We deals with the Monte Carlo simulation of the Townsend discharge current in a vicinity of minimum of Paschen’s curve when the high reduced electric fields, E/p = (200–660) V/(cm Torr), are achievable. The main idea in description of the steady-state mode is that a great number of electron avalanches simultaneously generated in the gap provides the current in the external electric circuit. The impact ionization coefficient α (the first Townsend coefficient), obtained in modeling, is in a good agreement with the experiments on measuring the prebreakdown current. However, the interpretation of the process of electron multiplications needs in corrections. The classical impact ionization coefficient is conventionally introduced with invoking the notion of electron drift velocity. In the case of high reduced electric fields, the runaway electrons, which move in the regime of continuous acceleration, essentially contribute both in the expansion of the electron cloud and in the ionization multiplication of the initial emission current.

In a source based on a non-self-sustaining high-voltage glow discharge with the extraction of a large cross section electron beam (2900 cm2) into the atmosphere, the dynamics of the anode plasma during the generation period of an auxiliary glow wire discharge with a hollow cathode is demonstrated. A repetitively pulsed generation mode with a frequency of 10 kHz was used. The features of the formation and evolution of the auxiliary discharge anode plasma were studied by measuring the distribution of the electron beam current density in the atmosphere. By applying a negative potential bias to the anode grid by (50–200 V) relative to the walls of the vacuum chamber, the possibility of increasing the electron beam extraction coefficient into the atmosphere during the discharge generation pause is shown. New modes of electron beam generation and its more efficient extraction into the atmosphere open new prospects for using electron sources of this type both in science and in industrial applications.

The results of measuring the current density distribution of a wide-aperture electron beam outputted into the atmosphere and generated in an accelerator based on a non-self-sustaining high-voltage glow discharge (HVGD) are described. A self-sustained glow wire discharge with a hollow cathode was used to generate the emission plasma. A comparison was made of the inhomogeneity of the beam current density in the atmosphere in continuous and pulse-periodic emission generation modes.

The paper proposes and experimentally demonstrates a method for estimating the coefficient of electron extraction from a plasma emitter based on a low-pressure arc discharge with layered/grid stabilization of the emission plasma boundary. The method is based on the exclusion of the emission current from the total current in the accelerating gap by "sharp" switching off the arc discharge current. The condition for the applicability of the method is an insignificant change in the concentration of the anode plasma during the cutoff of the discharge current pulse. The preliminary data obtained testify in favor of a change in the electron extraction coefficient by up to 20% during a discharge current pulse with a duration of 150 μs.

A new method is presented for increasing the coefficient of electron beam extraction into the atmosphere for accelerators with a non-self-sustained high-voltage glow discharge, which is characterized by high-frequency (tens of kilohertz) generation of an auxiliary discharge. An increase in the efficiency of beam extraction into the atmosphere is achieved by using a pulsed auxiliary glow discharge of the orbitron type, also called wire discharge, and by controlling the pulse duty factor of this discharge while stabilizing the average value of its current. Using the example of the accelerator under study, the possibility of increasing the output coefficient by 0.3 relative to the mode with direct current is shown.

Описаны результаты по измерению распределения плотности тока широкоапертурного электронного пучка, выведенного в атмосферу и сгенерированного в ускорителе с несамостоятельным высоковольтным тле ющим разрядом.Для генерации эмиссионной/анодной плазмы использовался тлеющий разряд с полым катодом орбитронного типа с частотой генерации десятки килогерц.Проведено сравнение неоднородности плотности тока пучка

В работе представлена теория, описывающая влияние скорости движения волны ионизации на характеристики пробоя. Проведена экспериментальная проверка зависимости падения напряжения в передающей линии, происходящего вследствие движения волны ионизации, от скорости ее движения. Результаты экспериментального исследования согласуются с построенной теорией.

The paper proposes numerical and analytical approaches to modeling the generation of an electron beam in a repetitively pulsed regime of a wide-aperture electron accelerator based on secondary ion-electron emission with a plasma emitter. With the help of known codes and the developed approach, separate modes are simulated. Qualitatively, the obtained experimental results are qualitatively explained, which are in good agreement with the developed approach to modeling.

In this work, the region of the auxiliary discharge of an electron accelerator based on a non-self-sustained high-voltage glow discharge is investigated. The paper presents the analytical approach and the results of test modeling of a glow discharge, which plays the role of an auxiliary discharge for plasma generation with subsequent extraction of ions from it into the main high-voltage gap. In this glow discharge, the anode is two tungsten wires, and the cathode is the inner walls of the chamber. In the model, these regions are considered as separate independent nodes, which are connected by a layer of conducting plasma that matches the boundary conditions. A model based on the representation of an auxiliary glow discharge in the form of matched electron and ion diodes makes it possible to estimate the range of possible voltages for each of the near-electrode layers and to determine the plasma potential.

The evolution of the spatial form of the flow versus the Rayleigh number in layers with two rigid horizontal boundaries and layers with a free surface is experimentally investigated depending on the Rayleigh and Marangoni numbers. The experiments were carried out with layers of ethyl alcohol and water. A thermal imager was used to measure temperature fields on free surfaces of liquids.

