Macroparticle Dynamics during Multi-Cathode-Spot Vacuum Arcs

Macroparticle dynamics in multi-cathode-spot (MCS) vacuum arcs were studied by utilizing laser Doppler anemometry (LDA) methods for in situ measurement of the cathodic macroparticle velocities and relative emission rates. Arc current pulses having peak values of 1-2 kA at either 6 or 1 ms after arc initiation were investigated. Systematic dependence of the macroparticle dynamics (i.e., speed and direction of flight) on cathodic thermophysical properties, location of the measurement probe in the interelectrode region, instantaneous value of the arc current, arc current waveform, and macroparticle size was determined. It was found that the macroparticle velocity increased with the melting temperature of the cathode metal, distance from the cathode surface, and the instantaneous value of the arc current, and decreased with macroparticle size and the rise time of the current waveform. All the above dependencies may be understood as direct indications of the plasma-macroparticle interaction during the discharge. The measured instantaneous relative emission rates were found to peak later than the arc current but before the peak average cathode surface temperature, which was estimated using a semi-empirical model. This result may be an indication of the dependence of cathodic erosion in the form of molten metal droplets on the average cathode surface temperature.     

Macroparticle filtering of high-current vacuum arc plasmas

The transport of vacuum arc plasmas through a 90° curved magnetic macroparticle filter was investigated using a high-current pulsed arc source with a carbon cathode. The peak arc current was in the kiloampere range, exceeding considerably the level of what has been reported in the literature. The main question investigated was whether magnetic macroparticle filters could be scaled up while maintaining the transport efficiency of small filters. In front of the cathode, we found that arc current dependent total ion saturation currents were in the range from 10% to 23% of the arc current. The best relative transmission was 25% (time integrated output/time integrated input) at a duct wall bias of 12.5 V and at an axial magnetic field of about 100 mT. The measured relative transmission of the used high-current arrangement is comparable to what has been observed with other low-current filters. The absolute measurable ion saturation currents at the filter exit reached 70 A at an arc current of about 1000 A     

Macroparticle acceleration by a laser-plasma stream

A physical model is proposed for macroparticle acceleration in an expanding laser plasma. Two acceleration mechanisms are considered, based on absolutely inelastic interaction of the laser-plasma ions with the macroparticle surface: acceleration due to the ion momentum and reactive acceleration due to evaporation of the surface layer. The processes are mathematically described with the aid of the gasdynamics equations with thermal conductivity. Approximate analytic expressions are obtained and permit an estimate of the laser-plasma energy as well as of the quasiparticle momentum as functions of the parameters of the laser pulse, of the target, and of the macroparticle. The analytic solutions are compared with numerical computations using the RAPID program; the comparison confirms that the results agree qualitatively and quantitatively.Quantum Radiophysics Division, Lebedev Physics Institute. Translated from Preprint No. 209 of the Lebedev Physics Institute, Academy of Sciences USSR, Moscow, 1990.     

Characteristics of Macroparticle Emission from a High-Current-Density Multi-Cathode Spot Vacuum Arc

Macroparticle mass transport, size distribution, and spatial distribution were studied in a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc. The macroparticle erosion rate was determined to be 105 ?g/C, and together with ionic emission, accounted for most of the cathodic erosion. The number of macroparticles emitted decreased exponentially with macroparticle diameter, with 20-80-?m macroparticles carrying the bulk of the mass transport. Macroparticles are emitted preferentially at an angle of 20° with respect to the cathode surface. In comparison to previous investigations, higher macroparticle erosion rates, a larger proportion of large macroparticles, and a higher emission angle are observed, and the differences are attributed to the large current density used in the present experiment.     

Macroparticle reflection from a biased substrate in a vacuum arcdeposition system

The reflection of macroparticles, generated by a vacuum arc, from a substrate biased negatively with respect to the surrounding plasma is considered. Charging of macroparticle in the near-substrate sheath and its influence on the macroparticle motion were taken into account. It was found that macroparticles can be either reflected or attracted to the substrate depending strongly on the ion current density. The possibility of macroparticle reflection increases with negative bias voltage and saturates at about a few hundred volts     

In Situ Determination of Macroparticle Velocities in a Copper Vacuum Arc

The velocity components of individual macroparticles (molten droplets) moving through the interelectrode plasma of copper vacuum arc were measured by applying the forward-scattering laser Doppler anemometry method (LDA). The arc was sustained between two cylindrical copper electrodes, 14 mm in diameter and spaced 4 mm apart. Two current waveforms, with rise times to peak currents of 1 and 6 ms and duration of about 5 and 30 ms, respectively, were used in the experiment, while in both cases peak currents were either 1 or 2 kA. Macroparticles traversing through the ellipsoid shaped "probe-volume," which was produced by the intersection of the two He-Ne laser beams, scattered the laser light, through a monochromator, used as a 1.7-A bandpass filter, onto a photomultiplier. The Doppler-frequency component of the photomultiplier was recorded after appropriate filtering and amplification. The macroparticle velocity component obtained from the Doppler frequency was in the plane defined by the illuminating laser beams and directed perpendicularly to the optical axis. Macroparticles were detected during the whole period of the discharge, and their velocity was determined either at the instant of peak current or when the current decreased to 10 percent of its peak value, at several spatial locations inside the discharge volume. The measured macroparticle velocity components ranged from about 10-20 m/s up to about 700 m/s, showing a systematic dependence on the instantaneous value of the arc-current and on the spatial position of the "probe-volume," e. g.     

Excitation and Ionization of Ethylene by Charged Projectiles

Using the time dependent local density approximation, applied to valence electrons, coupled non-adiabatically to molecular dynamics of ions, the collision process between ethylene and fast charged projectiles is studied in the microscopic way. The impact of ionic motion on the ionization is explored to show the importance of treating electronic and ionic degrees of freedom simultaneously. The number of escaped electrons, ionization probabilities are obtained. Furthermore, it is found that the ionic extensions in different directions show the different patterns.     

