空气中等离子光栅诱导探测光丝三次谐波辐射放大的实验研究

对探测光丝的不同位置与等离子光栅相互作用和探测光丝作用到等离子光栅不同位置引起三次谐波的增强进行了实验研究.研究发现,探测光丝的三次谐波信号强度对于探测光丝不同位置与等离子光栅相互作用和等离子光栅内部钳制的激光强度具有极强的依赖关系.与等离子光栅相互作用,三次谐波信号与等离子光栅基波信号的相位匹配与否是解释探测光丝三次谐波信号强度变化的关键.控制探测光丝以小角度与等离子光栅相互作用是实现探测光丝三次谐波信号有效放大的最佳途径.     

碰撞等离子体中电中性条件对单探针测量的约束

在碰撞等离子体中使用单探针测量系统测得的伏安特性曲线会发生畸变. 实验中分别测量了非碰撞条件下(20 Pa)和碰撞条件下(400 Pa)氩气(Ar)电感耦合等离子体的单探针伏安特性曲线, 并进一步通过在400 Pa下测量不同位置处的单探针伏安特性曲线和引入干扰电极调节整体电中性约束的程度的方法来分析研究曲线的畸变现象. 结果表明碰撞等离子体中整体电中性约束条件会对非碰撞探针测量系统进行约束, 并通过理论分析和实验证明:在单探针测量过程中, 以真空室壁为电位参考点, 等离子体将通过改变自身等离子体电位来满足整体电中性的要求, 此时, 单探针测得的伏安特性曲线就不是理论上的单探针的鞘层伏安特性.     

On the Boil-Off Time of Probe Surfaces in Plasmas

The time required for the melting of the surface of a glass-insulated probe inserted in a plasma is computed including the influence of plasma mass motion. This effect is relevant for probe measurements in moving current sheaths of electrical gas discharges. For dense plasma focus experiments, it is shown that the time of formation of a transition layer of evaporated material is shorter than the ion relaxation time in the plasma and the transit time of the current sheath by the probe position.     

脉冲射频容性耦合氩等离子体的发射探针诊断

利用工作在浮点模式下的发射探针,对500 Hz脉冲调制的27.12 MHz容性耦合氩气等离子体的空间电位和电子温度的时变特性进行了诊断.等离子体空间电位是通过测量强热状态下的发射探针电位获得的,而电子温度则是由发射探针在冷、热状态下的电位差来估算得到.测量结果表明:脉冲开启时,空间电位会快速上升并在300μs内趋于饱和;当脉冲关断后,空间电位经历了快速下降后趋于稳定的过程.电子温度在脉冲开启时存在过冲并趋于稳定的特征;而在脉冲关断期间,电子温度在300μs内则快速下降到0.45 e V后略有上升.无论在脉冲开启或关断期间,空间电位基本上都随功率和气压的变化存在有线性的依赖关系;而放电功率对脉冲开启期间过冲电子温度与稳态电子温度差异的影响较大.针对空间电位和电子温度在各阶段及不同放电条件下的时变特性,给出了相应的解释.     

Direct measurements of the plasma potential by katsumata-type probes

By taking advantage of the inherent difference between the gyro-radii of ions and electrons in a magnetised plasma, we developed a method to measure the plasma potential directly. The principle is based on the concept of the Katsumata probe. The probe collector is hidden inside a tube which screens an adjustable fraction of the electron flux whereas it lets pass most of the ions. In this paper an arrangement of three Katsumata-type probes with different diameters is used in the edge plasma region of the CASTOR tokamak to investigate the influence of the dimension of the probe. The results show almost no influence of the probe diameter on the values of the plasma potential.     

Is the Collecting Surface of a Flat One-Sided Probe Constant when Measuring the Ion Distribution Function?

The correctness of the known plane single-ended probe method for measuring the anisotropic ion distribution functions in a gas-discharge plasma has been considered. Analysis has been performed for positive probe potentials relative to the plasma with magnitudes on the order of the mean ion energy, which as a rule is much lower than the mean electron energy. We have analyzed the dependence of the collection surface area of a plane probe on its potential in this range. The structure of the near-probe layer has been determined for an isotropic electron distribution function of the Maxwellian or Druvestein type and an anisotropic ion distribution function. These results are used to derive analytic relations for the correction to the second derivative of the probe current with respect to the plane probe potential. It has been shown that, when the ion distribution function is measured in a wide range of conditions in the gas-discharge plasma, when the approximation of a collisionless probe layer is applicable, and the probe does not perturb the plasma, the dependence of the collection surface area of the probe on the potential can be disregarded in this range.     

