Volume effect of laser produced plasma on X-ray emissions

An investigation of x-ray emission from Cu plasma produced by 1.054 μm Nd:glass laser pulses of 5 ns duration, at 2 × 10¹² − 2 × 10¹³ W cm⁻² is reported. The x-ray emission has been studied as a function of target position with respect to the laser beam focus position. It has been observed that x-ray emissions from ns duration plasma show a volume effect similar to subnanosecond plasmas. Due to this effect the x-ray yield increases when target is moved away relative to the best focal plane of the laser beam. This result supports the theoretical model of Tallents and has also been testified independently using suitably modified theoretical model for our experimental conditions. While above result is in good agreement with similar experimental results obtained for sub-nanosecond laser produced plasmas, it differs from result claiming filamentation rather than pure geometrical effect leading to x-ray enhancement for ns plasmas.     

利用单飞秒激光脉冲驱动类氖钛X射线激光的研究

提出了一种利用单飞秒激光脉冲驱动类氖钛X射线激光的物理方案.利用自相似方法研究了不同脉冲前沿的单飞秒激光辐照钛平板靶产生的类氖钛X射线激光等离子体的特性,得到了电子温度、电子密度和定标长度三者的定标律,讨论了给定输入参数下各定标律曲线的特性.研究表明,利用单个飞秒激光能够实现X射线激光的产生,而且脉冲前沿强度随时间增长平缓的飞秒激光有利于驱动X射线激光.本研究为实验上实现单飞秒激光脉冲驱动X射线激光提供了一种新的方案.     

Investigation on the hydrodynamics of slab x-ray laser plasma by nonuiform line focused laser irradiation

Based on the two-dimensional model,this paper compares the hydrodynamics of slab x-ray laser plasma produced by different nonuniform line focused irradiations.It finds that the average intensity and the duration of laser pulse and the overall shape of the intensity distribution in the focal line have different influences on the plasma.Calculations show that the evolution of temperature variation is more sensitive to the pulse duration and the electron density variation is more sensitive to the pulse intensity.Pulses with duration of 200 ps to 500 ps and with intensity of 0.2 TW/cm 2 to 1.0 TW/cm 2 are proved acceptable in slab x-ray lasers.     

预-主短脉冲激光驱动的高增益类氖氩软X射线激光

 用数值模拟程序计算了脉宽1.5ps预脉冲和主脉冲激光，经1.2ns的时间延迟先后泵浦喷射的密度为6.632×10^-3g/cm³的氩气柱，以获得高增益的类氖离子电子碰撞激发3p-3s跃迁激光的可能性。结果表明：在等离子体中能够形成宽的类氖离子丰度大于50%的区域，并且具有大于10²⁰ cm^-3的自由电子密度。主脉冲到达之后迅速加热自由电子，高密度的自由电子与类氖离子碰撞激发形成具有很高增益系数的增益区，增益区的半高宽度大约100μm，一维理论计算的增益系数为200~300cm^－1，持续时间大约相当于泵浦激光的时间宽度，在1~2ps之间。     

激光辐照固体靶产生等离子体反冲研究

提出一种通过诊断等离子体反冲动量来计算激光加载产生冲击压强的方法. 当强激光辐照固体靶表面时, 所产生的高速喷射的等离子体对靶具有反冲作用, 通过诊断等离子体反冲动量的变化可以计算激光辐照固体靶产生的冲击压强变化. 本文利用辐射流体力学软件研究了这种诊断方法, 模拟采用的激光功率密度为5×10¹²-5×10¹³ W/cm², 激光脉宽选取纳秒量级. 模拟结果表明该方法是有效且可行的.     

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.     

Resonantly photo-pumped Fe16+ soft X-ray laser

A resonantly photo-pumped soft x-ray laser process is studied for neon-like iron ions with an automatic line match scheme. The special advantages of this scheme are shown. The temporal characteristics of lasing gains and their dependences on the pump strength and electron density are investigated. Unlike the electron-collisional excitation and some other photo-pumped excitation schemes, it is demonstrated that main photopumped gains come from 2p-4d-3p-3s transition channels. Using the experimental data of pump strengths from a non-LTE plasma produced in our experiment as input data, our model yields a peak gain of 5.6 cm^–1, 2.8 cm^–1 and 1.9 cm^–1 on the dominant lasing transitions at 38.74 nm, 25.47 nm and 20.46 nm.     

Investigations of the mega-ampere multiwire X pinch

The results of experimental investigations into the dynamics of a plasma produced in the multiwire X pinch at currents up to 2.3 MA are presented. The materials, diameters, and the number of wires are varied. At such currents, the power of the soft x-ray radiation with the photon energy from 1 to 2 keV increases to 120 GW, and, since the size of a hot spot is less than 20 μm, it corresponds to a source brightness of ～10¹⁵ W/(cm² sr). The energy recorded in lines of neon-like molybdenum (in the range of 2.5–3 keV) is higher than 10 J. Hard x-ray radiation detected in experiments with tungsten and molybdenum X pinches has the photon energy ≥800 keV.     