Fragmentation Cross Sections of 158 AGeV Pb Projectiles

We investigate the fragmentation cross sections of 158 AGeV ^207Pb projectile interacting with different targets using CR39 nuclear track detectors. The exposure is made at SPS beam facility of CERN. The detectors are chemically etched to obtain data in the form of etched cone length. The nuclear and electromagnetic contributions to the measured total charge-changing cross sections are investigated. The estimated results are compared with those found in the literature for similar research work.     

弹丸在混凝土靶中运动轨迹的数值模拟

 针对混凝土的破坏模式，将最初用于研究岩石破碎的连续损伤模型加入到三维非线性有限元动力分析程序LS-DYNA中。利用数值模拟方法研究了弹丸对混凝土的斜侵彻过程，对弹丸在混凝土靶中的几种可能运动轨迹进行了分析，得到了影响弹丸轨迹的几个因素。     

高速动能弹温度场数值模拟

随着兵器发射技术和空气动力学技术的发展,动能弹的发射初速和飞行状态正从超声速向高超声速发展,由此产生了气动热问题.准确预测动能弹温度场是其气动力和热防护设计的关键技术.采用CFD预测温度场的方法,包括平衡流流动控制方程及差分格式,构造平衡流通量Jacob矩阵,在差分格式矢通量分裂过程中嵌入平衡流真实气体模型模拟温度场,获得平衡流气体状态方程.对典型高速动能弹热环境进行验证,考察方法的合理性.对设计的一种新型高超声速动能弹温度场进行数值模拟,为其气动设计及热防护提供了较可靠的数据.     

自动聚焦系统中聚焦窗口的选择及参量的确定

在使用图像分析进行自动聚焦时,聚焦的准确性和有效性至关重要。聚焦窗口过大不仅增加计算量,且背景图像太多会引起误判;窗口太小则会导致目标偏离窗口,亦会引起误判。深入研究了聚焦窗口的选择问题,提出了一种新的选择方法,有效解决了上述问题。引入非均匀采样,可以保证中央的高分辨率和较大的视场范围,既保证目标图像位于聚焦窗口之内,又显著减小了背景图像的负面影响。同时对采样参量做了进一步的研究,以获得更好的效果。实验结果表明该方法有效提高了对焦的准确性和有效性而且减少了数据量,没有影响系统的实时性。     

Four-Body CDCC Analysis for Breakup Reactions of Three-Body Projectiles

We present a new method of smoothing discrete breakup cross sections calculated by the continuum-discretized coupled-channels method based on the complex-scaling method. One of the advantages of this approach is to be applicable to many-body breakup reaction systems. In this work, we apply the new smoothing method to analyses of ¹²C(⁶He, nn ⁴He) and ²⁰⁸Pb(⁶He, nn ⁴He) reactions at 240 MeV/A.     

Focusing of time-reversed reflections

Recently time-reversal techniques have emerged as a new, important and fascinating discipline within wave propagation. Many of the problems involved can best be understood, analysed and optimized based on a random field model for the medium. Here we discuss stable refocusing of second-order time-reversed reflections. This phenomenon may appear as surprising at first. However, we show how it can be understood in very simple terms viewing the wavefield as a stochastic process. We give sufficient conditions on Green's function of the propagation problem for the phenomenon to happen. In particular we discuss acoustic wave propagation in the regime of weak random medium fluctuations and explicitly give the derivation of stable refocusing in this case, illustrating it with numerical examples.     

Focusing holographic systems

A focusing holographic system for the shadow visualization of a cross-section through a transparent irregularity is described. The focusing depth of the system is about 20 mm. Correction of large aberrations is achieved, and the system may also be used to obtain interference patterns in various observation directions. The method may be used in flow dynamics.     

Focusing diffused waveguides

Focusing optical fibre and thin film guide can be made to have nearly the same group velocity for every mode if the optimum distribution of refractive index or thickness of the film is realized. A focusing optical fibre is manufactured from a multilayer glass rod by solid state diffusion. A thin film waveguide is prepared from nitrocellulose by controlling the airflow distribution. Undulation of light propagating in the film is observed, and it is confirmed that the focusing property is determined by the distribution of the film thickness. The field distributions of the TE modes have been analysed for an optical waveguide fabricated by solid state diffusion. Glass waveguide has been manufactured with the appropriate concentration of lead ions to give the maximum propagation constant. A low loss waveguide is fabricated by the diffusion of lead or thallium ion into glass followed by selective etching.     

Focusing of singular beams

The process of focusing of an inhomogeneously polarized beam passed through a system consisting of a uniaxial crystal and a polarization filter is considered. It is shown that the action of this system is equivalent to the action of a phase transparency with a complex relief of the refractive index. In particular, if the angle between the axes of the quarter-wave plate and the polarizer is equal to 45°, this transparency has a helicoidal relief. Simultaneously, two singular beams with different curvatures of their wavefronts arise in the transparency. The action of the lens consists in formation of a combined singular beam having three focal waists. The boundary wave arising upon focusing is able not only to form a new spectrum of toroidal vortices, but also to straighten the focusing region, forming a fairly extended range with a weak divergence.     

Focusing Components in Typical Chaotic Billiards Should be Absolutely Focusing

We demonstrate that the defocusing mechanism fails to work if not all focusing components of the boundary are absolutely focusing. More precisely, we construct billiard tables with arbitrary long free path away from a non-absolutely focusing component such that a nonlinearly stable periodic orbit exists. Therefore the only known standard procedure of constructing chaotic ergodic billiards works in general only if all focusing boundary components are absolutely focusing.