A theory of the positive column constriction in the noble gas discharges at medium pressures

A theory explaining the positive column constriction in terms of a thermal effect is presented. As a consequence of the heating the neutral gas becomes more rarified near the tube axis and a locally increased molecular ion production ensues. If this is balanced by the dissociative (volume) recombination a constriction results at higher currents and pressures. The density profiles and other physical quantities are computed and compared with the available experimental data. All computations are performed for a non-Maxwellian plasma with the electron Coulomb interaction neglected. In contrast to the theories based on an ionization mechanism the thermal theory predicts also a constriction of the density profile.     

极性对自保护药芯焊丝电弧焊光谱特征的影响

由于自保护药芯焊丝具有抗风性以及优异的焊缝性能,已广泛应用于野外管道焊接以及大型机械的修复过程。电极极性是影响焊接过程的重要工艺参数。为了研究电极极性对电弧等离子体的影响机理,设计电弧等离子体空域中各点逐步扫描的同步采集系统,通过光谱特征谱线的分析,采用Stark谱线轮廓法计算电子密度,并且基于Boltzmann作图法计算电弧等离子体的温度,同时针对Al和Mg活性元素的分布特征进行分析。结果表明,靠近电极处,沿y轴负方向,直流正接时(焊丝接电源负极性),弧柱中心区电弧电子密度、电弧温度和活性元素呈现“水滴状”分布。而直流反接时(焊丝接电源正极性),弧柱中心区电弧电子密度、电弧温度和活性元素的分布特征表现为“手指状”分布。根据“自磁收缩”的原理,直流正接条件下,活性元素在径向方向受到的电磁力较小,整体分布呈现发散状。直流反接条件下,活性元素在径向方向受到的电磁力较大,收缩较为严重,整体表现为收缩状态。采用相同的电参数时,直流反接条件下弧柱中心区的电弧电子密度、电弧温度均大于直流正接条件下得到的电子密度和电弧温度,其中电子密度分布特征和带电粒子的电离程度是影响电弧温度的主要因素。在相同的电...     

Attenuation properties of EM wave by magnetized nonuniform plasma slab coated on perfect conductor plane

The total amplitude reflection coefficient of EM wave in a nonuniform plasma slab coated on perfect conductor plane is newly derived by using the scattering matrix method (SMM), and the attenuation is calculated. Three types of plasma electron density profile, that is the hyperbolic, sinusoidal, and linear profiles, are used. The external magnetic field and plasma parameters such as the maximum electron density and collision frequencies are discussed to calculate the attenuation of EM wave. The calculation results show that the plasma electron density profile and maximum electron density exert a large effect to the attenuation of EM wave and the attenuation under the uniform external magnetic field is taken place by the electron cyclotron resonance absorption, the up hybrid resonance absorption and geometric resonances absorption.     

Experimental Study of Planar Langmuir Probe Characteristics in Electron Current-Carrying Magnetized Plasma

Experimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current-free magnetized plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface.     

Low frequency ion acoustic instability in the positive column of A D.C. discharge

The one-dimensional dispersion equation of the electroacoustic instability of the low pressure column is analyzed numerically. The column steady state parameters were obtained from the plasma balance equations based on a Maxwell distribution. The computed curves are compared with the experimental dispersions measured for spontaneously existing anode directed wave observed at the cathode end of the column. Although the agreement between the observed and computed dispersion curves is fairly good the current driven electroacoustic instability does not provide a universal explanation for all of these waves.     

Measurement of Temperature and Density Profiles of the Plasma at PR-2 Facility

A new system of probe diagnostics at linear plasma simulator PR-2 is described, allowing us to measure the profiles of plasma temperature and density in different cross sections of the plasma column. The Langmuir probe fixed to the movable part of the two-coordinate positioning system built into the PR-2 passes the region of the discharge area during the process of measuring plasma parameters. The overall dimensions of the positioning system make it possible to mount electrical probes (magnetic probes, optical fibers, and other diagnostic equipment) covering almost the entire volume of the vacuum chamber between the magnetic mirrors of the device. We present the measurement results of local plasma parameters of the beam-plasma discharge (BPD) for different values of the input power and working gas pressure. The boundaries of appearance of discharge of three types were determined: the diffusive BPD mode, the BPD mode, and the arc mode. Dependences of the plasma concentration and temperature on the input power for different values of pressure have been also determined.     

