Influence of heating effect in Thomson scattering diagnosis of laser-produced plasmas in air

A state diagnosis of laser-produced plasma in air generated by a 1064 nm pulse laser was investigated by the Thomson scattering(TS)method.The evolutions of the electron temperature and electron density were obtained as a function of the time delay which ranged from 300-3200 ns.The heating effect produced by the 532 nm probe beam with different energies on the air plasma at different interaction times was further studied using a time-resolved optical emission spectroscopy technique.The influence of the probe beam on the electron density was found to be negligible,whereas its influence on electron temperature is evident.In addition,the heating effect of the probe beam on the plasma strongly depends on the energy of the probe beam,and gradually weakens with increasing time delay.Our results are helpful for further understanding the TS method and its application in plasma diagnostics.

Numerical Simulation of the Thomson Scattering Experiment Performed with 0.351μm Laser-Produced Aluminum Plasmas

The evolutions of the electron temperatures of Muminum plasmas produced with 0.351 μm laser are simulated by means of one-dimensional hydrodynamic code. The simulations show that the plasma geometry has strong influence on the electron temperature＇s evolution while the effect of the flux limiter is not so significant. The simulations are in good agreement with the experiments only at some spatial points. A full comparison between the simulations and experiments indicates that the one-dimensional code is not accurate enough to characterize the laser-produced plasmas. A post-processor code based on the hydro code is developed to generate the streak image of the Thomson scattering spectra, which can be directly compared with the experimental data.

Measurement of electron density and electron temperature of a cascaded arc plasma using laser Thomson scattering compared to an optical emission spectroscopic approach

As advanced linear plasma sources, cascaded arc plasma devices have been used to generate steady plasma with high electron density, high particle flux and low electron temperature. To measure electron density and electron temperature of the plasma device accurately, a laser Thomson scattering（LTS） system, which is generally recognized as the most precise plasma diagnostic method, has been established in our lab in Dalian University of Technology. The electron density has been measured successfully in the region of 4.5？×10^19m^-3 to7.1？×10^20m^-3 and electron temperature in the region of 0.18 eV to 0.58 eV. For comparison,an optical emission spectroscopy（OES） system was established as well. The results showed that the electron excitation temperature（configuration temperature） measured by OES is significantly higher than the electron temperature（kinetic electron temperature） measured by LTS by up to 40% in the given discharge conditions. The results indicate that the cascaded arc plasma is recombining plasma and it is not in local thermodynamic equilibrium（LTE）. This leads to significant error using OES when characterizing the electron temperature in a non-LTE plasma.

Present Status of the Nd:YAG Thomson Scattering System Development for Time Evolution Measurement of Plasma Profile on Heliotron J

A new high repetition rate Nd：YAG Thomson scattering system is developed for the Heliotron J helical device. A main purpose of installing the new system is the temporal evolution measurement of a plasma profile for improved confinement physics such as the edge transport barrier （H-mode） or the internal transport barrier of the helical plasma. The system has 25 spatial points with -10 mm resolution. Two high repetition Nd：YAG lasers （〉 550 m J@ 50 Hz） realize the measurement of the time evolution of the plasma profile with ~10 ms time intervals. Scattered light is collected by a large concave mirror （D----800 mm, f/2.25） with a solid angle of -100 mstr and transferred to interference filter polychromators by optical fiber bundles in a staircase form. The signal is amplified by newly designed fast preamplifiers with DC and AC output, which reduces the low frequency background noise. The signals are digitized with a multi-event QDC, fast gated integrators. The data acquisition is performed by a VME-based system operated by the CINOS.

