磁场约束下激光诱导等离子体光谱强度演化研究

对磁场约束下激光诱导铜等离子体光谱强度演化进行了实验研究,分析了在磁场约束环境下的等离子体光谱强度演化过程以及激光能量对光谱增强的影响.实验结果表明:在磁场约束下铜等离子体内原子光谱和离子光谱均有所增强,在磁场约束下Cu I 510.55 nm谱线强度时间演化过程中在1.2—5.7μs时间范围内附近出现双峰结构,在距离靶材表面0—1.4 mm空间范围内磁场约束Cu I 510.55 nm光谱增强明显.Cu I510.55 nm和Cu I 515.32 nm光谱增强因子随激光能量的增加呈单调递减变化,激光能量20 mJ时增强因子最大分别为11和8.对磁场约束下等离子体发射光谱强度增强的物理原因进行了探讨.     

模拟溢油样品激光诱导荧光光谱的偏振特性研究

激光诱导荧光是海洋溢油探测的有效手段之一,但该技术的应用易受自然水体中叶绿素、CDOM等物质荧光信号的干扰。为了寻求排除自然水体荧光干扰的方法,基于532 nm连续激光器搭建了激光诱导偏振荧光实验装置,并针对六种不同密度的模拟溢油样品和自然水体开展了荧光光谱偏振特性研究。实验结果发现,与自然水体的荧光光谱不具有明显的偏振特性不同,所有模拟溢油样品的诱导荧光均具有明显的偏振特性,这一结论说明激光诱导荧光光谱的偏振特性可以作为排除叶绿素、CDOM等物质荧光干扰的依据。实验还发现,溢油样品的荧光偏振性质因样品种类而异。在线偏振光激发下,原油样品荧光偏振度随波长逐渐降低,其中重质原油样品偏振度降低幅度最大,轻质原油样品幅度最小,而柴油样品荧光偏振度没有明显变化;当周期性改变激发光的偏振状态时,所有模拟溢油样品的荧光偏振度随之发生趋势相同的周期性波动,中质样品荧光偏振度波动的幅度低于重质样品,但明显超过轻质样品。这一结果说明,模拟溢油样品诱导荧光偏振度的波长变化特性及对激发光偏振态的响应特性均与样品密度存在一定关联,其偏振特性可以作为辅助油种识别的重要参量。     

缓冲气体对激光等离子体光谱特性影响的实验研究

准分子激光(波长:308 nm,脉宽:10 ns)诱导Al等离子体.详细研究了缓冲气体对激光等离子体光谱特性的影响,测量了不同延时下激光诱导Al等离子体的电子温度和不同缓冲气压下光谱线的Stark展宽并由此计算了等离子体的电子密度,最后根据电子碰撞激发理论对实验结果进行了讨论.     

激光诱导等离子体光谱空间分布特性

激光波长和激光入射角是影响激光诱导等离子体空间分布和光谱强度空间分布特性的重要因素.基于流体动力学和SAHA方程,仿真了激光诱导等离子体的二维空间演化过程,研究了激发等离子体的辐射光谱空间分布特性及激光波长、入射角度等参数对等离子体特征谱线空间分布特性的影响.研究结果表明:波长为1064 nm的激光在不同延时条件下,最佳激光入射角度均为0°.当入射角度为0°时,所激发的等离子体辐射在不同的探测角度处均有较强的光谱信号,且在100,500,1000 ns延时条件下,最佳探测角分别为±41°、±11°和±12°.对于不同的波长,当延时分别为100 ns和500 ns且激光以0°入射时,长波长激光所激发的等离子体光谱在不同探测角处的强度均强于短波长激光.当延时为100 ns时,1064 nm波长激光所激发的光谱在最佳探测角位置的强度约为532 nm和266 nm波长激光所激发的光谱在各自最佳探测角位置强度的2倍.随着探测角绝对值的减小,等离子体辐射光谱强度先增大,到达最佳探测角后强度再减小.入射波长分别为532 nm和1064 nm的激光诱导击穿光谱实验结果验证了仿真结果.     

