Formation of a cathode crater in a low-voltage cold-cathode vacuum arc

The cathode crater in a low-voltage cold-cathode vacuum arc is planar. The area of the crater and the density of the current flowing through it depend on the thermal and electrical properties of the cathode metal. Pis’ma Zh. Tekh. Fiz. 24, 14–19 (July 12, 1998)     

Influence of the type of cathode on the dynamic characteristics of ionization waves

The influence of different types of cathodes on the dynamic characteristics of ionization waves is determined experimentally. It is established that when the discharge is initiated by negative pulses, the maximum wave propagation velocity is observed for a cold cathode while the maximum peaking of the output pulse leading edge is observed for a heated cathode. The mechanism for this effect is discussed. Pis’ma Zh. Tekh. Fiz. 24, 39–42 (July 12, 1998)     

Depolarization of ferroelectrics by nanosecond electrical pulses

It is shown that the ordinary pulsed action on a ferroelectric cold cathode, which induces electron emission, results in depolarization of the ferroelectric. This phenomenon must be taken into account in the development and use of cold cathodes. Pis’ma Zh. Tekh. Fiz. 23, 71–74 (January 26, 1997)     

Long-lived explosive-emission cathode for high-power microwave radiation generators

The operation of cold explosive-emission cathodes having a current density of ∼10⁴ A/cm², fabricated using various materials, was investigated under a large number of switching cycles. The cathode voltage was ∼500 kV, the maximum current ∼5 kA, and the pulse duration ∼20 ns. It is shown that when the number of switchings is small (⩽10³ pulses), cathodes having similar geometry exhibit similar emission properties. For most of the materials studied, as the number of switching cycles increases (⩾10³ pulses), the current rise time increases (as far as the pulse duration) and the maximum vacuum diode current decreases. When a graphite cathode was used, the maximum current remained unchanged up to 10⁸ switching cycles. The mass removed from the cathode was determined for various materials. The results were used to achieve continuous operation of a relativistic 3 cm backward-wave tube having an output power of 350–400MW and an almost constant power level during 10⁸ pulses at a repetition frequency of 100–150 Hz. Pis’ma Zh. Tekh. Fiz. 25, 84–94 (November 26, 1999)     

Estimation of a crater volume formed by impact of a projectile on a metallic target

The crater volume has been an important factor in ballistics and has many influences such as material strength, initial projectile velocity, angle of incidence, and nose shape. The goal of this research is to predict the resulting crater volume of a long rod penetration based on the initial projectile velocity and mass. Mooney’s (Bull Seism Soc Am 64(2):473, 1974) displacement equations were used to calculate the elastic crater volume for a given impulse force, P, varying as a delta function in time on the surface of an isotropic, semi-infinite solid. This estimated elastic volume, V_elastic is linearly related to the experimental ballistic volume, V_experimental by an “energy dissipation factor”, k. V_elastic = kV_experimental. The energy dissipation factor “lumps” together elastic and plastic deformation mechanisms. Terminal ballistic data for a steel long-rod projectile into “semiinfinite” steel or aluminum target will be compared to the crater volume calculated through the use of k.     

Identification of self-organizing structures by the numerical simulation of laminar three-dimensional flow around a crater on a plane by a flow of viscous incompressible fluid

Three-dimensional steady-state vortex structures which undergo self-organization during flow around a deep spherical crater on a plane are analyzed by using a factorized finite-volume method to solve the Navier-Stokes equations in a simplified approach based on an analytic definition of metric coefficients. Pis’ma Zh. Tekh. Fiz. 24, 6–12 (March 26, 1998)     

Investigation of the action of ultrahigh pressures on carbon nanotubes

An investigation is made of the possibility of obtaining technical-grade diamonds in ultrahigh-pressure apparatus by using the cathode deposits formed during fullerene production in an arc discharge as raw material. It is shown that carbon nanostructures present in the cathode deposit increase the number of diamond crystallization centers compared with standard graphite. Pis’ma Zh. Tekh. Fiz. 25, 18–21 (March 12, 1999)     

