首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Plasma jets were produced using a high repetition rate laser by laser-ablation of coatings on the surface of conical impressions machined into solid blocks of an aluminium alloy. The ablating plasmas emerged into background gases generating shock waves. The jet-shock system was diagnosed using interferometry. The use of a high repetition rate laser allowed examination of a large number of combinations of jet materials, background gases and gas pressures.  相似文献   

2.
In this paper, recent results obtained on highly radiative shocks generated in a xenon filled gas cell using the GEKKO XII laser facility are presented. Data show extremely high shock velocity (??150 km/s) never achieved before in gas. Preliminary analyses based on theoretical dimensionless numbers and numerical simulations suggest that these radiative shocks reach a new radiative regime where the radiative pressure plays a role in the dynamics and structure of the shock. A major effect observed is a strong anisotropic emission in the downstream gas. This unexpected feature is discussed and compared to available 2D radiation hydrodynamic simulations.  相似文献   

3.
We present the results of experiments in which jets are created through the collision of two laser-produced plasmas. These experiments use a simple ‘v-foil’ target design: two thin foils are placed at an angle of 140° to each other, and irradiated with a high-energy laser. The plasmas from the rear face of these foils collide and drive plasma jets moving with a velocity of ～300 km?s?1. By choosing the foil thickness and material to suit the laser conditions available, it has proven possible to create plasma jets for which the relevant scaling parameters show significant overlap with those of outflows associated with young stellar objects (YSOs). Preliminary results are also shown from experiments to study the effect of an ambient gas on jet propagation. Nominally identical experiments are conducted either in vacuum or in an ambient medium of 5 mbar of nitrogen gas. The gas is seen to increase the jet collimation, and to introduce shock structures at the head of the outflow.  相似文献   

4.
We have performed a series of experiments examining the properties of high Mach number blast waves. Experiments were conducted on the Z-Beamlet laser at Sandia National Laboratories. We created blast waves in the laboratory by using ∼1000 J laser pulses to illuminate millimeter scale solid targets immersed in gas. Our experiments studied the validity of theories forwarded by Ryu and Vishniac (1987, 1991) and Vishniac (1983) to explain the dynamics of perturbations on astrophysical blast waves. These experiments consisted of a systematic scan of the decay rates of perturbations of known primary mode number induced on the surface of blast waves by means of a regularly spaced wire array. The amplitude of the induced perturbations relative to the radius of the blast wave was tracked and fit to a power law in time. Measurements were taken for a number of different mode numbers in a mixed gas consisting of 7.5 Torr xenon and 2.5 Torr nitrogen and the results are compared to theoretical predictions. It is found that two of the three mode numbers imply one polytropic index while the third case, which is the most complicated for several reasons, implies a higher polytropic index.  相似文献   

5.
We present experimental results of plasma jet, interacted with an ambient medium, using intense lasers to investigate the complex features of astrophysical jets. This experiment was performed in France at the LULI facility, Ecole Polytechnique, using one long pulse laser to generate the jet and a short pulse laser to probe it by proton radiography. A foam filled cone target was used to generate high velocity plasma jet, and a gas jet nozzle produced the well known ambient medium. Using visible pyrometry and interferometry, we were able to measure the jet velocity and electronic density. We get a panel of measurements at various gas density and time delay. From these measurements, we could underline the growth of a perturbed shape of the jet interaction with the ambient medium. The reason of this last observation is still in debate and will be presented in the article.  相似文献   

6.
We describe experiments that investigate the capability of an experimental platform, based on laser-driven blast waves created in a medium of atomic clusters, to produce results that can be scaled to astrophysical situations. Quantitative electron density profiles were obtained for blast waves produced in hydrogen, argon, krypton and xenon through the interaction of a high intensity (I ≈ 1017 Wcm−2), sub-ps laser pulse. From this we estimate the local post-shock temperature, compressibility, shock strength and adiabatic index for each gas. Direct comparisons between blast wave structures for consistent relative gas densities were achieved through careful gas jet parameter control. From these we investigate the applicability of different radiative and Sedov-Taylor self-similar solutions, and therefore the (ρ,T) phase space that we can currently access.  相似文献   