Design of Thomson scattering diagnostic system on linear magnetized plasma device

In addition to the magnetic confinement fusion plasma,Thomson scattering has been applied to measure electron density and temperature of low-temperature plasmas.Based on a linear magnetized plasma device,a set of Thomson scattering diagnostic system is designed to diagnose the plasma with n_(e)=10^(18)–10^(19)m^(-3)and T_(e)=2–5eV.Due to low plasma temperature and density,this diagnostic system needs high spectral resolution and collection efficiency to meet the requirements of electron velocity distribution function measurements.Through the bench test,it is confirmed that the spectral resolution reaches 0.01 nm,and theoretical collection efficiency is high enough to obtain a Thomson scattering spectrum by 1000 accumulations.

Design of Thomson Scattering Diagnostic System on EAST

Primary physical design of the Thomson scattering system for EAST, including the configuration of the system and the design considerations of different sections of the system, is presented. The expected measurability of this design, namely an electron temperature of 513 eV to 5 keV and a plasma density beyond 0.5× 10^19 m^-3, fulfills the requirements of the EAST operation.

Demonstration of X-ray Thomson Scattering on Shenguang-Ⅱ Laser Facility

X-ray Thomson scattering technique for diagnosing dense plasma was demonstrated on Shenguang-Ⅱ laser facility. Laser plasma x-ray source of titanium He-a lines （-4.75 keV）, generated by laser beam （1.5 kJ/527 nm/2 ns） heated titanium thin foil, was used as x-ray probe beam. The x-ray probe was then scattered by cold CH foam column of 1 g/cm^3 density. The scattered radiation at 90° was diffracted by polyethylene terephthalate （PET） crystal and recorded on x-ray charge-coupled device. Well-defined scattering spectra were obtained with good signal to noise ratio.

Thomson scattering off a pair （electron-positron） plasma

Thomson scattering off a pair （electron-positron） plasma is theoretically investigated in the collisionless and collisional limits respectively. Our calculations show that the power spectrum of the Thomson scattering off a collisionless pair plasma is just proportional to the velocity distribution function of the particles in the plasma. Collective modes in the plasma do not have any effects on the Thomson scattering spectrum because of the correlation between the negatively- and positively-charged particles. In the collisional limit, the power spectrum of the Thomson scattering presents three spikes： two peaks correspond to two contra-propagating sound waves and one peak corresponds to an entropy wave.

Characterization of a microsecond pulsed non-equilibrium atmospheric pressure Ar plasma using laser scattering and optical emission spectroscopy

A non-equilibrium atmospheric pressure argon(Ar)plasma excited by microsecond pulse is studied experimentally by laser scattering and optical emission spectroscopy(OES),and theoretically by collisional-radiative(CR)model.More specifically,the electron temperature and electron density of plasma are obtained directly by the laser Thomson scattering,the gas temperature is measured by laser Raman scattering,the optical emissions of excited Ar states of plasma are measured by OES.The laser scattering results show that the electron temperature is about 1 eV which is similar to that excited by 60 Hz AC power,but the gas temperature is as low as 300 K compared to about 700 K excited by 60 Hz AC power.It is shown that the microsecond pulsed power supply,rather than nanosecond ones,is short enough to reduce the gas temperature of atmospheric pressure plasma to near room temperature.The electron temperature and electron density are also obtained by CR model based on OES,and find that the intensities of the optical emission intensity lines of 727.41,811.73,841.08,842.83,852.44 and 912.86 nm of Ar can be used to characterize the behavior of electron density and electron temperature,it is very useful to quickly estimate the activity of the atmospheric pressure Ar plasma in many applications.

激光等离子体中相干Thomson散射的实验研究

在“星光 II”装置上进行了相干 Thomson散射实验。实验中以 0 .3 5μm激光为作用束 ,以 0 .53 μm激光作为探针光束 ,散射光采用反射式收光系统配光谱仪、条纹相机和CCD记录 ,得到较高时间和空间分辨散射光谱 ,分析得出时间变化的激光等离子体参数 ZTe和 ui。