State-of-the-art in analytical characterization of high purity solid samples by different spectroscopic methods

Facilities and some results of several spectroscopic methods which have potential applications in the field of analysis of solid high purity substances and which have been elaborated in Russia, will be discussed in this paper. Laser nondispersive atomic fluorescence method with glow discharge cathode sputtering atomiser, may be used for trace element determination as well as a tool for the investigation of technological processes, viz. deposition of thin films. Investigations on reduction of a background level in the new hollow cathode ion source for mass-spectrometry have been carried out. Laser mass spectrometry with tandem laser mass reflectron is successfully designed and applied for gaseous impurities determination in high pure silicon with limit of detection of 10⁻³–10⁻⁵ ppm wt. Several results of the layer-by-layer and bulk trace analysis of solids by high resolution mass spectrometry with radio frequency powered glow discharge ion source with the limits of detection at 10⁻¹–10⁻³ ppm wt will be presented here. The traditional arc and spark emission technique still finds considerable use. One of the examples considered in the paper is the analysis of metalfullerenes. To overcome the calibration problem the fluorination process inside the electrode crater using zinc fluoride has been investigated.     

Influence of the magnetic self-field on the formation of a cathode plasma jet in steady-state vacuum arcs

It is shown that magnetic pinching of a cathode plasma jet is initiated after various microjets have merged to form a single jet at a distance r>10³ d from the cathode (d is the diameter of the cathode microspot). The plasma jet acquires the form of a paraboloid of revolution whose surface oscillates about the equilibrium position because of the inertia of the plasma transverse motion. At this point, cooling of the jet ceases. Pis’ma Zh. Tekh. Fiz. 23, 47–53 (June 12, 1997)     

Observation of carbon segregation and evolution of vacancy defects in a surface layer of iron exposed to a low-energy high-current electron beam

For the first time, Rutherford backscattering (RBS) and proton-induced x-ray emission (PIXE) using a proton microbeam and a slow positron beam were used to analyze (scan) the surface and near-surface layers of α-Fe after exposure to a low-energy high-current electron beam. Grain-boundary segregation of carbon and the formation of impurity complexes were observed. Proton microbeam scanning revealed regions of reduced local density which are sources of crater formation. Pis’ma Zh. Tekh. Fiz. 24, 13–20 (May 12, 1998)     

Simulating gas-discharge processes at a single cold microscopic point

The development of ionization avalanches in nitrogen at atmospheric pressure near a single cold microscopic point on a cathode surface has been simulated under the conditions of E/P ≫ 1 kV/(cm Torr), where E is the electric field strength and P is the gas pressure. It is established that a layer of dense gas-discharge plasma with a density of ∼10¹⁶ cm⁻³ is formed within a period of ∼1 ps as a result of the gas ionization by electrons emitted from the cathode. The current of fast electrons, which appears due to gas ionization is more than ten times greater than the field emission current and can reach I ∼ 1 A for one microscopic point.     

Condition for a time-constant current in a high-current planar diode

The conditions required to achieve a constant current from an explosive cathode during the pulse in a high-current planar diode are calculated. Pis’ma Zh. Tekh. Fiz. 24, 63–69 (March 12, 1998)     

Possible transport of an electron beam with above-limit current in a vacuum channel with plasma partitions

Computer modeling by the particle-in-cell method is used to demonstrate that a high-current electron beam with an above-limit current can be transported in a vacuum channel with plasma partitions, whereas without these partitions a virtual cathode forms, blocking the channel. Pis’ma Zh. Tekh. Fiz. 24, 86–90 (February 12, 1998)     

Heating of a current filament and formation of constrictions in a pulsed vacuum discharge

