Laser-based optical emission studies of barium plasma

We present the optical emission characteristics of the barium plasma produced at the surface of barium hydroxide Ba(OH)₂, also known as baryta, generated by the first harmonic (1,064 nm) of a Q-switched Nd:YAG laser. The laser beam was focused on target material by placing it in air at atmospheric pressure. The experimentally observed line profiles of neutral barium have been used to extract the electron temperature using the Boltzmann plot method, whereas the electron number density has been determined from the Stark broadening. The electron temperature is calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser energy. Besides, we have studied the variation of number density as a function of laser energy as well as its variation with distance from the target surface. It is observed that electron temperature and electron number density increase as laser energy increases.     

Optical emission studies of sulphur plasma using laser induced breakdown spectroscopy

We present the optical emission studies of sulphur (S) plasma generated by the first (1064 nm) and second (532 nm) wavelengths of a Q-switched Nd:YAG laser. The target material was placed in front of laser beam in air at atmospheric pressure. The experimentally observed line profiles of neutral sulphur have been used to extract the electron temperature (T _e ) using the Boltzmann plot method, whereas the electron number density (N _e ) has been determined from the Stark broadening. The electron temperature is calculated by varying, distance from, the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. Beside we have studied the variation of number density as a function of laser irradiance as well as its variation with distance from the target surface. It is observed that electron temperature and electron number density increases as laser irradiance is increased.     

Spectroscopic studies of sodium nitrate plasma produced by nanosecond laser ablation

We present the optical emission characteristics of the sodium plasma produced at the surface of sodium nitrate (NaNO₃) also known as Chile saltpeter. We used a Q-switched Nd:YAG (Quantel Brilliant) pulsed laser having a pulse duration of 5?ns and 10?Hz repetition rate which is capable of delivering 400?mJ at 1064?nm and 200?mJ at 532?nm. The target material was placed in front of laser beam in air (atmospheric pressure). The experimentally observed line profiles of neutral sodium have been used to extract the electron temperature using the Boltzmann plot method, whereas the electron number density has been determined from the Stark broadening. The electron temperature is calculated by varying the distance from the target surface along the line of propagation of the plasma plume and also by varying the laser irradiance. Besides, we have studied the variation of number density as a function of laser irradiance as well as its variation with the distance from the target surface. It is observed that electron temperature and electron number density increase as the laser irradiance is increased.     

Spatial and time resolved analysis of CN bands in the laser induced plasma from graphite

Analysis of the emission bands of the CN molecules in the plasma generated from a graphite target irradiated with 1.06 μm radiation pulses from a Q-switched Nd:YAG laser has been done. Depending on the position of the sampled volume of the plasma plume, the intensity distribution in the emission spectra is found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as function of distance from the target for different time delays with respect to the incidence of the laser pulse. The translational temperature calculated from time of flight is found to be higher than the observed vibrational temperature for CN molecules and the reason for this is explained.     

激光脉冲对内层等离子体辐射源的影响

本文的基本思想是设计双层金铝薄膜靶以检测激光脉冲宽度与等离子体消融深度的关系,找出有效的等离子体加热方法以产生更强更亮的等离子体辐射源.由于有预脉冲激光的存在,表层金薄膜首先被消融,由主脉冲携带的大能量就能较易穿过表层金等离子体将能量聚焦在内层铝靶上,由此产生内层高温等离子体.又由于外层低温等离子体存在,其将有效的阻碍内高温等离子体因膨胀而引起的能量损失.对无预脉冲而言,直接入射激光能量都沉积在靶表层形成表层高温等离子体.但是激光直接入射而产生的等离子体辐射总强度只比由预脉冲情况下产生的金等离子体辐射强度增加15%.而预脉冲能量只占激光总能量的2%.实验结果显示Al光谱线主要来自类氢,类氦离子跃迁.Au等离子体光谱线主要来自它的N带,O带和P带谱.我们也观察到一个明显的软X射线短波发射极限.所有结果显示由于预脉冲的存在将对靶各层等离子体辐射产生极大的影响     

真空环境下激光烧蚀铝靶冲量耦合系数的数值模拟

 通过分析不同情况下激光与固态靶、气化物质的作用机理,利用激光体烧蚀模型,采用流体力学理论和1维Lagrange有限差分的计算方法,对真空条件下不同激光参数下气化物质对靶产生冲量的过程进行了数值模拟,模拟计算结果与实验测量结果、Phipps定标关系符合较好。计算结果表明,在等离子体的情况下冲量耦合系数随着激光强度增大而减小。     

