The influence of the discharge parameters on the plasma spatial structuring in argon DBDs

The plasma parameters, discharge plasma uniformity and filamentation processes in high pressure (near atmospheric pressure) dielectric barrier discharges (DBD) in argon are studied using the developed two-dimensional 2D(r, z) model. The applied voltage frequency, the voltage shape, the dielectric layers material and its thickness are varied and the effects of such variations on plasma uniformity, discharge structure and operation are studied. The DBD discharges with different dielectric layers thickness, dielectric constants and secondary electron emission coefficients are simulated. It was shown that the dielectric layer thickness is an important parameter for producing high pressure discharges uniform over the radius. The possibility of the radially uniform discharges at atmospheric pressure was shown in the present study.     

高密度氩等离子体电子密度的计算

用屏蔽氢离子模型计算了冲击压缩产生的温度T～2.0eV、密度ρ～0.01g/cm3～0.49g/cm3范围内氩等离子体的电子密度,探讨了不同温度、密度范围内的高密度氩等离子体中粒子之间相互作用对电子密度的影响.     

Measurements of electron density and plasma current distributions in Tokamak plasma

A submillimeter-wave, phase-modulated polarimeter/interferometer is used for simultaneous time-dependent measurement of line-averaged electron density and poloidal field-induced Faraday rotation along chords of the plasma column in ISX-B tokamak. Heterodyne detection and hollow dielectric waveguide are utilized to achieve the high sensitivity required for the multichord experiment. A data analysis code has been developed to reconstruct the asymmetric distributions of plasma density. The validity of the code is examined, and the result shows good agreement with density profiles measured by Thomson scattering.Work sponsored by the Division of Magnetic Fusion Energy, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation.     

The effect of the plasma density gradient on current filamentation instability

The filamentation instability is one of the basic beam-plasma instabilities that play a significant role in the energy deposition mechanism of the relativistic electrons generated by the laser-plasma interaction in the fast ignition scenario. In this paper, the effect of the density gradient into plasma on the filamentation instability was investigated in the Weibel unstable plasma, where the plasma temperature anisotropy can play an important role. Results indicated that the density gradient enhances the instability growth rate so that decreasing the density gradient from the critical surface to the core of fuel leads to instability for longer regions in k space. Also, investigations in the region close to the critical surface showed that for decreasing the beam number density n_b ≤ 0.01n₀, the instability occurs for while this can be different for higher values. Increasing the beam relativistic factor causes a decreasing peak of instability growth rate because of a reduction in beam current, whereas the initial thermal spread of plasma amplifies the filamentation instability.     

Mode-coupling effects in the density dependence of the shear viscosities of dense argon and methane

We argue that the density dependence of the shear viscosities of dense argon at 174, 223, and 301 K and of dense methane at 298 K can be understood on the basis of the mode coupling theory for hard spheres, in particular near the fluid-solid phase transition.     

Ion current density in an expanding laser plasma

The dependence of the ion current density in an expanding laser plasma on the parameters of the laser radiation, the target material, and the distance from the target is investigated theoretically. Calculations needed to design laser ion sources for accelerator technology are performed. An explanation for the two-peaked shape of the collector signals observed in some experiments with a laser plasma is given. Additional possibilities for obtaining information on the experimental conditions from collector signals are considered. Zh. Tekh. Fiz. 68, 28–32 (April 1998)     

The spatial distribution of argon clusters in gas jet

The spatial distribution of argon clusters in gas jet is tested using the Rayleigh scattering method. A pulsed laser is used to acquire the whole evolution of the cluster inside one event. The measured results at a fixed axial position show that the argon clusters grow in less than one millisecond after the nozzle is opened and the cluster size keeps constant during the whole open period of 20 ms. Further results show that the scattering signal along the radial direction is almost Gaussian-distributed and the full width half maximum (FWHM) increases almost linearly when the distance from nozzle increases. The scattering signal along the axial direction is Landau-distributed and the area near the nozzle is most effective for laser cluster interaction.     