超高速碰撞2024-T4铝靶产生等离子体的朗缪尔双探针诊断

 有些等离子体,本身并不存在电极和参考点,而该电极或参考点是提供偏压朗缪尔探针所必需的。为了获得超高速碰撞所产生等离子体的特征参量,建立了一种新的静电探针诊断技术,该技术不需要扫描频率,其探针可用于测量与时间相关的电子温度、电子密度。该诊断系统基于双通道电路,电流和电压谱通过数字示波器同步输出。研究的主要目的是,应用双朗缪尔探针诊断2024-T4铝弹丸超高速碰撞2024-T4铝靶时产生的瞬态等离子体。     

Plasma Diagnostics and Nonlinear Wave Studies with a Gridded Electrostatic Energy Analyzer

A simplified theory relating the collector current to the grid bias and other plasma parameters has been developed for a multiple-gridded electrostatic energy analyzer. Experiments have shown that the theory is only approximately valid, but the temperature obtained agrees closely with that from a Langmuir probe. It is found that temperature measurements depend on the grid bias potentials and the ratios of grid hole radius to electron Debye length. As a result, the conditions for the best performance of the energy analyzer as an accurate diagnostic device have been determined. A theory has also been worked out for the second-order change in the electron distribution function due to a cyclotron damped wave. This, together with experimental results, will provide valuable information on the thermal anisotropy of a plasma.     

The model of internal column of hollow cathode vacuum discharge

Analyzes the characteristics of rarefied, nonequilibrium-state plasmas in the internal column of a hollow cathode discharge (HCD). The analysis is based on the theory of plasma disintegration in a strong electric field (Dreicer 1959, 1960). It is demonstrated that this process has a crucial influence upon the forming of directed flux of electrons with energy values 20-30 eV at the exit of the hollow cathode. The obtained values significantly exceed the energy of thermal motion of electrons in the plasma disintegration zone. A new method is suggested of calculating electron density and electric field intensity in respect to the axis of the internal column in the channel model of the discharge. In addition, a method is presented of calculating the length of the internal column and the energy of the directed electron flux at the exit of the hollow cathode on the basis of HCD fundamental parameters     

光谱法诊断大气压等离子体电子密度

采用发射光谱方法对大气压氩气介质阻挡放电(DBD)系统中的电子密度进行了诊断。通过考虑放电等离子体中的各种加宽机制, 采用自编的非对称卷积程序对氩原子发射谱线的线型进行分析拟合, 再通过反卷积的方法将各种加宽机制分离开来, 最终将Stark展宽分离出来进行大气压氩气介质阻挡放电电子密度的计算。诊断结果表明, 在大气压氩气介质阻挡放电中当有三个放电丝存在, 电子温度为10000 K时, 电子密度约为4.06×1021 m-3, 诊断结果和模拟结果符合得很好。此方法不仅可以应用在大气压介质阻挡放电中, 还可以用于其他含有非氢气体的大气压等离子体电子密度的测量。     

电子回旋共振离子源等离子体的密度测量

 诊断电子回旋共振离子源等离子体的传统方法是采用传统的单探针无发射时测量伏安曲线,并根据曲线的拐点由理论公式计算出的等离子体密度。本文设计并研制了等离子体密度的测量装置。采用单根朗缪尔探针（该探针可以用来发射电子）测量等离子体的伏安特性。在探针有发射和无发射两种状态下测量得到两条伏安曲线,根据这两条曲线的"分叉点"得到等离子体电位,然后根据该电位直接由计算机计算出电子温度、电子密度。采用该新方法,测量得到的等离子体参量空间电位约为17 V,悬浮电位约为-5 V,电子温度约为4.4 eV,离子密度为1.10×10¹¹cm^-3,与传统方法计算出的等离子体1.12×10¹¹cm^-3相比,两者相差仅1.8％,但新方法效率和精度更高。     

Probe Measurements in a Beam Plasma Discharge

The influence of the beam energy and beam current density on the parameters of the plasma is investigated. The efficiency of carrier generation by collective interactions (Langmuir-oscillations) is compared with that by the single collision mechanism. The additional negative charging of the static probe caused by the beam electrons is used for the determination of the beam electron density.     

An Experimental and Theoretical Study of the Creation Plasma Process in dc Pulsed Discharges

An experimental study of the evolution along the creation process of the plasma parameters characterizing a dc pulsed Argon discharge has been developed. The diagnostic of these parameters is based on the I — V probe characteristic technique. The electron density evolution values so measured are in a good agreement with those obtained from a theoretical model of this process, so the validity of the assumptions involving the model are confirmed.     