激光直接加热自背光法辐射不透明度测量方法探索

本文提出一种基于单束激光直接加热多层平面靶开展 稠密等离子体辐射不透明度特性研究的靶物理设计并对其进行了实验验证. 在XG-II激光装置上, 采用三倍频束匀滑激光辐照Au/CH/Al/CH多层平面靶产生背光源和Al样品等离子体, 通过观测背光源经样品等离子体衰减后的透过谱得到样品等离子体的辐射吸收性质. 采用Multi-1D程序对激光加热多层靶进行了辐射流体力学数值模拟, 给出了样品等离子体状态及其时间演化过程. 利用细致谱项模型 (DTA) 对实验测量的Al等离子体吸收谱进行理论分析, 表明等离子体温度在20–70 eV之间, 该结果与辐射流体力学模拟结果基本一致. 关键词： 吸收光谱 自背光 激光等离子体     

The role of radiative transfer in pulsed laser plasma-target interactions

The plasma generated when a 2024 aluminum target was irradiated with the beam from a cold cathode electron beam laser (CCEBL) has been investigated with spectroscopic techniques. The target was designed to form the end plate of a differentially pumped slit assembly attached to a McPherson Model 225 visible-u.v.-v.u.v. monochromator. A pinhole in the center of the plate permitted plasma radiation to enter the monochromator from the back side of the target. Calibration with tungsten, deuterium and argon standards permitted quantitative plasma radiance measurements to be made in the region 115–600 nm using a range of diffraction gratings and photomultiplier tubes. The total radiant fluence observed (1 J/cm²) was in reasonable agreement with the 1-1.2J/cm² of absorbed thermal fluence determined by independent thermal coupling measurements. Photographic spectra of the plasma (100–620 nm) were recorded and many features identified.     

Analysis of short-wavelength charge-exchange lasers via plasma-gas beam clashing

The coupled particle and radiation rate equations have been solved for the population inversion and laser intensity which result when a plasma is incident on a vapor target. For the specific case of a helium plasma incident on a cesium vapor target, a laser intensity of order 10⁶W/cm² at 584A?is calculated. The numerical results are in basic agreement with a simple analytical model. Experimental considerations and progress are discussed.     

Optical spectroscopic studies of titanium plasma produced by an Nd : YAG laser

We present optical emission characteristics of the titanium plasma produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q-switched Nd: YAG laser using laser induced breakdown spectroscopy (LIBS). The experimentally observed line profiles of neutral titanium (Ti I) have been used to extract the electron temperature (T _e ) using the Boltzmann plot method. The electron number density (N _e ) is calculated using the Stark broadening profile of 368.73 nm spectral line. Beside we have studied the spatial variation of electron temperature and number density as a function of laser energy for titanium plasma by placing the target material in air (at atmospheric pressure). We have determined the electron temperature and the electron number density along the axial position of the plasma plume.     

不同气压背景下激光烧蚀Al靶产生等离子体特性分析

用时间和空间分辨诊断技术研究了脉冲激光烧蚀不同气压环境下金属Ａｌ靶过程中产生的等离子体羽的特性。     

用斜入射方法计算碳纤维靶的激光增益

 用斜入射方法并考虑了激光被折射和P极化激光的共振吸收,计算了碳纤维靶的等离子体状态和激光增益。激光斜入射获得的类H碳的n=2 、3 跃迁的激光增益系数,比按垂直入射的计算的值略大,但是,增益系数的空间分布变得更窄。激光增益发生在更靠近靶的外表层。     

激光辐射半圆柱凹槽靶产生等离子体特点

在利用激光产生的等离子体作为增益介质,实现X射线激光器的研究中,等离子体的密度分布形状及其时间演化起着重要作用,它不仅决定了等离子体中的原子分子过程,还决定了X射线的传输特性。在一般的软X射线自发辐射放大的实验里,如线聚焦激光辐照薄箔实验,在脉宽为数十微微秒,功率密度约为10~(14)W/cm~2的高功率激光照射下,产生的靶等离     

Plasma line emission from short pulse laser interactions with dense plasmas

Plasma line emission is observed in simulations of dense plasma irradiated by moderately intense light pulses of duration some tens of femtoseconds, and its scaling with density and with laser intensity is studied. Plasma emission is recorded both during the pulse where it is observed against the background spectrum of harmonics of the laser frequency as well as postpulse. Harmonics of the plasma line up to the fifth have been observed. An unexpected feature present in most of the reflected light spectra appears on the blue side of the plasma line with a central frequency omega approximately 1.5omega(p).     