某些类氖离子和类镍离子等离子体状态对应关系的研究

利用一维拉格朗日流体动力学程序Med103详细研究了产生类镍Ag,Cd,In,Sn, Sb离子和类氖Fe,Co,Ni,Cu,Zn离子的等离子体状态.通过对电子温度、离子丰度、离子数密度等状态参量的分析比较，得到了这两类等离子体状态之间的对应关系.即类镍银的等离子体状态与类氖铁 的相近，类镍镉的与类氖钴的相近，依次类推.利用这一对应关系可以方便地由类氖机理x射线激光的等离子体状态来对相应元素的类镍x射线激光的等离子体状态进行判断. 关键词： 类氖 类镍 等离子体状态 x射线激光     

强激光束与空腔靶相互作用实验研究

1.06μm波长的强激光束辐照Au材料制作的空腔靶,采用目前国内最先进的诊断设备。对腔内高温等离子体现象演变规律进行了实验观察,获得了反射激光、能量吸收、X光转换、亚千X光能谱及时空特性、辐射温度、超热电子等重要物理信息,并就实验结果作了必要的分析和讨论     

Evolution Characteristics of the Laser-Plasma Electron Temperature of Heavy Metal Cr in Liquid Matrix

Using the nanosecond single-pulse laser-induced breakdown-spectroscopy (LIBS) technique, we determine the LIBS spectrum of a mixed solution of soluble compounds as a sample. Based on the LIBS measurement system established and the best experimental parameters, we measure the plasma emission spectrum of some lines of the Cr element. When the ICCD gate delay is measured from 500 to 2500 ns every 500 ns, and the liquid sample flow rate is set in the range of 35 ml · min^?1?55 ml · min^?1 with an interval of 5 ml · min^?1, the partial emission line of the Cr element in the laser plasma is measured, and the Boltzmann oblique line is drawn after the integrated intensity obtained. We calculate the evolution characteristics of the laser-plasma electron temperature with experimental parameters. The results show that the electron temperature values obtained from the intensity of the spectral lines of different elements are consistent with each other, indicating the reliability of the experimental measurement data. The range of electron temperature is obtained from the intensity of the Cr element line in the plasma range from 4071.0 K to 5628.4 K. As the flow rate continues to increase, the plasma electron temperature also increases but gradually becomes gentle. We select Cr 357.87 nm as the analytical line in the experiment and measure the particle density ratios of the Cr element in the ground state at different ICCD gate delays and sample flow rates. It provides experimental parameter support for further LIBS quantitative analysis of trace heavy metal elements in a liquid matrix.

     

Measurement of the electron temperature via incoherent emission from thin-foil laser-produced plasmas

Summary Time-resolved images of the rear surface of thin metallic foils irradiated by 5 ns pulses of Nd-glass laser have been recorded within a spectral window from 5800 ? to 6900 ?. By approximating the plasma to a blackbody, the recorded intensities have been converted into temperature giving electron temperature up to 10⁸ K and the results are compared with the predictions of the computer code, MEDUSA. The authors of this paper have agreed to not receive the proofs for correction.     

激光等离子体温度测量方法设计

为了获得高时间和空间分辨率的激光等离子体温度测量结果,基于等离子体温度测量的二谱线法,设计了一种激光等离子体温度测量方法。介绍了该方法的测量原理及时间空间分辨率实现方法。利用这种方法获得了空间分辨率微米量级、时间分辨率纳秒量级的激光等离子体温度测量结果,测量得到的激光等离子体温度为10^4K量级。实验结果表明,该方法能够获得具有很高时间空间分辨率的温度测量结果,具有较好的可行性。     

Analysis of characteristic X-ray generation induced by laser plasma electrons accelerated by an electric field

The results of experiments devoted to the study of spectral, spatial, and time characteristics of a spectrally bright point x-ray source based on a vacuum diode with a laser-plasma cathode and a titanium needle anode with a photon energy approximately equal to 4.5 keV are presented. The experimental results revealed a considerable difference between the electron emission from laser plasma in a strong electric field and the explosive electron emission and demonstrated the effectiveness of laser plasma as an electron source. The optimization of the laser radiation power density, the accelerating voltage, and the interelectrode spacing made it possible to create a point x-ray source whose spectral brightness exceeds available sources in the class of small-size pulse x-ray instruments (tubes with explosive cathodes). It has been proved experimentally that the maximum contrast of the characteristic lines of the anode material is attained in the case of an optimal choice of accelerating voltage. The x-ray source has the following parameters: (1) spectral brightness of the K-lines of titanium of the order of 10²¹ photons/cm² s sr keV; (2) emitting region size of 250 mm; and (3) laser pulse duration less than 20 ns.     