基于光偏振旋转效应的碱金属气室原子极化率测量方法及影响因素分析

利用原子自旋效应能够实现超高灵敏度的惯性和磁场测量。一类操控原子自旋处于无自旋交换弛豫态的器件可以进行物理参数测量。碱金属气室为该类器件的敏感表头。碱金属原子密度与原子极化率是碱金属气室的重要参数,对研究原子自旋处于无自旋交换弛豫态有着重要的作用。光的偏振效应在量子计算和原子物理研究中发挥了重要作用。利用光的偏振效应能够实现对碱金属原子密度与原子极化率的检测。提出一种基于光偏振旋转效应的碱金属原子极化率测量方法。首先对碱金属气室加恒定磁场,利用激光作为检测光,根据光偏振旋转原理,检测通过气室的偏振光的法拉第旋转角,得到碱金属气室原子密度。然后将碱金属原子抽运,利用激光作为检测光,检测通过气室的偏振光的偏转角,得到碱金属原子极化率。该方法在测量原子极化率的过程中也测量了碱金属原子密度,实现利用一套系统测量两个重要参数,具有快速测量和高灵敏度等特点,简化了实验设备及过程。对两种偏转角进行仿真分析,得到该方法实验时检测激光波长变化对偏转角的影响,根据仿真图得到检测激光波长的可取范围,验证了该方法的可行性。最后分析激光器波长波动与磁场波动对其测量精度的影响,提出实验对激光器与磁场的要求。     

Er:YAP晶体多波长激光运转及偏振特性研究

研究了b轴10 at.%Er∶YAP晶体的偏振吸收和输出特性以及多波长激光运转特性。首次测量了Er∶YAP晶体的偏振吸收光谱,结果显示晶体对偏振方向平行于a轴和c轴的偏振光在~4I_(11/2)能级对应的吸收峰附近的最大吸收系数分别为4.606 3 cm~(-1)(975.2 nm处)和2.936 6 cm~(-1)(967.6 nm处),因此选择合适波长的线偏振光作为泵浦光有利于提高晶体对泵浦光的吸收效率,从而改善激光性能;在氙灯泵浦的激光实验中,自由运转条件下实现了2 710,2 728,2 795和2 918 nm等4条谱线的激光输出,并分别研究了各谱线的偏振特性和起振的阈值特性。通过在谐振腔内加JGS石英片、云母片、 K9镜片等选择性吸收片分别实现了2 918 nm单波长, 2 710,2 821,2 837和2 862 nm等四波长和2 710 nm单波长的激光输出。测量了自由运转和不同选择性吸收片条件下的激光光谱,并与不同选择性吸收片的透过谱及之前报道的荧光光谱进行对照分析,证明通过调节谐振腔能够对Er∶YAP晶体中的起振谱线进行选择。偏振激光实验结果表明除谱线2 918 nm是偏振方向平行于a轴的线偏振光,谱线2 728 nm有时是偏振方向平行于c轴的线偏振光,有时又是偏振椭圆长轴平行于YAP晶体a轴的部分偏振光外,谱线2 710和2 795,2 821,2 837和2 862 nm均为偏振方向平行于c轴的线偏振光;在LD端面泵浦条件下,得到2 710,2 728,2 750和2 795 nm四条谱线的激光输出,其中谱线2 750 nm首次在Er∶YAP晶体中实现激光输出,这四条谱线均为偏振方向平行于晶体c轴的线偏振光;此外,首次测量了该晶体在8 K低温下的吸收光谱,利用Gauss函数对光谱进行分峰拟合,根据拟合结果对激光上下能级各斯塔克子能级进行了能级指认,并结合Er∶YAP晶体激光光谱、荧光光谱对可能的谱线跃迁进行了辨认。Er∶YAP晶体偏振特性和多波长讥光运转特性的研究以及对后续Er∶YAP晶体调Q等技术的实现和电光调Q晶体的选择具有一定的指导意义。     

氩气中飞秒超强紫外激光成丝的实验研究

中心波长808 nm,脉宽70 fs的钛蓝宝石激光系统三倍频后产生的中心波长268 nm、带宽1.5 nm、单脉冲能量0.58 mJ的紫外超快激光光源经凹面镜聚焦后注入到样品池氩气中,由强场非线性效应诱导形成了等离子体通道,并对紫外激光脉冲光谱进行展宽.实验着重研究了268 nm波段的紫外激光成丝特点,以及在不同气体压强、聚焦长度、气体种类等条件下紫外光丝对光谱的调制作用.在2.2×105Pa氩气气压、焦距1000 mm条件下,可以获得光谱宽度3.3 nm,加宽为人射紫外脉冲的2.2倍.实验观察到压强和聚焦长度的增加都有利于等离子体通道的增加,有利于光谱展宽.紫外激光诱导的等离子体通道为在紫外波段内获得极端超快激光脉冲提供了有效的途径.     