Experimental investigation of stochastic pulsation and formation of light bullets with megagauss magnetic fields by an intense laser pulse propagating in a preionized plasma

The generation of extremely stable light bullets in a preformed plasma near critical density has been observed experimentally during the interaction of intense picosecond laser beam with a metallic target in air. Optical probing measurements indicate the formation of pulsating channels, typically of about 5 microm in diameter, directed towards a heating laser beam, as well as of disconnected massive plasma blocks moving also towards the laser beam. The velocities of the dense plasma blocks reach the values of 4.5x10(8) cm/s. The blocks are stable during their acceleration and propagation in air. Self-generated magnetic fields up to 4-7 MG were observed by means of the Faraday rotation of a probe laser beam.     

强激光辐照凹槽靶引起的高阶电离离子的丰度增强

高功率激光辐照固体靶时,靶面上的激光辐射场基本上可以与原子内壳层的电子束缚场相比拟。激光迅速地将元素大部分电子剥离从而产生高温高密度的等离子体并发射丰富的软X射线谱。当激光功率进一步提高时,等离子体的离子将进一步地被剥离、处激发态上的电子可能进一步被激发到更高的能级上并发射更为丰富、更强的X射线。     

Recombination effects during the laser-produced plasma expansion

The precise characterization of plasmas generated by laser irradiation is needed for the development of ion sources. As the characteristic parameters of the expanding plasma vary with both the distance and the time, an experimental study of their evolution is appropriate for a deeper knowledge of the plasma. The purpose of this work is to study the same characteristics of laser plasma produced by ablation of a pure Cu target as a function of the distance from the target along the propagation axis of the plasma plume. We irradiated the target by a KrF laser and a lens of 15 cm focal length. As the diagnostic system, a small Faraday cup array and an axial Faraday cup were utilized to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed due to the recombination of charged species, which occurs within a critical distance, relatively close to the target, where the plasma density is high enough. The critical distance was determined for different laser fluences; beyond the critical distance, the collisions among plasma particles are negligible and the ion charge remains frozen. It was observed that the critical distance increases as the laser fluence increases.     

Fractional ionization in plasmas produced by pulsed laser ablation

A pulsed infrared laser (Q-switched Nd:Yag) is employed to irradiate different metal targets having atomic number from Z =13 up to Z =82. The high laser fluence deposited on the metals, of the order of 100 v J/cm, produces high ablation yield and a plasma generation at the target surface. The emitted species are neutral and ionized atoms. Both components have been investigated in terms of yield emission, time-of-flight measurements and angular distribution. Results indicate that the main emission occurs mainly along the normal to the target surface, that the etching, at high fluence, is of the order of 10 v atoms/ pulse, that the atoms velocities are of the order of 10 v m/s, that the maximum ion energies are of the order of keV. During the laser irradiation, expanding and non-equilibrium plasma is produced in front of the target. The plasma has a fraction ionization depending by the metal species and generally within 10% and 80%. The plasma'temperature, at high fluence, can be theoretically calculated and reaches about 10 v K. The fractional ionization of the plasma, experimentally measured, has been investigated as a function of the laser fluence and of the energy binding of the target molecules. The ion emission yield is presented and discussed in view of the possibility to realize a laser ion source for ion accelerators.     

Parametric study of plasma active medium and gain saturation region in a Ne-like soft X-ray laser

Plasmas created by the interaction of high power optical laser with a target surface can be used as a source of soft X-ray lasers. Plasma and pump laser characteristics play significant role in achieving high gain coefficient for such plasma based on soft X-ray lasers. In the present work, the plasma active medium parameters for germanium element at a wavelength of 19.6 nm irradiated by a double-pulse pump laser have been studied using MED103 hydrodynamic code. For this purpose, first, the effects of laser intensity, pulse width and delay time of two pulses on the gain coefficient have been investigated and the optimum conditions for the maximum gain extent of Ne-like germanium soft X-ray laser are obtained. Then, in order to calculate the intensity of such high gain lasers in which Linford equation is invalid, we have adopted the general formula of amplified spontaneous emission intensity, which is valid in all range of intensities even at much higher intensities than saturation intensity. Finally, the soft X-ray laser intensities in the saturated areas for different plasma lengths have been calculated. The results show that the output of soft X-ray laser intensity with 294 cm⁻¹ gain coefficient can reach to about several times saturated intensity by applying a 1–2 mm plasma length as the active medium.     

Evidence of ultrashort electron bunches in laser-plasma interactions at relativistic intensities

The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.