子产生和内爆产生的时间差,实现了对内爆过程前后不同时刻的照相等。模拟表明,D³He反应产生的14.7 MeV的准单能质子具有的脉宽短、尺寸小的优点,可用于ICF内爆过程的动态照相。     

Fresnel波带板编码成像技术

 介绍了Fresnel波带板编码成像原理,并设计出高空间分辨率的Fresnel波带板编码相机,用双孔可见光源对编码相机的成像进行了实验研究,表明该编码相机可以真实地还原源区的空间大小和形状。     

Time-dependent electron thermal flux inhibition in direct-drive laser implosions

We simulate direct-drive CH target implosions with square laser pulses by a one-dimensional Fokker-Planck solver combined with a hydrodynamic code, and compare the results with those simulated by the flux-limited Spitzer-H?rm model. We find that the electron thermal flux inhibition is time dependent, resulting in longer density scale length, larger laser absorption, and smaller growth of Rayleigh-Taylor instability. The time of peak neutron production calculated from Fokker-Planck simulations agrees with experiments for both 1-ns and 400-ps pulses.     

X-ray imaging measurements of capsule implosions driven by a Z-pinch dynamic hohlraum

The radiation and shock generated by impact of an annular tungsten Z-pinch plasma on a 10-mm diam 5-mg/cc CH(2) foam are diagnosed with x-ray imaging and power measurements. The radiative shock was virtually unaffected by Z-pinch plasma instabilities. The 5-ns-duration approximately 135-eV radiation field imploded a 2.1-mm-diam CH capsule. The measured radiation temperature, shock radius, and capsule radius agreed well with computer simulations, indicating understanding of the main features of a Z-pinch dynamic-hohlraum-driven capsule implosion.     

基于D3He反应产生的单能质子对ICF内爆过程的照相模拟研究

为实现单能质子对惯性约束聚变(ICF)内爆过程的诊断,使用FLUKA蒙特卡罗程序模拟分析了影响质子对内爆过程诊断的因素。通过设置不同初始质子数目,来分析质子产额对小球图像质量的影响,结果发现,要得到清晰图像,需要109的质子数。参照实验中常用的靶参数,模拟了直接驱动和间接驱动条件下的照相情况,分析了间接驱动情况下,金属腔对质子束的散射会造成内爆小球图像模糊,说明质子照相不适合间接驱动内爆。给出了一种时间分辨的质子照相方法,通过调整质子产生和内爆产生的时间差,实现了对内爆过程前后不同时刻的照相等。模拟表明,D3He反应产生的14.7 MeV的准单能质子具有的脉宽短、尺寸小的优点,可用于ICF内爆过程的动态照相。     

FP-1装置铝套筒内爆动力学过程的一维磁流体力学模拟

作为一种重要的柱面会聚冲击和准等熵压缩加载源,磁驱动固体套筒内爆技术已广泛应用于高能量密度物理实验研究.针对FP-1装置驱动的固体套筒内爆动力学过程,建立了含强度的一维磁流体力学模型,并对典型实验进行了模拟.计算获得的套筒内爆速度同实验结果较为相符.模拟结果显示,该装置在40 kV充压条件下,可以将直径3 cm,厚0.5 mm的铝套筒加速至1.1 km/s,内壁速度超过1.5 km/s,同时保持大部分材料为固体状态.内爆套筒与相同材料靶筒碰撞产生的冲击压力约9 GPa.改变靶筒内部填充气体的压力,可以获得不同的靶筒运动速度、轨迹以及反弹半径,以满足不同类型实验的研究需要.     

Hohlraum-driven ignitionlike double-shell implosions on the omega laser facility

High-convergence ignitionlike double-shell implosion experiments have been performed on the Omega laser facility [T. R. Boehly, Opt. Commun. 133, 495 (1997)] using cylindrical gold hohlraums with 40 drive beams. Repeatable, dominant primary (2.45 MeV) neutron production from the mix-susceptible compressional phase of a double-shell implosion, using fall-line design optimization and exacting fabrication standards, is experimentally inferred from time-resolved core x-ray imaging. Effective control of fuel-pusher mix during final compression is essential for achieving noncryogenic ignition with double-shell targets on the National Ignition Facility [Paisner, Laser Focus World 30, 75 (1994)].     

