纳秒激光诱导空气等离子体的红外辐射特性研究

纳秒激光诱导空气等离子体存在从紫外、可见、近红外乃至射频微波的宽谱段辐射,但目前的研究大多关注紫外到可见波段的光谱辐射。激光等离子体作为一种新型的红外辐射源具有很多优势,相比于红外干扰弹以及红外干扰手段而言,空气等离子体红外辐射源可以灵活布置,成本低廉,因此研究空气等离子体的红外辐射特性就很有必要。针对目前脉冲激光诱导空气等离子体的红外干扰研究需要,对激光波长为532 nm的纳秒脉冲激光诱导空气等离子体的红外辐射特性进行实验研究,探讨激光能量对空气等离子体红外辐射强度的影响规律,以及空气等离子体红外辐射的角度分布特性,分析了等离子体红外辐射的可能产生机制。实验结果表明,激光诱导空气等离子体在950～1 700 nm范围内的红外光谱为线状谱和连续谱的叠加。其中线状谱主要是氮和氧的中性原子谱线,并且氮原子红外辐射占主导。随着激光能量的增加,由于空气击穿产生的氧和氮原子数量增加,导致空气等离子体红外辐射的谱线强度逐渐增大。随着红外探测角度的变化,在探测角度为75°时,OⅠ 1 128.63 nm和NⅠ1 246.96和1 362.42 nm谱线强度达到最大,在探测角度为120°时,NⅠ 1 011.46和1 053.96 nm谱线强度达到最大,这是因为空气等离子体红外辐射强度随探测角度变化呈现空间非对称性,表明空气等离子体内不同粒子的空间分布呈现非对称性。     

短脉冲激光产生的等离子体X射线辐射

此文着重研究短脉冲强激光作用下的等离子体Ｘ射线辐射强度随入射激光能量或功率的变化，等离子体辐射主要由激光功率决定，实验显示了辐射曲线的突变区域和他和区域结构。此外，本结果表明在激光以２０Ｐｓ入射条件下的等离子体相对论吸收效应还不太明显，在等离子体中主要是逆韧致吸收。     

等离子体天线表面电流分布与辐射特性研究

建立了表面波驱动等离子体天线装置,提出了一种等离子体天线表面电流指数分布模型,并利用该表面电流分布模型计算了天线的辐射方向图.研究结果表明,等离子体密度随轴向距离的增大呈指数衰减趋势.正常工作状态下表面波波矢虚部随等离子体密度的增大而下降,遵循与等离子体密度类似的指数衰减规律,但其实部则基本保持不变.等离子体天线的表面电流呈振幅指数衰减行波模式.利用该表面电流分布模型计算得到的辐射方向图呈现典型的8字形分布,与实验测量结果良好符合.当射频功率减小,等离子体天线辐射方向图宽化.     

气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.     

Compton散射对激光等离子体通道天线特性的影响

应用多光子非线性Compton散射模型和数值计算方法,研究了激光等离子体通道天线传播和辐射特性,结果表明:随通道周围介质损耗和传输模式阶数的增大,传输模式THnm衰减常数明显增大.这是因散射使通道内外电场和磁场增强,粒子间碰撞频率增大,电场使更多分子电离而吸收更多能量的缘故.随模式阶数增大,电性有耗介质使相移常数明显减小.这是因散射使高阶模式可能存在被耦合电场俘获的缘故.等离子体耦合频率为0.7附近,衰减常数随频率增大而剧烈增大.这是因散射使介质分子发生二、三阶电离,更多电子被耦合电场急剧加速的缘故.随天线长度增加,天线辐射方向图主瓣和副瓣数量、宽度和最大辐射方向发生明显变化.这是因散射使天线频率增大,辐射波长变短,粒子电离几率增大,辐射波能量和频率成分增大的缘故.     

