Mechanism of Runaway Electron Generation at Gas Pressures from a Few Atmospheres to Several Tens of Atmospheres

The mechanism of runaway electron generation at gas pressures from a few atmospheres to several tens of atmospheres is proposed. According to this mechanism, the electrons pass into the runaway mode in the enhanced field zone that arises between a cathode micropoint—a source of field-emission electrons—and the region of the positive ion space charge accumulated near the cathode in the tails of the developing electron avalanches. As a result, volume gas ionization by runaway electrons begins with a time delay required for the formation of the enhanced field zone. This process determines the delay time of breakdown. The influence of the gas pressure on the formation dynamics of the space charge region is analyzed. At gas pressures of a few atmospheres, the space charge arises due to the avalanche multiplication of the very first field-emission electron, whereas at pressures of several tens of atmospheres, the space charge forms as a result of superposition of many electron avalanches with a relatively small number of charge carriers in each.     

Simulation of oscillatory processes in a beam-plasma system with a virtual cathode in gas-filled interaction space

Physical processes occurring in an intense electron beam with a virtual cathode in an interaction space filled with neutral gas are studied in a two-dimensional model. A mathematical model is proposed for investigating complicated self-consistent processes of neutral gas ionization by the beam electrons and the dynamics of an electron beam and heavy positive ions in the common space charge field with allowance for the two-dimensional motion of charged particles. Three characteristic dynamic regimes of the system are revealed: complete suppression of oscillations of the virtual cathode as a result of neutralizing its space charge by positive ions; the pulsed generation regime, in which the ions dynamics repeatedly suppresses and restores the virtual cathode oscillations; and the continuous generation regime with an anomalously high level of noisy oscillations.     

Dynamics of a relativistic electron beam in the vicinity of a spherical injecting body in space plasma

The dynamics of a relativistic electron beam in the vicinity of an injector in the form of a spherical conducting body in a space plasma is considered. An equation describing the radial evolution of a steady electron beam with a self-similar density profile in the electric field of the injector is formulated. A method for calculating the radial evolution of a relativistic electron beam in the vicinity of an injector is developed. The method is based on the numerical integration of a set of ordinary differential equations for the beam radius and field potential in the space charge region under the relevant boundary conditions at the injector surface. Results are presented from numerical simulations of the radial dynamics of an electron beam in the vicinity of a spherical screen system for neutralizing the electric charge carried away by the beam. The numerical results show that the electric field of the injector hastens the beam expansion.     

Nonlinear dynamics and chaotization of oscillations of a virtual cathode in an annular electron beam in a uniform external magnetic field

Results are presented from a numerical study of the effect of an external magnetic field on the conditions and mechanisms for the formation of a virtual cathode in a relativistic electron beam. Characteristic features of the nonlinear dynamics of an electron beam with a virtual cathode are considered when the external magnetic field is varied. Various mechanisms are investigated by which the virtual cathode oscillations become chaotic and their spectrum becomes a multifrequency spectrum, thereby complicating the dynamics of the vircator system. A general mechanism for chaotization of the oscillations of a virtual cathode in a vircator system is revealed: the electron structures that form in an electron beam interact by means of a common space charge field to give rise to additional internal feedback. That the oscillations of a virtual cathode change from the chaotic to the periodic regime is due to the suppression of the mechanism for forming secondary electron structures.     

Unsteady processes during stimulated emission from a relativistic electron beam in a quasi-longitudinal electrostatic pump field

The problem of stimulated emission from a relativistic electron beam in an external electrostatic pump field is studied. A set of nonlinear time-dependent equations for the spatiotemporal dynamics of the undulator radiation amplitude and the amplitude of the beam space charge field is derived. The beam electrons are described by a modified version of the macroparticle method. The regimes of the single-particle and collective Cherenkov effects during convective and absolute instabilities are considered. The nonlinear dynamics of radiation pulses emitted during the instabilities of the beam in its interaction with the forward and backward electromagnetic waves is investigated.     