双端泵浦Nd~(3+)掺杂MgO:LiNbO_3正交偏振双波长连续激光调控

报道了一种采用双端泵浦的Nd~(3+)离子掺杂MgO:LiNbO_3正交偏振双波长激光器,并对正交偏振双波长激光输出进行调控.基于晶体的偏振荧光光谱,对1084与1093 nm的双波长激光振荡机理进行分析,建立晶体热透镜焦距与受激发射截面比之间的关系,并推导出1084及1093 nm双波长共振区间,给出通过改变谐振腔腔型结构调控双波长激光输出的方法.在实验中采用813 nm的半导体激光器双端泵浦a切的Nd:MgO:LiNbO_3晶体,测量了1084与1093 nm两种波长的输出规律,并对输出波长进行调控.最终得到了6.02 W的1093 nm和3.02 W的1084 nm单波长激光输出,在X,Y方向上的光束质量分别为M_X~2=1.70和M_Y~2=1.81.在28 W泵浦注入功率下获得了4.58 W的双波长激光输出,实验结果与理论分析相符合.为正交偏振双波长的可控输出及应用奠定了理论和实验基础.     

生物战剂气溶胶激光雷达遥测技术研究

基于激光诱导荧光（LIF）原理和弹性散射作用于非球形粒子上形成的偏振特性,设计并研制由3个激光波长、2个接收望远镜、退偏组件和荧光光谱仪等组成的生物战剂/气溶胶Mie散射、荧光和偏振激光雷达。退偏比的水平测量结果表明:1） 退偏比表现出较强的波长依赖性;2） 多波长退偏比测量可以显著提高生物战剂的鉴别能力。退偏比的垂直测量结果表明:气溶胶在边界层内总体上维持在较低的水平,在温湿压风等气象条件和光生化条件的作用下,对流层底层空变化明显,在部分云团处,退偏振比可达0.3;气溶胶的Mie散射时空分布表明,355 nm波长的测云能力在激光出射能量相当时较532 nm强。     

激光回馈波片位相延迟测量的误差源及消除方法

激光回馈法是一种新型的波片位相延迟测量方法。将波片放置在激光器与回馈镜之间,可使激光回馈波形产生偏振90°旋转（即跳变）现象,而两偏振态所占的周期比（占空比）与波片的位相延迟相关。通过测量占空比即可由计算机自动给出波片位相延迟。采用双向扫描回馈镜,2次获得并测出占空比,这样可以很好地消除两偏振态损耗的波动造成的测量误差,提高了该方法长期测量的稳定性。测量的重复性达到0.5°。该方法结构简单,在线测量精度高,满足工业化生产的需要。     

Laser-induced fluorescent measurements of magnetic field contoursin a low-pressure discharge

Faraday rotation of a laser beam and emission spectroscopy to resolve Zeeman splitting provide information about the plasma magnetic field, integrated along the line of sight. Information about the local magnetic field strength can be obtained using a dye laser tuned off the center of an atomic or ionic transition by an amount δλ_z. If the absorption linewidth of the transition probed is less than the Zeeman splitting, only those atoms/ions residing in a magnetic field where the Zeeman splitting is δλ_z will resonantly absorb energy from the laser and fluoresce. The feasibility of this magnetic field contour technique was studied in a low-pressure neon discharge. A conductor insulated from the discharge generated a large magnetic field in the discharge free of the Stark broadening effects associated with large plasma currents. The laser-induced fluorescence (LIF) intensity profile measured along the laser beams had peaks at those spatial locations where local magnetic fields, inferred from the conductor current, agreed with the Zeeman shifted wavelength of the laser     