60MHz电容耦合等离子体中电子能量分布函数特性研究

甚高频(频率大于30 MHz)耦合放电源由于能产生大面积高密度的等离子体而受到了人们的广泛关注. 采用电流、电压探针以及朗缪尔探针诊断技术对60MHz射频激发产生的容性耦合等离子体的放电特性及电子行为进行了研究. 实验结果表明,等离子体的等效电阻/电容随着射频输入功率的增加而减小/增加;等离子体中电子行为不仅依赖于射频输入功率,还与放电气压密切相关;放电气压的增加导致电子能量概率分布函数(EEPF)从双温Maxwellian分布向Druyvesteyn分布转变,而且转变气压远低于文献所报道的数值,这主要是由于在60MHz容性耦合等离子体中电子反弹共振加热效率大为降低. 关键词： 甚高频容性耦合等离子体 朗缪尔探针诊断 电子加热模式     

电子在激光驻波场中运动产生的太赫兹及X射线辐射研究

采用单电子模型和经典辐射理论分别对低能和高能电子在线偏振激光驻波场中的运动和辐射过程进行了研究. 结果表明: 垂直于激光电场方向入射的低速电子在激光驻波场中随着光强的增大, 逐渐从一维近周期运动演变为二维折叠运动, 并产生强的微米量级波长的太赫兹辐射; 高能电子垂直或者平行于激光电场方向入射到激光驻波场中, 都会产生波长在几个纳米的高频辐射; 低能电子与激光驻波场作用中, 激光强度影响着电子的运动形式、辐射频率以及辐射强度; 高能电子入射时, 激光强度影响了电子高频辐射的强度, 电子初始能量影响着辐射的频率; 电子能量越高, 产生的辐射频率越大. 研究表明可以由激光加速电子的方式得到不同能量的电子束, 并利用电子束在激光驻波场的辐射使之成为太赫兹和X射线波段的小型辐射源. 研究结果可以为实验研究和利用激光驻波场中的电子辐射提供依据.     

Compton散射对激光等离子体通道天线特性的影响

应用多光子非线性Compton散射模型和数值计算方法, 研究了激光等离子体通道天线传播和辐射特性,结果表明: 随通道周围介质损耗和传输模式阶数的增大, 传输模式TH n m衰减常数明显增大。这是因散射使通道内外电场和磁场增强, 粒子间碰撞频率增大, 电场使更多分子电离而吸收更多能量的缘故。随模式阶数增大, 电性有耗介质使相移常数明显减小。这是因散射使高阶模式可能存在被耦合电场俘获的缘故。等离子体耦合频率为0.7附近, 衰减常数随频率增大而剧烈增大。这是因散射使介质分子发生二、三阶电离, 更多电子被耦合电场急剧加速的缘故。随天线长度增加, 天线辐射方向图主瓣和副瓣数量、宽度和最大辐射方向发生明显变化, 这是因散射使天线频率增大, 辐射波长变短,粒子电离几率增大, 辐射波能量和频率成分增大的缘故。     

等离子体云团宽带电磁辐射源的实验研究

通过工质爆燃,产生了具有很宽频谱的电磁辐射。用4个宽带天线从不同方向对球形工质产生的电磁辐射进行了接收测量,并对信号作了频谱分析。实验结果表明:150 g工质爆炸产生的电磁脉冲持续时间为50 ns,上升沿很陡,频谱连续,频率范围为1~4 GHz,峰值频率为2 GHz,功率为2 W。对爆燃型工质产生电磁辐射的机理进行了初步理论探讨,认为等离子体辐射可能是产生电磁辐射的源。     

不同能量水下激光诱导等离子体的轴向辐射分布特性研究

激光诱导击穿光谱(LIBS)技术作为一项新兴的水下原位探测技术,备受海洋探测技术领域的关注。将这项技术推向实用化的关键之一是改善LIBS的远程探测能力,因此需要采用超击穿阈值的高能量探测激光。为观察超击穿阈值情况下的等离子体辐射和动态击穿特性,采用图像与光谱相结合的方法,以KCl溶液为样品进行了系列实验研究。通过对1～20 mJ不同能量激发下的等离子体图像分析,获得了不同激发条件下总辐射的轴向跨度和最亮点位置信息。随激光脉冲能量增大,等离子体长度增加,从1 mJ时的0.49 mm增加到20 mJ时的1.83 mm,同时辐射最亮点位置向激光入射方向移动了0.79 mm。结合光谱探测分析,得出等离子体特征辐射的轴向空间分布也对激光能量有明显的依赖性。虽然不同能量下谱线强度呈相似的轴向空间分布,但钾原子辐射最强处的位置和相应强度均随能量变化,在5 mJ激发下获得最佳辐射强度。实验结果表明,为满足远程LIBS应用需求,提高激光能量时应考虑其对原子辐射的影响。还对不同能量下的谱线的半高宽和信背比进行了观测分析。     

