Range of existence of self-sustained, subcritical microwave streamer discharges

Data are presented from a study of subcritical microwave streamer discharges in air in an electromagnetic beam. It is shown that, depending on the degree of subcriticality, this kind of discharge can have two forms: a self-sustained discharge and a discharge that is attached to the initiating trigger. The range of subcriticality for the initial field is determined as a function of air pressure within which the self-sustained developed discharge form exists. Zh. Tekh. Fiz. 69, 19–24 (November 1999)     

Stability of a bubble in a dielectric liquid in an external electrostatic field

Critical instability conditions are found for a gas bubble in a liquid dielectric in a uniform external electrostatic field E ₀. It is shown that they depend both on the magnitude of E ₀ and on the properties of the liquid, as well as on the gas pressure in the bubble. In a linear approximation with respect to the square of the eccentricity of an equilibrium spheroidal form, the equilibrium eccentricity of the bubble exceeds the equilibrium eccentricity of a drop in the field E ₀. The gas pressure in the bubble lowers the critical electric field E ₀ for development of an instability in the bubble. Zh. Tekh. Fiz. 69, 43–48 (August 1999)     

Influence of the Emission of Charged Particles on the Characteristics of Glow Discharges with Oscillating Electrons

The influence of the extraction of charged particles on the conditions of sustaining and the characteristics of a glow discharge with oscillating electrons is considered. It is shown that there is some pressure-dependent optimum level for the extraction of ions at which the energy efficiency of the ion source reaches a maximum. Experimentally, it has been established that the sustaining of a discharge is adversely affected by the run-off of fast ionizing electrons from the discharge, whereas the emission of slow plasma electrons can facilitate the sustaining of the high-current variety of discharge and even lead to a stabilization of an unstable gas-discharge structure. It has been shown that due to the different character of the spatial distributions of fast and slow particles in discharges with electrons oscillating in a magnetic field it is possible to realize highly efficient electron emission without loss of discharge stability by extracting electrons from the near-anode region.     

Characteristics of the statistical distribution of the current transmitted by a cathode spot of a vacuum arc in variously oriented magnetic fields

The influence of variously oriented uniform magnetic fields on the cathodic attachment of a low-current vacuum arc with electrodes made of oxygen-free copper and CuCr30 composition is studied. It is found that, if the current is fixed, cathode spots in the arc attachment are distributed by the normal law in the entire range of variation of the amplitude of magnetic induction vector B and angle α between this vector and the normal to the cathode surface. The parameters of the distribution depend on the magnetic field and cathode material. The magnetic field dependence is appreciable only when angle α exceeds some critical value α* (α* ≈ 30° and ≈45° for cathodes made of copper and CuCr30, respectively). At α > α*, the parameters of the distribution become strongly dependent on α, while the B dependence remains weak. Only when α → π/2 does the field amplitude have a pronounced effect on the parameters of the distribution. From the obtained results, we determine the statistical characteristics of the distribution of the mean current transmitted by a cathode spot in variously oriented magnetic fields. The found relationships make it possible to explain the peculiarities of the structure of the cathodic attachment of the high-current vacuum arc stabilized by an external axial magnetic field.     

Potential surface waves at the boundary of a metal with a magnetoactive plasma at finite pressure

The propagation of surface-type potential waves along the interfacial boundary of a plasma with an ideally conducting metal in an external magnetic field perpendicular to the boundary is examined. It is shown that a necessary condition for the existence of these waves in the system is a finite gas kinetic pressure. Dispersion relations for these waves and expressions for the penetration depth of the wave fields into the plasma are obtained, and they are studied numerically for various plasma parameters. The frequency region for propagation of these waves is found. It is also shown that in a nonzero external magnetic field a system of this kind has a range of frequencies in which the wave is a generalized surface wave. Zh. Tekh. Fiz. 69, 30–33 (November 1999)     

Generation of a strong magnetic field inside a dielectric tube

Experiments on compression of an axial magnetic flux by a plasma liner are conducted with the MIG high-current generator. It is shown that, when the current passing through the liner is about 1 MA, a magnetic field with a strength of up to 50 T and a duration of 300 ns (in the steady state) can be achieved in a dielectric tube 3–4 mm in diameter that is placed on the axis. The physics of magnetic field evolution in the tube is discussed.     