发射光谱法诊断空气电弧的稳态电子密度分布

空气电弧是一种常见的等离子体,对其稳态阶段的微观参数进行研究,对准确把握等离子体内部的反应机制有重要意义。发射光谱法是一种成熟的诊断等离子体电子密度的技术,具有非接触式测量、瞬态探测、灵敏度高、响应速度快、仪器简单等优点。文中以H_(α)656.28 nm谱线做特征谱线,以石墨为电极材料在250 A、200 V的条件下对空气电弧进行发射光谱的测量,对实验获得的发射光谱作阿贝尔反变换和佛克脱线型拟合,反卷积得到斯塔克展宽,最终获得空气电弧电子数密度的径向分布。同时利用汤姆逊散射法测量相同参数条件下空气电弧的电子数密度以验证发射光谱法测量结果的正确性,经验证两种方法的测量结果具有良好的一致性。该实验为空气电弧等离子体电子数密度的测量提供了参考思路和方法。

激光惯性约束聚变中光学汤姆逊散射研究进展

当前,激光惯性约束聚变在越来越接近点火的极端能量密度条件下,实验与模拟的偏离逐渐增大,一个关键原因是缺乏对黑腔等离子体状态及其影响黑腔能量学和内爆对称性的细致研究和判断。光学汤姆逊散射主动式、诊断精确、参数完备的优点,使之成为激光惯性约束聚变黑腔等离子体状态参数精密诊断的标准方法。中国面向激光惯性约束聚变研究的光学汤姆逊散射实验技术的发展与神光系列激光装置的建设和在其上开展的物理实验紧密相关。近年来,四倍频汤姆逊散射实验技术在神光III原型和100 kJ激光装置上相继建立,部分实验结果不仅加深了对激光惯性约束聚变靶物理的认识,还反映了实验条件对汤姆逊散射诊断的影响,促进了实验技术的精密化发展。在未来,还需要进一步发展多支路汤姆逊散射、五倍频汤姆逊散射和超热相干汤姆逊散射等新技术,面向点火黑腔条件,大幅提升激光等离子体状态参数的诊断精度,开展新物理机制的探索和研究,在激光惯性约束聚变和其他高能量密度物理科学领域发挥更重要的作用。

微波等离子体炬光源基本特性的研究

用计算机化断层扫描成象技术研究了微波等离子体炬(MPT)放电的发射轮廓,证明该光源具有良好的对称性,并有一个有利于样品引入的中央通道,最佳分析区在炬管上方5～10 mm,用激光Thomson散射和Rayleigh散射技术测定了ArMPT和HeMPT放电的电子温度、电子密度和气体温度,证明MPT放电的电子温度很高而气体温度较低,是一种非热光源。其中的高能电子处于过布居状态,具有很高的激发能力。

汤姆逊散射技术在HL-2A装置上的应用进展

非相干激光汤姆逊散射诊断只需要假设电子速度满足Maxwell分布,测量得到的等离子体电子温度与电子密度的数据准确可靠,是托卡马克和其他磁约束核聚变研究装置上重要的诊断工具,并朝着高可靠性、高空间分辨和高重复测量频率的方向发展,其中高可靠性是前提。电子的汤姆逊散射截面很小,其总截面为σT=6.65×10-25 cm^2,通常使用电光调Q的Nd∶YAG激光器作为散射光源,激光脉冲宽度约10 ns、脉冲能量约3 J,用5~8通道的光谱仪对散射光谱进行测量与分析。如何对光电探测模块输出的散射脉冲进行数据采集,是激光散射诊断的关键问题之一。以前使用电流积分式的数据采集器(Q-ADCs,如CMC080模块),在一个确定的时间宽度(如50 ns)将散射脉冲信号积分在采样电容器上,从而得到散射信号的强度值,这种方法很难排除电路噪声和外来干扰。该研究通过使用高速数据采集器(纵向分辨率≥10 bits、采样频率f≥1 GS·s^-1,如V1742B模块)在包含散射信号在内的时间段(如300~500 ns)进行采集,获得散射脉冲信号、等离子体发光的扰动与背景噪声等叠加在一起的数据序列。利用最小二乘法,用高斯函数对散射脉冲的波形进行拟合,然后在50 ns时间宽度对散射波形脉冲进行数值积分,就得到散射信号的强度值。结果表明,高速同步采集技术的使用,能够用数字滤波技术排除大部分的干扰,从而提高信噪比,其幅度可以达到10倍左右。提取到更加准确可靠的光谱数据后,以置信水平95%、误差权重的最小二乘法开展数据处理,用A.C.Selden散射谱表达式对电子温度进行参数估计,得到了电子温度的测量值,其统计误差为3%左右,优于以前的10%左右。