7.
The physics of inertial confinement fusion is reviewed. The trend to short-wavelength lasers is argued, and the distinction between direct and indirect (soft X-ray) drive is made. Key present issues include the non-linear growth of Rayleigh–Taylor (R–T) instabilities, the seeding of this instability by the initial laser imprint, the relevance of self-generated magnetic fields, and the importance of parametric instabilities (stimulated Brillouin and Raman scattering) in gas-filled hohlraums. Experiments are reviewed which explore the R–T instability in both planar and converging geometry. The employment of various optical smoothing techniques is contrasted with the overcoating of the capsule by gold coated plastic foams to reduce considerably the imprint problem. The role of spontaneously generated magnetic fields in non-symmetric plasmas is discussed. Recent hohlraum compression results are presented together with gas bag targets which replicate the long-scale-length low density plasmas expected in NIF gas filled hohlraums. The onset of first Brillouin and then Raman scattering is observed. The fast ignitor scheme is a proposal to use an intense short pulse laser to drill a hole through the coronal plasma and then, with laser excited fast electrons, create a propagating thermonuclear spark in a dense, relatively cold laser-compressed target. Some preliminary results of laser hole drilling and 2-D and 3-D PIC simulations of this and the > 108 Gauss self-generated magnetic fields are presented. The proposed National Ignition Facility (NIF) is described.  相似文献   

8.
We review the recent developments of laser pair creation in the laboratory and their potential applications to astrophysics and other frontiers. Many astrophysical phenomena involving e+e− plasmas may be systematically investigated in the laboratory setting. We also discuss potential applications of dense positronium gas.  相似文献   

9.
Sometimes the most beautiful things are the hardest to understand. Pillars like those of the Eagle Nebula form at the boundary between some of the hottest (10000~K) and coldest (10~K) gas in the Galaxy. Many physical processes come into play in the birth and growth of such gaseous pillars: hydrodynamic instability, photoionization, ablation, recombination, molecular heating and cooling, and probably magnetic fields. High-quality astronomical observations, quantitative numerical simulations, and scaled laser experiments provide a powerful combination for understanding their formation and evolution. We put our most recent hydrodynamic model to the test, by creating simulated observations from it and comparing them directly to the actual radioastronomical observations. Successfully reproducing major characteristics of the observations in this manner is an important step in designing appropriate laser experiments.  相似文献   

10.
We discuss the design of jet-driven, radiative-blast-wave experiments for a 10 kJ class pulsed laser facility. The astrophysical motivation is the fact that jets from Young Stellar Objects are typically radiative and that the resulting radiative bow shocks produce complex structure that is difficult to predict. To drive a radiative bow shock, the jet velocity must exceed the threshold for strong radiative effects. Using a 10 kJ class laser, it is possible to produce such a jet that can drive a radiative bow shock in gas that is dense enough to permit diagnosis by x-ray radiography. We describe the design and simulations of such experiments. The basic approach is to shock the jet material and then accelerate it through a collimating hole and into a Xe ambient medium. We identify issues that must be addressed through experimentation or further simulations in order to field successful experiments.  相似文献   

11.
Supernovae launch spherical shocks into the circumstellar medium (CSM). These shocks have high Mach numbers and may be radiative. We have created similar shocks in the laboratory by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas; ablated material from the pin rapidly expands and launches a shock through the surrounding gas. Laser pulses were typically 5 ns in duration with ablative energies ranging from 1–150 J. Shocks in ambient gas pressures of ~1 kPa were observed at spatial scales of up to 5 cm using optical cameras with schlieren. Emission spectroscopy data were obtained to infer electron temperatures (< 10 eV). In this experiment we have observed a new phenomena; at the edge of the radiatively heated gas ahead of the shock, a second shock forms. The two expanding shocks are simultaneously visible for a time, until the original shock stalls from running into the heated gas. The second shock remains visible and continues to expand. A minimum condition for the formation of the second shock is that the original shock is super-critical, i.e., the temperature distribution ahead of the original shock has an inflexion point. In a non-radiative control experiment the second shock does not form. We hypothesize that a second shock could form in the astrophysical case, possibly in radiative supernova remnants such as SN1993J, or in shock-CSM interaction.  相似文献   