膨胀等离子体云的粒子密度时空分布

为了获得超高速碰撞产生等离子体粒子密度的时空分布特性,利用点电荷电场的1维理论模型,综合运用质量守恒方程、动量守恒方程、能量守恒方程和麦克斯韦方程,推导出了等离子体在膨胀过程中粒子密度的时空分布规律。通过对超高速碰撞2024-T4铝靶实验采集的原始数据分析,得到了超高速碰撞2024-T4铝靶产生膨胀等离子体云粒子密度的时空演化规律。     

膨胀等离子体云的粒子密度时空分布

为了获得超高速碰撞产生等离子体粒子密度的时空分布特性,利用点电荷电场的1维理论模型,综合运用质量守恒方程、动量守恒方程、能量守恒方程和麦克斯韦方程,推导出了等离子体在膨胀过程中粒子密度的时空分布规律。通过对超高速碰撞2024-T4铝靶实验采集的原始数据分析,得到了超高速碰撞2024-T4铝靶产生膨胀等离子体云粒子密度的时空演化规律。     

Electron density measurements during a current perturbation of a wall stabilized argon arc

The paper describes the application of a laser interferometric technique to the measurement of electron density changes which occur when a d.c. argon arc is subjected to a current step or a.c. modulation. The frequency of the latter was varied from 50 to 30,000 Hz. A first attempt is made to derive mobility measurements from the data.     

Number density and temperature measurements obtained using sensitive diode laser spectroscopy in an argon plasma

The absolute concentration and translational temperature of the 2p₁₀ and 2p₇ excited states of argon have been measured in an inductively coupled plasma chamber under a variety of operating conditions using both calibrated diode laser frequency modulation spectroscopy and cavity enhanced absorption spectroscopy. Accurate lineshape analysis of frequency modulation signals has been employed to extract the desired information, and is corroborated by cavity enhanced measurements. Temperatures are found to vary linearly with pressure from ∼400 K at 20 mTorr (2.7 Pa) to ∼510 K at 90 mTorr (12 Pa) in a 200 W discharge while concentrations peak at 3.25×10⁸ cm^-3 at 30 mTorr (4 Pa) (also in a 200 W discharge). The uncertainty in the recovered temperature is 7%, dominated by uncertainties in the calibration of the frequency scale. PACS 42.62Fi; 52.70kz     

Density effects on the spectral emission of a high temperature argon plasma

In order to explain the detailed features of radiation spectra obtained from dense argon plasmas, an ionization-radiation model for argon has been constructed which calculates time-integrated spectra as a function of plasma temperature, density and size. The model describes a plasma in collisional-radiative equilibrium (CRE) by solving the time-dependent atomic rate equations for ground state and selected excited state populations of argon coupled with a probability-of-escape radiation transport scheme for both bound-bound and bound-free photons. Results are presented which illustrate basic changes in the X-ray spectra of an argon plasma as density is increased, in particular, relative intensities of resonance, satellite and intercombination lines as well as the free-bound continuum. In addition, temperature and density profiles from 1-D MHD calculations characterizing the peak emission from argon puffed-gas plasmas (≈10¹⁹ions/cm³) and argon-seeded laser-imploded microballons (?10²²ions/cm³) are post-processed using the model and the resulting spectra are discussed.     

The observation of sonic range velocities of normal zone propagation in high current density superconductor

The sonic range velocities of normal zone propagation in high current density superconductor were observed. The experimental results were obtained on samples of 0.16–0.11 mm thickness of a monofilamentary NbTi wire with critical current density J_c about 10⁶ A/cm². The s→n change of the sample sections was recorded after the normal zone stimulation by the magnetic field impulse. The voltage increasing speed reached 1.5×10⁷ V/s, and the normal zone propagation velocity exceeded 5±1 km/s. This high velocity can be caused by the detonation-like propagation of normal phase in superconductor with high current density.     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

稠密Ar等离子体不透明度的计算

用屏蔽氢离子模型计算了冲击压缩产生的温度T～1.8 eV,密度ρ～0.0044 g/cm3稠密氩等离子体随光子能量变化的辐射不透明度,并与实验作了比较,探讨了冲击压缩产生的稠密等离子体中自由-自由吸收、束缚-自由吸收和束缚-束缚吸收对不透明度的贡献.计算结果表明,对冲击压缩产生的稠密等离子体,自由-自由吸收对不透明度的贡献非常大,特别当光子能量较低时(hv～2.0 eV )自由-自由吸收为不透明的主要部分,因此较好地计算自由-自由吸收项对冲击压缩产生的稠密等离子体不透明度的研究是非常重要的.     