Elementary Many‐Particle Processes in Plasma Microfields

The effect of electric and magnetic plasma microfields on elementary many‐body processes in plasmas is considered. As detected first by Inglis and Teller in 1939, the electric microfield controls several elementary processes in plasmas as transitions, line shifts and line broadening. We concentrate here on the many‐particle processes ionization, recombination, and fusion and study a wide area of plasma parameters. In the first part the state of art of investigations on microfield distributions is reviewed in brief. In the second part, various types of ionization processes are discussed with respect to the influence of electric microfields. It is demonstrated that the processes of tunnel and rescattering ionization by laser fields as well as the process of electron collisional ionization may be strongly influenced by the electric microfields in the plasma. The third part is devoted to processes of microfield action on fusion processes and the effects on three‐body recombination are investigated. It is shown that there are regions of plasma densities and temperatures, where the rate of nuclear fusion is accelerated by the electric microfields. This effect may be relevant for nuclear processes in stars. Further, fusion processes in ion clusters are studied. Finally we study in this section three‐body recombination effects and show that an electric microfield influences the three‐body electron‐ion recombination via the highly excited states. In the fourth part, the distribution of the magnetic microfield is investigated for equilibrium, nonequilibrium, and non‐uniform magnetized plasmas. We show that the field distribution in a neutral point of a non‐relativistic ideal equilibrium plasma is similar to the Holtsmark distribution for the electrical microfield. Relaxation processes in nonequilibrium plasmas may lead to additional microfields. We show that in turbulent plasmas the broadening of radiative electron transitions in atoms and ions, without change of the principle quantum number, may be due to the Zeeman effect and may exceed Doppler and Stark broadening as well. Further it is shown that for optical radiation the effect of depolarization of a linearly polarized laser beams propagating through a magnetized plasma may be rather strong. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高通鸟笼线圈激发螺旋波等离子体特性研究

研制了高通鸟笼线圈,使用氩气作为工质气体对射频天线的工作性能进行了初步评估。利用COMSOL5.4模拟出了鸟笼天线在13.56MHz的工作频率下,电场和磁场呈线性极化分布。对鸟笼线圈进行了电路结构解剖,推导出了其谐振频率计算公式。利用热耦合红外测温仪测试了正常工作状态下的鸟笼线圈外表温度明显低于传统射频天线,电容器最高温度仅65.8°。使用光谱仪对螺旋波等离子体放电光谱特性进行诊断。通过朗缪尔探针诊断了不同压强和磁场强度下螺旋波等离子体密度,在1.0Pa、600Gs、射频功率700W条件下等离子体密度达到1.62×10¹⁸m^-3。诊断了正向功率和反向功率对应的等离子体密度,其与磁滞现象变化趋势雷同。测试了螺旋波等离子体的径向密度分布,其在轴心处密度达到最高。探究了无磁场条件下等离子体特性,其密度值不会发生大幅度跃迁,纵向磁场是引发螺旋波等离子体放电的关键因素,低压条件下有利于得到更低的电子温度,最低达到2.67eV。表明鸟笼线圈低热耗、高馈入的特性使其在激发大体积的高密度螺旋波等离子体方面具有明显优势,可以投入到下一阶段氢气螺旋波等离子体的激...     

High quality electromagnetically induced transparency spectroscopy of ~(87)Rb in a buffer gas cell with a magnetic field

We have studied the phenomenon of electromagnetically induced transparency(EIT) of ~(87)Rb vapor with a buffer gas in a magnetic field at room temperature. It is found that the spectral lines caused by the velocity selective optical pump effects get much weaker and wider when the sample cell is mixed with a 5-Torr N_2 gas while the EIT signal is kept almost unchanged. A weighted least-square fit is also developed to remove the Doppler broadening completely. This spectral method provides a way to measure the Zeeman splitting with high resolution, for example, the Λ-type EIT resonance splits into four peaks on the D_2 line of ~(87)Rb in the thermal 2-cm vapor cell with a magnetic field along the electric field of the linearly polarized coupling laser. The high-resolution spectrum can be used to lock the laser to a given frequency by tuning the magnetic field.     

Ion velocity distribution function investigated inside an unstable magnetized plasma exhibiting a rotating nonlinear structure

The frequent situation where a strongly nonlinear rotating structure develops in a linear magnetized plasma column is investigated experimentally with emphasis on the ion velocity distribution function (IVDF). Most often, a mode m=2 appears exhibiting a large density and potential perturbation with angular frequency slightly above the ion cyclotron frequency. For the first time the spatiotemporal evolution of the IVDF is studied using time-resolved laser induced fluorescence to explore the ion's interaction with the nonlinear wave propagating inside the column and at the origin of plasma transport outside the limiter. The ion fluid exhibits an alternance from azimuthal to radial velocity due to the electric field inside the rotating structure. A fluid model also allows us to locally reconstruct the self-consistent electric field evolution which contradicts all existing theories.     