Plasma modulation of harmonic emission spectra from laser-plasma interactions

We report results from particle-in-cell simulations of the interaction of intense laser light with overdense plasma designed to examine the effects of plasma waves generated by pulses of fast electrons on high-order harmonic emission from the plasma. We show that the emission spectrum is modulated at the plasma frequency and identify combinations of parameters and circumstances favorable for modulation. In particular, the observed modulation is shown to depend not only on the chosen plasma electron density and intensity of the incident light but on the density profile and pulse shape.     

Density and Temperature Profile Measurement on a Transferred Arc Plasma Using CW Laser Scattering

The electron density and temperature profile of a high-density (1023-m-3, 1.2-eV) transferred arc plasma are obtained with a CW laser scattering technique. Coherent synchronous detection is used to measure the collective scattered light spectra at the thermal level. Local plasma parameters are then inferred from the spectra using the BGK theory modified to account simultaneously for ion-ion as well as ion-neutral collisions in the same model. Those two collisional regimes are clearly identified in the plasma torch. The ability of the model to handle mixed collisional regimes allows a straightforward determination of the local density and temperature independently of position and current level.     

A Small Microwave Plasma Source for Long Column Production without Magnetic Field

A new HF device is described. It allows the production, without the use of a magnetic field, of long plasma columns from a small HF coupling structure situated at one end of the column. Its operation is based on the propagation of a cold plasma surface wave. This device can work (in argon for example) at pressures from 2 mTorr to 20 Torr with electron densities from 1010 cm-3 to 1013 cm-3, depending on plasma diameter and HF power. Typically, 80W of 500 MHz HF will produce a 25 mn diameter column of 1.8 m length. The plasma is quiescent (low electron density fluctuations), efficient (~ 100% absorbed power), and perfectly reproducible. It can be used as a substitute for a positive column, and some practical applications are foreseen in ion production, laser excitation, gas preionization and spectroscopic sources.     

Mega-Electron-Volt Electron Scattering and Radiography of Plasma

A Monte Carlo code is developed to study mega-electron-volt(MeV) electron scattering and transport in plasma based on multiple scattering.A scaling law relating the angular width of a scattered beam to the incident electron energy and the areal density of plasma is found,which may provide a method of MeV electron radiography for diagnosing the areal density of high-temperature,dense plasma under fusion conditions.The study on the MeV electron beam radiography also shows that plasma density interfaces could be discriminated by electron scattering.     

Vacuum Arc Plasma Centrfuge for Element and Isotope Separation

Vacuum arcs have been studied extensively in the past several decades with applications primarily in the areas of switching, vacuum remelting, and vapor deposition. Application of the vacuum arc for element and isotope separation has been studied recently and is reviewed in this paper. An arc was produced in a 30-cm-diameter 4-m-long cylindrical chamber with coaxially mounted electromagnets providing a 2.6-m-long constant axial magnetic field of up to 6 kG. The vacuum discharge between a solid cathode and a mesh anode was triggered electrically. A pulse-forming network (PFN) of 70-m? impedance provided nearly constant-current discharge pulses of several kiloamps and 6-12-ms duration. The magnetized plasma column, flowing axially from the anode with a typical velocity of 106 cm/s, rotated nearly as a solid body. This rotation was due to the E × B drift, produced by the axial magnetic field and the radial electric field across the column. A typical rotation frequency was 105 rad/s. The centrifugal effect due to the rotation caused a radial redistribution of ions within the plasma column, thereby producing elemental and isotope enrichment. The separation was observed to increase exponentially with the square of the radius. Enrichments of up to 300 percent were measured in a Cu-Zn plasma. The radial plasma density profile was found to be roughly Gaussian, with central electron densities of about 1013 cm-3. The radial potential profile across the column was measured and found to be parabolic with radius.     

Nonlinear Dynamics of Laser Interaction with High Z Plasma

The formation and evolution of a plasma in the interaction of laser radiation with gold targets are studied by computer simulations using MHD code (ESC-CASTOR). The nonlinear features of the interaction including the absorption of laser radiation, heating of the plasma components, and the deformation of the density profile under the action of the ponderomotive force are discussed. The role of these nonlinear effects together and the influence of the radiative and collisional transport in the evolution of the density profile are discussed based on the numerically obtained two-dimensional dynamical structures of the hydrodynamic parameter (electron density n_e, ion density n_i, electron temperature T_e, ion temperature T_i), radiation temperature T_r, ponderomotive force profile, and the space-time dependencies of the absorption efficiency and the ionization rates. The radiation spectrum is analyzed and the regularities in the motion of the critical density surface are clarified.