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应用电子和多光子集团非弹性碰撞模型和冷等离子体模型，研究了飞秒强激光与线性等离子体发生多光子非线性Compton散射时，散射激光与入射激光形成的飞秒耦合激光场对线性等离子体层中光场和电子密度分布的影响。研究发现，在耦合激光的有质动力作用下，电子密度分布和离子密度分布比Compton散射前的偏离更加严重，电子密度的变化比离子密度的变化更快，产生的静电场更强。即使耦合激光场非常弱，电子的运动仍表现出相对论效应，仍有静电场存在。     

Temperature and current density distributions at spark plasma sintering of inhomogeneous samples

We analyze the effect of inhomogeneity in the properties of a material on the conditions of obtaining thermoelectrics by spark plasma sintering. Inclusions localized and distributed over the volume of materials with different values of electric and thermal conductivities are considered. It is found that the presence of macroscopic inhomogeneities changes the current density distribution in the cross section of the sample being sintered. It is shown that inhomogeneity in the properties of materials during sintering do not substantially affect the temperature field in the sample at the macroscopic level, but change the current density distribution profile. The ranges of variation of the current density in the regions with inhomogeneous electric and thermal conductivities are determined for various types of macroscopically inhomogeneous inclusions and their distribution. The applicability of various models for describing spark plasma sintering is considered.     

The effects of trap density on current bistability and negative differential resistance in organic bistable devices

Current bistable properties and negative differential resistance (NDR) behaviors of organic bistable devices (OBDs) with a single layer were simulated by using Shockley–Reed statistics for the trap population. The current–voltage (I–V) curves were calculated to investigate the effects of the trap density on the NDR characteristics of current bistabilities in the OBDs. The simulation results of the I–V curves showed that the current bistability and the NDR behavior of the OBDs were dominantly attributed to the trapped electrons in the organic layer. The NDR behavior of the I–V curve appeared with increasing trap density, and the increasing rate of the internal electric field caused by the trapped electrons became larger than that of the external electric field due to the applied voltage. This resulted in the appearance of NDR behavior in the I–V curves. These results can help improve understanding of the effects of the trap density on the current bistability and the origin of the NDR behavior in the I–V characteristic in OBDs.     

Study and selection of plasma density optimal spatial distribution for magnetoplasma separation technologies

It is demonstrated that the spatial density characteristics of a plasma used in magnetoplasma separators considerably influence their operation. The forms of distribution profiles n _p = f(r) for plasma-beam and reflective discharges are found experimentally. Comparison between three ways of generating a separation plasma (plasma-beam discharge, helicon-based rf discharge, and reflective discharge) shows that the plasma density spatial distributions obtained in plasma-beam and reflective discharges are optimal in terms of maximal capacity of a separator.     

A new multi-channel heterodyne polarimeter for plasma current and electron density measurements

A spatially-scanning phase-sensitive heterodyne far-infrared (FIR) polarimeter/interferometer for measurement of plasma current and electron densities is described. The instrument utilizes a rotating diffraction grating wheel for the dual purpose of spatially scanning the plasma cross section and providing a Doppler frequency offset for heterodyne detection purposes. Only two detectors are required to obtain accurate interferometric and polarimetric information for 15 discrete spatial chords. A novel approach to the optical processing of the initially linearly polarized probe beam allows the Faraday rotation of the vibrational ellipse to be unambiguously encoded as a phase modulation on the intermediate frequency carrier signals extracted from the two detectors. The plasma birefringence can be recovered from the corresponding amplitude modulation.