CO2 laser ablative etching of polyethylene terephthalate

Films of polyethylene terephthalate (PET) can be successfully etched with 9 m radiation from a pulsed TEA CO₂ laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm^–2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.School of Chemistry     

X-ray absorption lines: Signature for preheat level in non-explosive laser implosions

The measured X-ray spectrum from thick glass shells imploded with two TW six-beam laser pulses displayed absorption by transitions of Si ions. This indicates the existence around the time of peak compression of a cooler (? 200 eV) layer surrounding the hot innermost glass layer, of density times thickness ?ΔR ～ 0.6 × 10^-3 g/cm². This temperature is indicative of the preheat level earlier in the implosion.     

Visible regime application of zone plate coded incoherent holography

Unlike conventional optical holography which requires coherent radiation, zone plate coded holography (ZPCH) offers the possibility of encoding 3D information of an incoherent radiation source. But ZPCH with its coherent optical reconstruction (COR) is unsuitable to be used in the visible regime. However, proposed digital ZPCH scheme for visible regime uses digital decoding instead of COR and overcomes this limitation. This paper discusses the issues of visible regime ZPCH and presents experimental results of applying the modified scheme to encode and successfully decode a visible source, for the first time, to our knowledge. In addition to imaging of incoherent radiation sources, visible regime incoherent holography may find use in constructing holograms of live cells or objects, like retina, where coherent illumination can be detrimental.     

Mass-profile and instability-growth measurements for 300-wire Z-pinch implosions driven by 14-18 MA

We present the first comprehensive study of high wire-number, wire-array Z-pinch dynamics at 14-18 MA using x-ray backlighting and optical shadowgraphy diagnostics. The cylindrical arrays retain slowly expanding, dense wire cores at the initial position up to 60% of the total implosion time. Azimuthally correlated instabilities at the array edge appear during this stage which continue to grow in amplitude and wavelength after the start of bulk motion, resulting in measurable trailing mass that does not arrive on axis before peak x-ray emission.     

High-resolution gamma-ray radiography produced by a laser-plasma driven electron source

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.     

Zone plate focused soft X-ray lithography

The electrical activity of a network of mammalian neurons is mapped with a Multi-Transistor Array (MTA) fabricated with extended CMOS technology. The spatial resolution is 7.4 μm on an area of 1 mm² at a sampling frequency of 6 kHz for a complete readout of 16,384 sensor transistors. Action potentials give rise to extracellular voltages with amplitudes in a range of 500 μV. On the basis of the high resolution in space and time, correlation algorithms are used to identify single action potentials with amplitudes as low as about 200μV, and to assign the signals to the activity of individual neurons even in a dense network.     

Micro coded aperture imaging applied to laser plasma diagnosis

We present a coded aperture imaging method which could be applied to the study of microplasmas obtained by laser. Based on the use of an annular aperture associated with a deconvolution processing, this method gives sections of the X-ray and emitted particle tridimensional distribution. The first experimental result shows the range of possibilities offered by this technique.     

All-semiconductor laser driven terahertz time-domain spectrometer

We present a terahertz time-domain spectrometer pumped by an all-semiconductor femtosecond laser system which emits 600 fs long optical pulses at 830 nm. Standard LT-GaAs antennas are used to generate and coherently detect THz radiation with a bandwidth of 1.4 THz. The measured time traces are in good agreement with simulations based on the Drude–Lorentz model. Spectroscopic measurements on polymers with different additive contents as well as THz imaging were carried out to confirm the successful operation of the spectrometer. Our approach holds great potential for the development of cost-effective, rugged and portable THz spectrometers for a variety of applications.