等离子体参数的激光探针诊断技术

激光探针诊断技术作为一种等离子体主动诊断技术,在等离子体参数诊断中具有其独特的优势。文章综述了激光作为探针诊断等离子体参数的干涉诊断技术、摩尔偏折诊断技术以及Thomson散射诊断技术,分别介绍了各种技术的原理和一般方法。

激光汤姆逊散射技术诊断辐射加热Fe/Al等离子体状态

在神光Ⅱ激光装置上,利用激光加热金腔产生的X射线辐射场加热Fe/Al样品,随后用激光探针光入射Fe/Al等离子体并在90°散射方向采集散射光谱,同时也记录下了样品的自发辐射光谱。实验观察到了清晰的散射光谱以及Fe等离子体自发辐射光谱,诊断结果表明,Fe/Al等离子体的平均电子温度为360eV,Fe的平均电离度为18.8。

圆柱形等离子体对微波散射的数值模拟与实验研究

模拟和实验研究了非均匀圆柱形等离子体及阵列对微波的散射作用。利用有限时域差分(FDTD)方法仿真得到了等离子体柱的密度、碰撞频率对微波传播系数的影响,并利用低气压放电产生的等离子体柱对微波的吸收和散射作用进行了验证。结果表明:电子密度中心高、周围低的非均匀等离子体柱可将微波散射至两个侧向;等离子体频率越大,散射的微波功率越强;增加碰撞频率使等离子体柱的微波散射功率减小、吸收增大。等离子体必须具有合适的密度,才能对微波反射产生较大影响。

EAST托卡马克实验中的L模和H模电子尺度湍流的特征

湍流是托卡马克等离子体反常输运的原因,可能对低约束模式(L-mode)和高约束模式(H-mode)的输运产生重要影响.通过集体汤姆逊散射诊断系统研究EAST(experimental advanced superconducting Tokamak)托卡马克实验中的L模和H模电子尺度湍流的特征.结果表明:第75286号炮放电中的H模的粒子输运得到很大改善而热输运改善不大;等离子体由L模进入H模后,梯度区湍流强度的下降比芯区湍流强度的下降更明显,不同空间区域的湍流强度变化存在差异;H模粒子输运和热输运的差异可能是TEM和ETG波数范围湍流的光谱差异导致的.

尘埃等离子体参数测量研究综述

尘埃等离子体物理是近二十年来兴起的一个物理学分支,实验室等离子体环境和空间等离子体等环境遇到的诸多问题可以用尘埃等离子体理论给予较好的解释.但理论的验证需要对尘埃等离子体内部参数进行测量.介绍了目前实验室中产生尘埃等离子体的常用方法和实验装置,对尘埃等离子体内部参数测量中存在的问题进行了分析说明.

Thomson散射诊断技术的新进展

Thomson散射是一种主动而无干扰地对等离子体进行诊断的方法 .它能够以较高的时空分辨率测量等离子体的参数 ,如电子与离子温度、密度以及等离子体的膨胀速度、电离程度、热流等参数 .文章从Thomson散射基本概念出发 ,介绍了Thomson散射诊断方法在研究激光与等离子体相互作用中的重要意义 ,并分别介绍了近年来Thomson散射诊断技术的新进展 ,如对高Z等离子体、两种离子种类的等离子体、多种形态等离子体以及高密度等离子体的研究 .