Combination of RF-plasma jet and Laser-induced plasma for breakdown spectroscopy analysis of complex materials

Laser-induced breakdown spectroscopy method is modified using an external radiofrequency (RF) plasma jet that overlaps spatially with the laser-induced plasma. Short UV laser pulses (wavelength 193 nm) are employed to ablate iron oxide ceramics, zinc oxide ceramics and polyethylene pressed pellets in ambient air background. An RF generator (frequency 13.56 MHz, power ≤1 kW) and a gas nozzle system are employed to generate a continuous jet of argon and nitrogen plasma expanding into air. The optical emission of the overlapped plasma is analyzed using an Echelle spectrometer equipped with an intensified CCD camera. The time dependence of optical emission intensities and of plasma parameters is investigated by varying the spectrometer delay time with respect to the laser pulse. The emission intensities of major (Fe, Zn) and minor (Mn, Al) elements are moderately increased with the RF plasma jet (≤2×). The intensities of atomic lines and molecular bands from ablated polymer material are increased also.     

大气压微波等离子体射流装置放电特性研究

基于同轴传输线结构设计了两种不同喷嘴结构的大气压微波等离子体射流(MW-APPJ)装置,其工作频率2.45 GHz,工作气体为氩气,分别研究了两种不同喷嘴结构对等离子体放电特性产生的影响。仿真结果表明,MW-APPJ在气体喷嘴处会产生高强度的电场,经过优化结构,实现在频率2.45 GHz下,喷嘴处的场强满足氩气电离的击穿场强阈值要求。同时,利用多物理场耦合仿真软件对装置的气流分布进行了稳态模拟,并通过实验对比分析了两种喷嘴结构下大气压氩等离子体射流的基本特性。实验结果表明,不同的喷嘴结构会影响等离子体装置的反射系数随输入功率的变化规律,但并不影响等离子体射流长度随输入功率的变化规律和反射功率随进气流量的变化规律;同时,在大气压下,稳态微波等离子体射流呈现出类金属性,等离子体中的电子只能在很薄的区域中吸收微波能量,因而造成微波的反射功率较大。     

ANALYSIS OF DUAL FREQUENCY CONFINED CAPACITIVELY COUPLED PLASMA AT LOW POWER

Confined dual frequency hydrogen plasma discharge has been investigated with microwave interferometer method and radial profiles are taken by Abel inversion technique. Dual radio-frequency sources, operating at 27.12MHz and 1.94MHz, are coupled to each other through the plasma. 27.12MHz RF power is used to enhance plasma density and 1.94MHz power is used to enhance ion acceleration energy to the electrode. Radial density profiles has been taken for comparing the effects of low frequency source that the secondary RF source causes reduction in plasma density due to the sheath expansion. Instead radial density profile is assumed as flat by most of the models, there is about 2.5eV of potential drop occurs from centre to boundary at 40W of primary source power. It has been observed that increasing sheath width (increasing the secondary source power to primary source power) reduces the bulk plasma volume and makes potential profile flattening in y direction. While the high frequency power is dissipated by electrons in the bulk plasma; low frequency power is mostly dissipated by ions in the sheath region. Using both high and low frequency power, we may control plasma density and ion acceleration energy to the electrode simultaneously.     

High-Power Modulated Intense Relativistic Electron Sources with Applications to RF Generation and Controlled Thermonuclear Fusion

Recent advances in the physics and technology of the modulated intense relativistic electron beams (IREB's) are reviewed in this paper. Bunched dense electron beams can be used to construct high-power RF sources, which may critically affect future progress in fusion technology. In this paper a system is described in which electrical energy can be converted from a single pulse of relatively long duration into a series of subpulses of short duration (nanosecond and subnanosecond) and of high power (~1010 W). This electrical system consists of an IREB propagating through passive structures. The mutual interaction between the electron beam and one passive structure modifies the IREB so that power compression and beam modulation occur. When the modified IREB interacts with the next passive structure, the kinetic energy of the electrons is converted into electrical energy or RF energy. The beam current modulation depends on the injected IREB and the structure parameters. A 100-percent modulation of the current has been achieved. A single-beam source may be used for exciting radiation in a frequency range of 60 MHz to 10 GHz. In the frequency range of 60-750 MHz a modulated beam with power ~1010 W has already been achieved. IREB modulation at a frequency of ~3 GHz was performed and RF energy was extracted from the bunched beam with power output of 5 × 108 W.     