12.
A series of experiments is underway using the Omega laser to examine radiative shocks of astrophysical relevance. In these experiments, the laser accelerates a thin layer of low-Z material, which drives a strong shock into xenon gas. One-dimensional numerical simulations using the HYADES radiation hydrodynamics code predict that radiation cooling will cause the shocked xenon to collapse spatially, producing a thin layer of high density (i.e., a collapsed shock). Preliminary experimental results show a less opaque layer of shocked xenon than would be expected assuming that all the xenon accumulates in the layer and that the X-ray source is a pure Kα source. However, neither of these assumptions is strictly correct. Here we explore whether radial mass and/or energy transport may be significant to the dynamics of the system. We report the results of two-dimensional numerical simulations using the ZEUS-2D astrophysical fluid dynamics code. Particular attention is given to the simulation method.  相似文献   

13.
Abstract— Mineral particles analogous to components of cosmic dust were tested to determine if their Raman signatures can be recognized after hypervelocity capture in aerogel. The mineral particles were accelerated onto the silica aerogel by light‐gas‐gun shots. It was found that all the individual minerals captured in aerogel could be identified using Raman (or fluorescence) spectra. The laser beam spot size was ?5 micrometers, and in some cases the captured particles were of a similar small size. In some samples fired into aerogel, a broadening and a shift in the wave numbers of some of the Raman bands was observed, a result of the trapped particles being at elevated temperatures due to laser heating. Temperatures of samples were also estimated from the relative intensities of Stokes and anti‐Stokes Raman bands, or, in the case of corundum particles, from the wave number of fluorescence bands excited by the laser. The temperature varied greatly, dependent upon laser power and the nature of the particle. Most of the mineral particles examined had temperatures below 200 °C at a laser power of about 3 mW at the sample. This temperature is sufficiently low enough not to damage most materials expected to be found captured in aerogel in space. In the worst case, some particles were shown to have temperatures of 500–700 °C. In addition, selected meteorite samples were examined to obtain Raman signatures of their constituent minerals and were then shot into aerogel. It was possible to find Raman signatures after capture in aerogel and obtain a Raman map of a whole grain in situ in the aerogel. It is concluded that Raman analysis is indeed well suited for an in situ analysis of micrometer‐sized materials captured in aerogel.  相似文献   

14.
Recent experiments in a gas embedded compressional Z-pinch are presented. The experiments have been carried out in H2 and D2, using a pulse power generator capable of delivering a dI/dt 1012 A/s. The pinch is initiated by a focused laser pulse, which is coaxial with a cylindrical DC microdischarge. This configuration results in double column pinch at early times, which at current rise evolves into a gas embedded compressional Z-pinch. Diagnostics used are Rogowskii coil, single frame holographic interferometry and holographic shadowgraphy, visible streak camera images from which, current, density, line density, pinch radius and plasma motion are obtained. The pinch is characterised by a maximum on axis density which is much higher than the expected value from filling pressure, with a Bennett temperature of 75 eV at 180 kA.  相似文献   

15.
窄线宽激光器在铯原子喷泉钟、光钟以及时间频率传递等方面有着重要的作用。主要介绍中国科学院国家授时中心开展的工作波长为698 nm窄线宽激光器的研究进展。实验中采用两级稳频方法将激光锁到初稳腔和高稳腔。通过第一级稳频,激光线宽为1.1 kHz;目前已初步实现了第二级稳频。通过逐渐优化第二级稳频,可以实现Hz量级线宽激光。  相似文献   