Calculation of the inverse bremsstrahlung absorption in unmagnetized plasmas

Inverse bremsstrahlung is one important way to deliver laser energy to the plasma in inertial confinement fusion. In this article, we study the collisional absorption rate as obtained from the Fokker–Planck treatment of an unmagnetized plasma in harmonic laser field. The electron–ion collision rate is considered in the Krook approximation, and the electron distribution function is considered a Maxwellian function. We evaluate the inverse bremsstrahlung absorption near the irradiated plasma surface in the critical layer. We observe that absorption increases with shorter laser wavelength and lower electron temperatures. When the maximum electron velocity in the limit of q → 1 reaches infinity, the q-non-extensive distribution function reduces to the standard Maxwell-Boltzmann distribution.     

Experimental results of helicon sources

Helicon plasma sources are known as efficient generators of uniform and high density plasma.A helicon plasma source was developed for the investigation of plasma stripping and plasma lenses at the Institute of Modern Physics, CAS.In this paper, the characteristics of helicon plasma have been studied by using a Langmuir four-probe and a high plasma density up to 3.9×10~(13)/cm~3 has been achieved with the Nagoya type Ⅲ antenna.In the experiment, several important phenomena were found: (1) for a given magnetic induction intensity, the plasma density became greater with the increase of RF power; (2) helicon mode appeared at RF power between 300 W and 400 W; (3) the plasma density gradually tended to saturation as the RF power increased to the higher power; (4) a higher plasma density can be obtained by a good match between the RF power and the magnetic field distribution.The key issue is how to optimize the matching between the RF power and the magnetic field.Moreover, some tests on the extraction of ion beams were performed, and preliminary results are given.The problems which existed in the helicon ion source will be discussed and the increase in beam density will be expected by extraction system optimum.     

Broadening of free-bound radiation thresholds in plasmas—II. Quasistatic theory for negative ions

Broadening and shifts of the long wavelength thresholds of the affinity continua of negative ions are investigated. A quasistatic theory is developed which treats the broadening as a plasma effect. The threshold shape is determined by convolution of the undisturbed negative ion cross-section with the distribution function of a component of the electric plasma microfield. This concept explains the absence of threshold shifts and shows that threshold broadening is proportional to the mean Holtsmark field strength in the plasma. Comparison of theory with experimental data shows good agreement.     

Anomalous plasma diffusion transverse to high magnetic fields in InSb

Investigations on the ambipolar diffusion of an electron-hole plasma transverse to a magnetic field have been carried out in InSb. A plasma layer, produced at the surface of the sample by a short laser pulse, was moved through the sample in crossed electric and magnetic fields by the Lorentz force. From the broadening of the plasma layer we found at 80K an enhanced diffusion coefficient which decreased proportional to 1/B for magnetic fields higher than 1T, constrary to the expected classical 1/B ² dependence. Furthermore, the diffusion coefficient was strongly dependent on the electric field. The ambipolar drift velocity, measured simultaneously showed a classical behaviour. Together with the enhanced diffusion we observed instabilites in the electric potential. The instability threshold decreased towards the cathode.     

Electrical characteristics of helicon wave plasmas

The external electrical characteristics of helicon wave plasmas have been studied over a wide range of magnetic fields, radio frequency (RF) power, frequencies, and Ar gas pressures. External parameters, such as antenna voltage, current, and phase shifts, and internal parameters, such as electron density, are measured. The equivalent discharge resistance, reactance, and power transfer efficiency are calculated through these measurements. The characteristics of helicon mode is compared with inductively coupled plasma (ICP) and low mode. The power efficiency of the helicon mode is better than that of other modes. Consequently, electron density of helicon mode is much higher than that of other modes. This means the existence of a mechanism where electrons are very efficiently accelerated by the electric field of the antenna in the helicon mode. The power efficiency of helicon mode is higher at lower RF frequency and at optimum gas pressure than that of other modes