Superspherical cumulation. Converging shock waves with amplitudes increasing faster than in spherical cumulation

Experimental, numerical, and theoretical investigations are made of a gas flow generated by a pulsed high-current discharge in an axisymmetric cavity bounded by a spherical lens adjacent to a flat plate. It is shown that the shock wave forming in the discharge and converging toward the axis is accelerated and amplified as it converges. The amplitude of the shock wave increases faster than does that of a spherical converging shock wave. Zh. Tekh. Fiz. 69, 10–18 (March 1999)     

The effect of the anode dimensions on the characteristics of a hollow-cathode glow discharge

An analysis is made of the effect of the anode dimensions and the gas pressure on the possibility of achieving various conditions of burning of a hollow-cathode glow discharge (with negative or positive anode fall), and also with an anode plasma and an electrostatic double layer. Analytical relations that agree with experimental data are found for the voltage across the cathode sheath and the double layer as a function of the anode area and the pressure. Simple expressions are derived for the critical pressure below which no discharge of a particular kind is possible. Zh. Tekh. Fiz. 67, 43–47 (May 1997)     

Quasistationary phase characteristics of a high-pressure volume discharge

This paper gives the results of investigating the effect of processes such as direct ionization by electron collision, recombination, and stepwise ionization in N₂ on the variation of E/p (E is the electric field strength and p is the gas pressure) in the plasma of a high-current volume discharge. It is shown both experimentally and theoretically that the process of stepwise ionization of molecules cannot be neglected in analyzing the energy of such discharges.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 52–56, December, 1976.     

Anomalous behavior of textures in a magnetic field

It is shown that a qualitatively new type of transition to a nonlinear phase is possible in two-dimensional two-phase textures in a magnetic field. The anomalous growth of the effective response (higher-order correlation functions of the current) near the critical magnetic field is studied. The dependence of the critical magnetic field on the angles of microinclusions is calculated. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 115–119 (25 July 1997)     

Plasma parameters of a nonself-sustained microwave discharge created by a programmed pulse

The kinetics of the electrons in the plasma of a nonself-sustained discharge formed at the focus of a microwave beam when the gas is exposed to electromagnetic radiation with an energy flux density that varies with time in a programmed-pulse mode are investigated. It is shown that the temperature of the electrons in the plasma of the localized microwave discharge is of the order of 1 eV and varies weakly during the pump pulse and as the air pressure is varied, while the electron density is an order of magnitude or more lower than the critical density and depends on the level of the pump generator power. It is shown that the degree of ionization of the plasma can be regulated by altering the programmed-pulse mode. Zh. Tekh. Fiz. 67, 19–23 (July 1997)     

Experimental study of the effect of a longitudinal magnetic field on the cathode parts of a glow discharge in helium

The effect of a longitudinal magnetic field on the electrical and optical properties of negative glow and the Faraday dark space of a glow discharge in helium is experimentally studied at a discharge current J_d=11 mA and a pressure of 100–150 Pa in the discharge chamber. An experimental setup is designed and described. A longitudinal electric field is found to be absent in the region of negative glow and the Faraday dark space in both the presence and the absence of the magnetic field. The magnetic field is shown to decrease the voltage of the glow discharge and to increase the total discharge luminosity and the intensities of some spectral lines by a factor of 10–12. The experimental results are explained.     

Model of pulsed electric discharge expansion in dense gas taking into account electron and radiative thermal conductivity. III. Limit discharge temperature

Based on the energy conservation law for the shell of a pulsed high-current electric discharge in dense gas (Xe, Kr, Ar, Ne, He, N₂, air) an equation for the relative temperature of the discharge channel was derived taking into account electron and radiative thermal conductivities and the energy expended to ionization of gas involved in the discharge. The condition under which the limit channel temperature T _LIM is achieved was determined by solving this equation, and the universal dependence between the temperature T _LIM and the ionization potentials of the above gas atoms was obtained. It was shown that this universal dependence is in agreement with experimental data.     

Cathode jets and the retrograde motion of arcs in magnetic fields

The influence of a transverse magnetic field on the jets of an impulse discharge was investigated. It was found that the jet trajectories show a curvature. The amount of the curvature depends on the pressure, the magnetic field intensity, the discharge current, the material and the polarity of the electrodes. The curvature of the jets was explained as a result of the action of forces arising in a plasma flow through a magnetic and electric field; their analysis was carried out on the bases of a MHD model. On hand of a derived solution, a possible influence of the jets on the origin of a retrograde motion is discussed. It is shown that the existence of certain discharge parameters and of a certain magnitude of the magnetic field brings about, in a plasma flying through a magnetic field, an induction of fields and currents under the action of which, on the one hand, the plasma itself flows in a retrograde direction and, on the other hand, there are influenced the trajectories of the carriers (particularly those of ions) of the current in the channel of discharge in front of the cathode spot. The proposed model of the retrograde motion is discussed and compared to the known results.     