高强度二倍频激光辐照银薄膜靶的烧蚀和X光辐射实验研究

激光聚变物理实验中,背光透视照相是靶丸内爆动力学过程观测的重要方法. Ag是一种重要的背光材料, 激光辐照产生的等离子体可以产生强L线辐射, 研究其烧蚀和辐射特性, 对提高内爆靶丸背光透视照相的图像质量具有十分重要的意义.在神光II装置上, 采用第九路输出的2 ns, ～ 5× 10¹⁴ W/cm², 526.5 nm激光均匀辐照Ag薄膜靶, 实验研究了其烧蚀特性, 获得了银薄膜靶在激光烧蚀驱动下的飞行轨迹和飞行速度的数据. 实验结果与一维辐射流体力学模拟结果相符. 火箭模型对实验数据进行拟合, 得到了Ag材料的质量烧蚀速率和烧蚀压的数据. 采用平面晶体谱仪和X射线二极管探测器阵列观测等离子体的辐射特性, 获得了Ag等离子体辐射光谱和L线转换效率, 实验结果对激光聚变内爆靶丸背光照相的实验设计具有重要的参考价值.     

Characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode

Temporal, spatial and spectral characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode are presented. Electrons from a laser-produced aluminium plasma were accelerated towards a conical point tip titanium anode to generate K-shell x-ray radiation. Approximately 10₁₀ photons/pulse were generated in x-ray pulses of ∼18 to ∼28 ns duration from a source of ∼300 μm diameter, athυ = 4.51 keV (K _α emission of titanium), with a brightness of ∼10²⁰ photons/cm²/s/sr. This was sufficient to record single-shot x-ray radiographs of physical objects on a DEF-5 x-ray film kept at a distance of up to ∼10 cm.     

Scaling of x-ray emission and ion velocity in laser produced Cu plasmas

The x-ray emission from slab targets of copper irradiated by Nd:glass laser (1.054 μm, 5 and 15 ns) at intensities between 10¹² and 10¹¹W/cm² has been studied. The x-ray emissions were monitored with the help of high quantum efficiency x-ray silicon photo diodes and vacuum photo diodes, all covered with aluminium filters of different thickness. The x-ray intensity vs the laser intensity has a scaling factor of (1.2–1.92). The relative x-ray conversion efficiency follows an empirical relationship which is in close agreement with the one reported by Babonneau et al. The ion velocities were monitored using Langmuir probes placed at different angles and radial distances from the target position. The variation of the ion velocity with the laser intensity follows a scaling of the form Φ ^β where β ∼0.22 which is in good agreement with the reported scaling factor values. The results on the x-ray emission from Cu plasma are reported.     

Non-uniform Influence of Prepulse Duration on a Nickel-Like Mo X-Ray Laser in XLIII

An experiment on a transient nickel-like Mo x-ray laser in the extreme light III (XL III) laser facility is analyzed, based on the two-dimensional hydrodynamic evolutions of a plasma under a non-uniform incident laser. Influences of the pulse duration and intensity on plasma scale length, electron density, temperature, as well as their distributions are investigated, based on which the pre-pulse character and delay time are determined according to the parameters of laser line focus on XLIII. It is found that the optimal intensity of the pre-pulse is 1.0TW/cm² with a duration of 500ps; a well modified pre-plasma can be obtained after 1.6ns under low quality line focus in the lab.     

利用x射线激光干涉诊断等离子体电子密度

x射线激光探针干涉方法是诊断高温高密度激光等离子体电子密度等信息的重要工具.利用神光Ⅱ装置输出激光驱动的类镍-银x射线激光作为探针，成功地进行了马赫-曾德尔干涉法诊断实验，获得了清晰的包含等离子体信息的动态干涉条纹图像，并据此给出了待测C8H8等离子体临界面附近电子密度的空间分布. 关键词： 等离子体电子密度诊断 x射线激光 马赫-曾德尔干涉仪     

The role of microwaves in the enhancement of laser-induced plasma emission

We studied experimentally the effect of microwaves (MWs) on the enhancement of plasma emission achieved by laser-induced breakdown spectroscopy (LIBS). A laser plasma was generated on a calcium oxide pellet by a Nd:YAG laser (5 mJ, 532 nm, 8 ns) in reduced-pressure argon surrounding gas. A MW radiation (400 W) was injected into the laser plasma via a loop antenna placed immediately above the laser plasma to enhance the plasma emission. The results confirmed that when the electromagnetic field was introduced into the laser plasma region by the MWs, the lifetime of the plasma was extended from 50 to 500 µs, similar to the MW duration. Furthermore, the plasma temperature and electron density increased to approximately 10900 K and 1.5×10¹⁸ cm^-3, respectively and the size of the plasma emission was extended to 15 mm in diameter. As a result, the emission intensity of Ca lines obtained using LIBS with MWs was enhanced by approximately 200 times compared to the case of LIBS without MWs.