It is shown that a rapid current rise in a pulsed vacuum discharge is accompanied by enhanced compression of the current filament by its self-induced magnetic field. As a result, a constriction forms at a distance L≃1 mm from the cathode and the electron temperature increases to 10²–10³ eV at currents of order 1 kA. This behavior explains the observed increase in the degree of ion charge and the appearance of x-rays as the current pulse length decreases. The criterion for a rapid rise is the condition τ<L/V≃10⁻²⁷ s, where τ is the characteristic current amplification time and V≃10⁶ is the velocity of the cathode plasma. Pis’ma Zh. Tekh. Fiz. 24, 50–56 (September 26, 1998)     

Pt–Au/C cathode with enhanced oxygen-reduction activity in PEFCs

Carbon-supported Pt–Au (Pt–Au/C) catalyst is prepared separately by impregnation, colloidal and micro-emulsion methods, and characterized by physical and electrochemical methods. Highest catalytic activity towards oxygen-reduction reaction (ORR) is exhibited by Pt–Au/C catalyst prepared by colloidal method. The optimum atomic ratio of Pt to Au in Pt–Au/C catalyst prepared by colloidal method is determined using linear-sweep and cyclic voltammetry in conjunction with cell-polarization studies. Among 3:1, 2:1 and 1:1 Pt–Au/C catalysts, (3:1) Pt–Au/C exhibits maximum electrochemical activity towards ORR. Powder X-ray diffraction pattern and transmission electron micrograph suggest Pt–Au alloy nanoparticles to be well dispersed onto the carbon-support. Energy dispersive X-ray analysis and inductively coupled plasma-optical emission spectroscopy data suggest that the atomic ratios of the alloying elements match well with the expected values. A polymer electrolyte fuel cell (PEFC) operating at 0·6 V with (3:1) Pt–Au/C cathode delivers a maximum power-density of 0·65 W/cm ² in relation to 0·53 W/cm ² delivered by the PEFC with pristine carbon-supported Pt cathode.     

Extended region of “anomalous” acceleration in the cathode jet of a vacuum discharge

It is observed that “anomalous” acceleration of ions in a pulsed vacuum discharge is initiated at a certain distance from the surface of the cathode and takes place in the bulk of the cathode jet. It is established that the average ion energy increases as the length l of the discharge gap increases. Pis’ma Zh. Tekh. Fiz. 24, 66–70 (February 26, 1998)     

Influence of feedback on the complex dynamics of an electron beam with a virtual cathode in a virtode

Results are presented of a numerical simulation of the nonlinear dynamics of a relativistic electron beam with a virtual cathode in a drift relativistic-electron-beam vircator system with and without external feedback. Pis’ma Zh. Tekh. Fiz. 24, 51–57 (March 12, 1998)     

Self-similarity and branching of wavelet transforms of microwave signals generated by a vircator

The turbulence regimes of an electron flow with a virtual cathode are studied. The microwave signals generated by a vircator are calculated by the method of large particles. The wavelet transforms of these signals are constructed, and it is shown that they have a complicated self-similar and branching structure. It is concluded that the turbulence of an electron flow is caused by macroscopic instabilities. Pis’ma Zh. Tekh. Fiz. 22, 92–96 (December 26, 1996)     

Fractal structure and some physical properties of carbon deposits obtained by sputtering of graphite in an electric arc

New experimental results are presented to show that when graphite is sputtered in an electric arc, a carbon deposit having a fractal structure is formed on the cathode. The superior microhardness and high microporosity indicate that this fractal carbon deposit may have applications in engineering. Pis’ma Zh. Tekh. Fiz. 23, 71–75 (July 12, 1997)     

Thermionic emission galvanomagnetic effect

Experiments are described which permit the existence of an appreciable thermionic emission current from a photocathode to be established and the dependence of the thermionic emission current density of the cathode on the temperature, magnetic field, and current passed through the photocathode to be measured. Pis’ma Zh. Tekh. Fiz. 24, 63–67 (January 26, 1998)