On the role of strong turbulence in low-pressure rf discharges in a magnetic field

The results of an experimental and theoretical investigation of strong turbulence produced by an external rf field in the plasma of a beam-plasma discharge are presented. The modulation instability of a rf electric field, perpendicular to a constant magnetic field, in the frequency range between the lower-hybrid and ionic Langmuir frequencies, has been investigated. It is shown that the low-frequency fields can be used to control beam relaxation. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 608–612 (10 November 1996)     

Operational characteristics of a plasma cathode with a grid stabilization in a two-stage ion source

The discharge characteristics and the parameters of the cathode plasma in a two-stage ion source with a grid plasma cathode and a magnetic trap in the anode region are investigated. It is shown that an increase in the gas pressure and the accompanying increase in the reverse ion current in the bipolar diode between the cathode and anode plasmas lead to an increase in the cathode plasma potential and a transition of the cathode into the regime of electron emission from the open plasma boundary. The dependence of the ion current extracted from the anode plasma on the area of the exit aperture of the hollow cathode and the mesh size of the grid plasma cathode is explained. The conditions at which the ion emission current from the anode plasma is maximum are determined. The potential difference at the bipolar diode is measured by using the probe method. It is shown that, when the gas pressures reaches a critical value determined by the mesh size of the grid plasma cathode, the discharge passes into a contracted operating mode, in which the ion current extracted from the anode plasma decreases severalfold.     

Nonlinear microwave response of YBCO single crystal in constant magnetic field

The generation of a microwave second harmonic by a YBCO single crystal in a dc magnetic field was studied. We found that the signal existed only when there was a direct screening current. As a result, the pinning current as a function of magnetic field can be derived directly from the second harmonic intensity versus the magnetic field. The experimental data were interpreted in terms of a generalized model of the critical state taking into account diffusion of vortices and the absence of a barrier stopping vortices from leaving the sample. We have shown that, in a decreasing dc magnetic field, the current density is considerably lower than both the critical and screening current densities in an increasing dc field. Our experiments indicate that vortices are not the sources of radiation at the double frequency. A relation between the mechanism of harmonic generation in the Meissner phase and modulation of the order parameter by the microwave magnetic field (Ginzburg-Landau nonlinearity) is discussed. It is remarkable that, by measuring the second harmonic intensity in the Meissner state versus temperature, one can obtain the magnetic field penetration depth as a function of temperature with fairly good accuracy. Zh. éksp. Teor. Fiz. 115, 2242–2253 (June 1999)     

Dynamics of pulsed magnetron discharge in high-current electron guns

High-speed video filming is applied to investigate low-pressure (argon 0.053–0.133 Pa) pulsed gas discharge dynamics in a planar magnetron built-in the explosive emission cathode of a high-current electron gun. It has been found that, in the beginning, the discharge starts at the side surface of the cathode and then spreads to its face. The stability of the discharge ignition instant at the cathode face as a function of the working pressure of the gas and the cathode design has been studied. It has been found that the pulsed longitudinal magnetic field results in the discharge switching over in the axial direction onto the electron gun collector. A qualitative explanation of the observed discharge behavior has been given.     

Charge-state distribution of ions in a vacuum arc discharge plasma in a high magnetic field

It is shown that the fraction of multiply charged metal ions generated in a vacuum arc discharge plasma grows substantially in a high magnetic field. This effect was observed for more than 30 different cathode materials. A relation is established between growth of the mean charge of the ions and increases in the burning voltage of the arc. It is demonstrated that the burning voltage of the vacuum arc can be ultimately increased to 160 V. Zh. Tekh. Fiz. 68, 39–43 (May 1998)     

Hysteresis in the presence of electron-cyclotron resonance

A method of determining the nonlinear interaction of microwaves with a magnetoactive lowpressure plasma is presented. It is shown by solving simultaneously the problems of the propagation of electromagnetic waves, the balance of charged particles, and the energy balance that near a critical value of the constant magnetic induction the character of the interaction of the wave and the plasma changes substantially: For magnetic fields above the critical field a much stronger interaction is observed, for which the penetration depth of the incident wave into the plasma increases. An investigation of the strong interaction regime showed the existence of hysteresis, which is accompanied by an abrupt transition from one standing-wave mode in plasma to another. It is shown that virtually complete absorption of the wave is possible. Zh. Tekh. Fiz. 69, 31–37 (December 1999)