The development of convective flow in a layer of ethyl alcohol when heating one of the vertical walls of a rectangular cavity was investigated experimentally. Thermal films were obtained, whose processing allowed plotting in time the distribution of temperature and temperature gradients on the free surface of the liquid layer and the opposite thin vertical wall of the cavity after the flow of heated liquid on it.

The processes of the development of free convection of ethanol in the regimes of heating the vertical wall of a rectangular cavity are experimentally investigated. The hydrodynamics in the cavity and the dependence of the temperature field on the opposite thin vertical wall are studied. Non-stationary temperature fields on a thin wall were measured with a thermal imager. The experiments were carried out at different densities of heat fluxes on the heater and at a discrete set of heights of the liquid layers.

The results of an experimental study of breakdown of a cone-to-plane gap, filled with nitrogen at a pressure of 12.5–400 kPa at a negative polarity, and its numerical simulation by the XOOPIC code are presented. The formation of a diffuse discharge is observed within the entire range of pressures. At the nitrogen pressures of up 200 kPa, a large-diameter streamer is formed in the gap. At higher pressure, two streamers of smaller diameters are observed to form. At low nitrogen pressures, the streamer starts forming at a certain distance from the cathode. The numerical simulation demonstrates that under these conditions the electrons rapidly leave the near-cathode region due to the high reduced electric field strength. The excitation of nitrogen molecules is taken into account in the simulation. Using the R 391/394 ratio of the emission intensities from the bands of the N 2 + molecular ion and the N 2 molecule, the dynamics of the electron temperature T e and the reduced electric field strength E/p in the plasma are determined. The streamer velocity is determined from the propagation velocity of the R 391/394 maximum along the gap. The results are compared with the numerical simulation results.

Unsteady thermal gravitational-capillary convection in a rectangular cavity with sudden heating of one of the vertical walls by electric current was experimentally investigated. The development in time of the spatial form of ethyl alcohol flows with the Prandtl number Pr = 16 at 20°C was studied. The development of the hydrodynamic boundary layer on the heated wall and the flow along the free surface of a liquid layer was examined. The profiles of the vertical and horizontal velocity components were measured during the development of boundary layers and flow in a volume of liquid. Evolution of temperature fields on the free surface of a liquid layer was studied using a thermal imager.

This paper addresses the simulation of electron avalanches in nitrogen by means of the Monte Carlo method. The data have been obtained for the conditions of classical measurements of the impact ionization coefficient in a wide range of reduced electric fields E/p. It is shown that at moderate E/p values, the results are in a good agreement with the generally accepted approaches in the description of the avalanches based on the drift motion of the electrons and on the diffusion. The high E/p is achieved in the vicinity of the minimum of Paschen's curve due to decreasing the gas pressure. Then the process of development of the single electron avalanche changes radically. The reason is that the runaway electrons appear, which leads to essential longitudinal expansion of the electron cloud. It is revealed that the average electron energy at the front of the cloud turns out to be larger than that at the backside of the cloud.

The evolution of convective flow in a layer of ethyl alcohol with a free surface after sudden heating of one of the vertical walls of a rectangular cavity is experimentally studied. The evolution of a non-stationary hydrodynamic boundary layer on a heated vertical wall in time is studied using digital video recording and computer processing of video films. The evolution of the flow along the free surface of the liquid layer is investigated. Using a thermal imager, the time evolution of the temperature field on a thin metal vertical wall after the flow of a heated liquid is investigated.

Experimental studies of the processes of crystallization of water as a liquid - simulator of melts with an inverse dependence of the density of melts on temperature - were carried out on two flat models. With the rapid cooling of the bottom of the cavity, substantial undercooling of the bottom layer of the liquid can be observed. The hydrodynamics in the process of growth of the ice layer at the bottom of the cavity was investigated. The effect of convective flow on the rate of crystallization and on the forms of the crystallization front has been studied. The evolution of the temperature field in the process of crystallization of water at the lower cooled boundary of the cavity is studied. It is shown that the evolution of temperature fields tracks the development stages of instability of the bottom layer of a liquid and the development of Rayleigh-Benard convection observed in a hydrodynamic experiment.

The paper deals with the development of the simulation methods applied to the low-pressure discharges in which the reduced electric field E/p is extremely high and there exists a problem for the correct description of the ionization processes in the classical electron avalanches. Here we use the PIC/MC code xoopic for calculation of the impact ionization coefficient alpha, the electron drift velocity and the electron diffusion coefficient D in the electron avalanches in nitrogen. The range of calculations covers the ratio E/p <= 1000 V.(cm.Torr)(-1). The simulation method was tested on a simple analytical model of an electron avalanche and showed good agreement in low and moderate E/p. It is shown that in conditions of high E/p the spatial distribution of electrons in the avalanche head based on the concepts of electron mobility during their drift motion and diffusion coefficient is not quite correct. The work confirmed that at the very low pressures pre-breakdown phenomena and the transition to the next stage of high-current breakdown cannot be based on ideas about classical electron avalanches.