射频辉光放电等离子体的电探针诊断及数据处理

Langmuir探针是等离子体诊断的一个重要方法.对探针I-V曲线进行求解二次微商是获得等离子体中的电子能量分布函数的关键.由Fourier变换导出一个求解微商的数值解方法.克服了现有方法所存在的缺点.实现了对探针I-V曲线求解二次微商的精确、自动运算.测量了硅烷射频辉光放电等离子体的平均电子能量(温度)和浓度随放电功率的变化. 关键词：     

激光等离子体无源电探针探测及光谱诊断的比较分析

提出一种利用无源电探针探测激光焊接光致等离子体的方法。采用光电同步采集系统对激光焊接光致等离子体进行研究,利用无源电探针和光纤式光谱仪探测光致等离子体,利用等离子鞘层理论分析电信号,并运用相对光强法计算出光致等离子体的电子温度,比较同步光电信号分析结果。将不涂覆表面物质以及表面分别涂覆KF和TiO₂三种情况下的计算结果进行对比,对影响结果准确性的因素进行分析。研究结果表明通过无源探针法计算等离子体温度与光谱信号计算结果基本吻合,准确度受等离子体离子质量的影响。无源电探针法能够反映激光焊接光致等离子体内温度变化,具有较好的实时性,可以作为激光等离子体监测手段。     

Use of magnetic probes for diagnostics of pulse plasma

Results of the measurement of proper magnetic field of a plasma cluster and of the distribution of current density in the cluster are given in this paper. The disturbances introduced into plasma by a probe are analysed and the applicability of internal magnetic probe is verified. The construction of probe is described.     

Plasma-wall interactions in DC discharges and sheath of Langmuir probes

The plasma-wall interactions in various DC discharges and sheath of Langmuir probe are analyzed and discussed. The methods of their investigations are discussed including fluid and PIC MC. Various assumptions used in fluid models e.g. plasma neutrality, Bohm criterion, Boltzmann electrons approximation, etc. are analyzed. Ion heating and electron cooling effect at the DC plasma wall is discussed and explained. Langmuir probes measurements in high-temperature and ion thruster plasma are analyzed. The secondary electron emission influences the IV characteristic of Langmuir probe especially at positive voltages. However, only elastic reflection processes really contribute significantly to the probe current. The elastic SEE processes reflect electrons from probe with the same relatively high speed. It was observed that the axial magnetic field influences probe characteristics and floating potential more significantly than radial field. The axial field deflects all electrons approaching probe.     

Electrostatic Probes for Studying Transport Properties in Tokamak Plasmas

Electrostatic probes for measuring the boundary plasma in tokamaks are reviewed and presented. Transport properties in JFT‐2, the ion temperature and the magnetic surface in JFT‐2M and floating potential fluctuations during the strong additional heating in JT‐60 are measured by several types of electrostatic probe the above‐mentioned purposes. The Langmuir probe including the double probe is applied to measure the spatial profile of boundary plasma in JFT‐2. The ion sensitive probe, the rotating cylindrical double probe, the asymmetric double probe and the differential double probe are applied to measure the ion temperature and magnetic surface in JFT‐2M. The reciprocating Langmuir probe applied to JFT‐2M observes the potential and density fluctuations and a new type probe is proposed for the quick diagnostic of core hot plasmas as a development of this probe. The fluctuation observed in JT‐60 is identified to be the ion cyclotron instability of the hot plasma caused by the strong anisotropy of the ion distribution function (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of Raman-shifted probe laser beam for plasma diagnosis using polaro-interferometer

Optical diagnostics of laser-produced plasma requires a coherent, polarized probe beam synchronized with the pump beam. The probe beam should have energy above the background emission of plasma. Though the second harmonic probe beam satisfies most of the requirements, the plasma emission is larger at the harmonic frequencies of the pump. Hence, at high intensities we need a probe beam at non-harmonic frequencies. We have set up a Raman frequency shifted probe beam using a pressurized hydrogen cell that is pumped by the second harmonic of the Nd glass laser that operates at only one Stokes line of 673.75 nm.     

Comparative measurements of the plasma potential with the ball-pen and emissive probes on the CASTOR tokamak

A novel approach to the direct measurement of the plasma potential in magnetized plasmas, using the so-called “ball-pen probe”, was recently tested in the CASTOR tokamak. Comparison with the standard technique of plasma potential measurement using the emissive probe is reported. It is found that the plasma potential determined by the emissive probe is systematically lower than that measured by the ball-pen probe. The difference is of the of the of kT _e/e. A possible reason of this difference is the space charge occurring in the proximity of the emissive probe.     