13.56 MHz/2 MHz柱状感性耦合等离子体参数的对比研究

通过Langmuir双探针和发射光谱诊断方法,对比研究了驱动频率为13.56 MHz和2 MHz柱状感性耦合等离子体中电子密度和电子温度的径向分布规律.结果表明:在高频和低频放电中,输入功率的增加对等离子体参数产生了不同的影响,高频放电中主要提升了电子密度,低频放电中则主要提升了电子温度.固定气压为10 Pa,分别由高频和低频驱动时,电子密度的径向分布均为"凸型".而电子温度的分布差异比较明显,高频驱动时,电子温度在腔室中心较为平坦,在边缘略有上升;低频驱动时,电子温度随径向距离的增加而逐渐下降.为了进一步分析造成这种差异的原因,在相同放电条件下采集了氩等离子体的发射光谱图,利用分支比法计算了亚稳态粒子的数密度,发现电子温度的径向分布始终与亚稳态粒子的径向分布相反.继续升高气压到100 Pa,发现不论高频还是低频放电,电子密度的径向分布均从"凸型"转变为"马鞍形",较低气压时电子密度的均匀性有了一定的提升,但低频的均匀性更好.     

On the possibility of the amplification of subterahertz electromagnetic radiation in a plasma channel created by a high-intensity ultrashort laser pulse

The evolution of the electron energy distribution function in a plasma channel in a xenon plasma at atmospheric pressure created by radiation of a KrF femtosecond laser has been considered. It has been shown that, owing to the existence of the Ramsauer minimum in the transport scattering cross section, such a channel can be used to amplify electromagnetic waves up to the terahertz frequency range at relaxation times of the energy spectrum of ～10^?7 s. The gain factor has been calculated as a function of the time and radiation frequency.     

Effect of electron-neutral collisions on the satellites of the Thomson scattering spectrum in arc plasmas

The effect of electron-neutral collisions on the high frequency spectrum of laser radiation scattered by the free electrons of a plasma is investigated for a partially ionized H₂ are plasma at atmospheric pressure. The calculations are carried out along Gorog's theory solving the linearized Boltzmann equation for electrons with a collision term. The collision integral is approximated by a Krook relaxation model with the collision frequency determined from experimental electron-atom scattering data. The collisions influence size, half width, and position of the high frequency satellites. In a H₂ arc plasma, the change of the satellites' position is negligible as well as the change in half width within experimentally attainable error limits. The change of size is of minor importance, since the general evaluation procedure uses only normalized scattering intensities. Thus, for laser scattering experiments in are plasmas the collision-free theory can be applied.     

Thermal and induced flow characteristics of radio frequency surface dielectric barrier discharge plasma actuation at atmospheric pressure

Thermal and induced flow velocity characteristics of radio frequency(RF) surface dielectric barrier discharge(SDBD)plasma actuation are experimentally investigated in this paper. The spatial and temporal distributions of the dielectric surface temperature are measured with the infrared thermography at atmospheric pressure. In the spanwise direction, the highest dielectric surface temperature is acquired at the center of the high voltage electrode, while it reduces gradually along the chordwise direction. The maximum temperature of the dielectric surface raises rapidly once discharge begins.After several seconds(typically 100 s), the temperature reaches equilibrium among the actuator's surface, plasma, and surrounding air. The maximum dielectric surface temperature is higher than that powered by an AC power supply in dozens of k Hz. Influences of the duty cycle and the input frequency on the thermal characteristics are analyzed. When the duty cycle increases, the maximum dielectric surface temperature increases linearly. However, the maximum dielectric surface temperature increases nonlinearly when the input frequency varies from 0.47 MHz to 1.61 MHz. The induced flow velocity of the RF SDBD actuator is 0.25 m/s.     

射频激励双通道不等长电极波导CO2激光器的外差频率调制研究

报道了采用不等长电极结构的射频激励双通道波导CO₂激光器,通过调节射频电源功率的方法可以调谐双通道输出激光的外差频率,其调谐率约为0.1 MHz/W。在未采取任何稳频措施时,其短期外差频率稳定性可达10-10。还在理论上分析了射频功率调谐外差频率的原理。