16.
Collisions between shocks are commonly found in many astrophysical objects, however robust numerical models or laboratory analogues of these complex systems remain challenging to implement. We report on the development of scaled laboratory experiments which employ new techniques for launching and diagnosing colliding shocks and high Mach number blast waves, scalable to a limited subset of astrophysically-relevant regimes. Use of an extended medium of atomic clusters enables efficient (>80%) coupling of 700 fs, 1 J, 1054 nm laser pulses to a “cluster” gas with an average density of ≈1019 particles cm−3, producing an initial energy density >105 J cm−3, equivalent to ≈5×109 J/g. Multiple laser foci are used to tailor the spatial profile of energy deposition, or to launch pairs of counter-propagating cylindrical shocks which then collide. By probing the collision interferometrically at multiple view angles in 5^ increments and applying an inverse Radon transform to the resulting phase projections we have been able to tomographicall reconstruct the full three-dimensional, time-framed electron density profile of the system.  相似文献   

17.
Methane spectral features in the visible to near-IR region are prominent in the spectra of the outer planets but laboratory data for the appropriate methane conditions are required to interpret the observational data. By use of the intracavity laser spectroscopy technique, a moderately high resolution (500,000) absorption spectrum of the 727 nm band of methane at 77 K is obtained. The methane absorption bands in the visible to near-IR region are very weak, but intracavity laser spectroscopy provides sufficient sensitivity to perform the measurements and to extract quantitative data for methane at low temperatures. Absorption coefficients are determined and are reported as averages at one Å intervals throughout the region 7127–7420 Å. By integrating over the band, an intensity of 753 cm–1 km–1 am–1 is obtained. The results compare well with previous low resolution measurements on methane at room temperature, with gas phase results calculated using the absorption spectrum of liquid methane, and with absorption coefficients derived from methane features observed in the spectra of the outer planets and Titan.  相似文献   

18.
介绍了2010年上海天文台卫星激光测距的常规观测和白天千赫兹激光测距情况。在国际上首次对同步轨道卫星开展了千赫兹激光测距,作用距离达38800多千米,测距系统性能达到国际先进水平。利用2套独立的激光测距系统,开展了激光收发分离测距实验,为行星际异步应答式激光测距模拟试验提供了实测数据。最后概述了空间碎片目标激光测距进展情况。  相似文献   

19.
月球激光测距是国内外所瞩目的宏伟目标 ,代表着单光子探测技术的高峰。本论文的目的是探索提高激光测月回波光子数的新方法 ,进而增加激光测月成功的几率。其思路是源于一个新的想法 :在激光测月过程中引入大气波前倾斜量实时补偿的技术。首先介绍激光测月的现状和其难度所在 :回波光子数太少 ,基本上属于亚光子探测范畴。在现有技术条件下 ,本文对影响激光测月回波光子数的因素逐一进行分析讨论 ,提出应该把激光束截面能量分布和大气湍流效应包括进去。为此分析讨论了大气湍流和大气中光场的基本统计性质、激光束在大气中传输时所受大气湍流的影响以及大气湍流对激光测距的影响 ,得出大气湍流特别对激光测月有着明显影响的结论。在此基础上对传统的激光测距方程进行了修正 ,使其应用到激光测月时更符合真实的情况 ,从而指导补偿的进行。涉及到在激光测月中对受大气湍流而畸变的激光束进行补偿 ,本文抓住重点 ,通过分析看出对大气倾斜量的实时补偿是提高激光测月回波光子数的重要因素。结合激光测月以及云南天文台现有测距系统的实际情况 ,本文独创性地提出利用激光测月目标近旁一定大小区域的扩展面源探测与计算大气倾斜量 ,然后对激光测月中的激光束进行大气倾斜量实时补偿的新技术方法。在分析比较  相似文献   

20.
657nm外腔半导体激光器的实现   总被引：2，自引：0，他引：2  
对一种Littman-Metcalf外腔结构半导体激光器的实现过程作了报道.利用这种结构的光反馈特性,使原来自由运转时波长在660nm附近的半导体激光器在常温下能够调节到657nm附近,并且把激光线宽从原来的30MHz压窄到100kHz左右,连续调节范围大约为10GHz.这种窄线宽的激光器将用在钙原子Ramsey光谱的实验中.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号