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针对气压大于133Pa的气体放电时等离子体鞘层及粒子碰撞对微波共振探针密度测量的影响变大的问题,对微波共振探针模型进行了改进,推导了鞘层修正因子和碰撞修正因子,探讨了微波共振探针尺寸对测量等离子体密度的影响。结果表明,针丝半径选择0.2mm,并且选用较大宽度的探针有利于减小测量误差。     

等离子体鞘层及粒子碰撞对微波共振探针诊断的影响

针对气压大于133Pa的气体放电时等离子体鞘层及粒子碰撞对微波共振探针密度测量的影响变大的问题,对微波共振探针模型进行了改进,推导了鞘层修正因子和碰撞修正因子,探讨了微波共振探针尺寸对测量等离子体密度的影响。结果表明,针丝半径选择0.2mm,并且选用较大宽度的探针有利于减小测量误差。     

Measurement of Mach probe on plasma flow velocity in highly collisional plasma jet

A normalized plasma flow velocity in highly collisional plasma formed by a microwave plasma jet, which is dimensionless unit for plasma flow velocity/ion acoustic velocity, was measured by the parallel Mach probe. To deduce the normalized plasma flow velocity under highly collisional plasma conditions, the collisional model of a Mach probe was proposed. In addition, neutral gas flow velocity which assumed to be plasma flow velocity was calculated by the turbulent model. The results for the two different models were compared with those for the collsionless models of the Mach probe. The turbulent model produced 2–4 times reduced values than by measurements with collsionless models. The measured results with the collisional model were shown as approximately 100–250% lower than those for collsionless models. They were obtained to be in good agreement with difference rate of 10–30% when compared to those for the turbulent model.     

甚高频激发的容性耦合Ar+O_2等离子体电负特性研究

利用探针辅助的脉冲激光诱导负离子剥离诊断技术对掺入5%O_2的容性耦合Ar等离子体电负特性进行了诊断研究.首先详细解析了脉冲激光剥离后探针的电信号,分析了探针偏压在低于或高于空间电位下的探针收集信号特征;根据探针偏压与探针收集信号之间的依赖关系,用来描述Ar+O_2等离子体电负特性的等离子体电负度被定义为脉冲激光剥离出的电子电流与偏压高于空间电位的探针收集到的背景电子电流的饱和比值,并对等离子体电负度随放电气压、射频功率以及轴向位置的变化进行了诊断测量.实验结果表明等离子体的电负度随着射频功率的增加而减小、随着放电气压的上升而变大;由于非对称电极的分布特性,在轴向方向上靠近功率电极时等离子体电负度有升高的趋势,这种趋势可能与鞘层边界附近二次电子的动力学行为以及负离子的产生与消失过程有关.     

Particle‐in‐cell/Monte Carlo simulation of electron and ion currents to cylindrical Langmuir probe

Electron and ion currents to a cylindrical Langmuir (electrostatic) probe were calculated using the particle‐in‐cell/Monte Carlo (PIC/MC) self‐consistent simulation for a neutral gas in the pressure range 2–3,000 Pa. The simulation enables us to calculate the probe currents even at high neutral gas pressures when the collisions of collected charged particles with neutral gas particles near the probe are important. The main aim of this paper is the calculation of probe currents at such high gas pressures and the comparison of the results with experimentally measured probe currents. Simulations were performed for two cases: (a) probes with varying radii in a non‐thermal plasma with high electron temperature at low neutral gas pressure of 2 Pa (in order to verify the correctness of our simulations), and (b) probe with the radius of 10 μm in the afterglow plasma with low electron temperature and a higher neutral gas pressure (up to 3,000 Pa). The electron probe currents obtained in case (a) show good agreement with those predicted by the orbital motion limited current (OMLC) theory for probes with radii up to 100 μm for the given plasma conditions. At larger probe radii and/or at higher probe voltages, the OMLC theory incorrectly predicts too high an electron probe current for the plasma parameters studied. Additionally, a formula describing the spatial dependence of the electron density in the presheath in the collisionless case is derived. The simulation at higher neutral gas pressures, i.e. case (b), shows a decrease of the electron probe current with increasing gas pressure and the creation of a large presheath around the probe. The simulated electron probe currents are compared with those of measurements by other authors, and the differences are discussed.     

A theory of probes in high-pressure strongly-ionized plasmas

A theory is developed for a probe in a strongly ionized high-pressure plasma. The situation discussed is typical of high-pressure plasmas, where the plasma occupying most of the near-probe layer is in a state of local ionization equilibrium with a common temperature for electrons and heavy particles. The possibility of determining the parameters of the unperturbed plasma by analyzing the probe characteristics on its ion saturation segment, transient segment, and for a floating probe potential is discussed. The calculations are carried out for the example of a strongly ionized xenon plasma at atmospheric pressure. Zh. Tekh. Fiz. 67, 16–20 (December 1997)     

Determining railgun plasma current distribution using Jansson'smethod to deconvolve B-dot probe signals

The authors present a method of determining the plasma armature current distribution from B-dot probe signals. This method utilizes the knowledge that the B-dot probe signal can be shown to be a convolution of the plasma armature current distribution with the impulse response function of the B-dot probe. Jansson's (1984) relaxation-based nonlinear deconvolution technique is developed for use with recorded B-dot probe data. Results obtained with this technique indicate that significant improvement in estimations can be obtained over previously used linear deconvolution methods