Perturbation of a collisional plasma by exterior bodies in a uniform external electric field

The charging of a metal sphere in a weakly ionized collisional plasma in a uniform external electric field is investigated with allowance for the effect of the space charge field and ionization-recombination processes. The sphere charge and the spatial distributions of charged plasma particles are calculated both numerically and analytically (for some particular cases) for the case of a strong external field. It is found that the size of the perturbed plasma region is determined by the external field and the intensity of recombination processes. It is shown that the total electric charge (the sphere charge plus the plasma space charge) is zero in accordance with predictions of the theory of static currents in a conducting medium.     

Space charge lens for focusing negative-ion beams

A space charge lens is proposed to focus intense beams of negative hydrogen ions. The focal length of the lens is determined as a function of the parameters of the beam and the gas medium. It is demonstrated experimentally that the lens efficiently focuses H-ion beams with currents of up to ～30 mA and energies of ～10 keV. The measured focal lengths are in good agreement with the calculated ones.     

Two-component structure of the current pulse of a ranaway electron beam generated during electric breakdown of elevated-pressure nitrogen

Conditions are investigated at which two current pulses of ranaway electron beams are generated in elevated-pressure nitrogen during one voltage pulse. It is shown that the regime with two runaway electron beam current pulses takes place at decreased values of the electric field strength E in the gap (or decreased values of the parameter E/p, where p is the gas pressure). The regime with two runaway electron beam current pulses is observed both at high (1500?C3000 Torr) and low (below 100 Torr) pressures. It is shown that, for the second runaway electron beam current pulse to form, the voltage across the gap should be partially reduced during the first pulse. At low nitrogen pressures (~10 Torr), the regime in which two runaway electron beams are generated can be implemented by increasing the breakdown strength of the gap and/or increasing the value of E/p. In experiments carried out in atmospheric-pressure air with a picosecond time resolution, a rather complicated structure of the beam current pulse is observed at a voltage rise time of ~300 ps.     

Interaction between two conducting spheres in weakly ionized collisional plasma

Charging of two conducting spheres in a weakly ionized collisional plasma flow is considered. The spheres are arranged along the flow, and the plasma is assumed to consist of two ion species with the charges equal in magnitude but opposite in sign. The problem is analyzed with allowance for the external electric field, charging of the spheres due to the sedimentation of plasma ions on them, the fields of the sphere charges, the space charge field, and the processes of recombination and molecular diffusion. The interaction between the spheres and plasma is studied by numerically solving a time-dependent problem in a bispherical coordinate system by the finite difference method. The steady-state values of the sphere charges and the distributions of the space charge and ion densities in the ambient plasma are found as functions of the plasma parameters and the distance between the spheres. The electrostatic forces acting on the spheres are determined, and the effects of the external field, the space charge fields, and the fields of the sphere charges are comparatively analyzed. It is shown that, for the considered plasma parameters, the main electrostatic effect in the interaction between two spheres is their mutual approach in the external field due to the difference in their charges (one sphere catches up with the other). Due to the friction force with the neutral gas, this mutual approach is much slower than all other processes in the system. For widely spaced spheres, the results coincide with the solution obtained previously for a solitary sphere.     

Charging of Dust Grains in a Nuclear-Induced Plasma at High Pressures

The process of dust-grain charging in plasmas produced by radioactive decay products or spontaneous fission fragments in air and xenon at high pressures is studied numerically in the hydrodynamic approximation. It is shown that, at sufficiently high rates of gas ionization, the dust grains in air are charged by electrons rather than ions, so that the grain charge in air is comparable to that in electropositive gases. The results of numerical calculations based on a complete model agree well with the experimental data. The time evolution of the grain charge is investigated, and the characteristic time scales on which the grains acquire an electric charge are established. The validity of approximate theories of dust-grain charging in electropositive and electronegative gases at high pressures is examined.     

Dielectric permittivity of a plasma in an external electric field

The ion contribution to the dielectric function of a plasma in an external electric field is determined by applying a kinetic approach to the ions in a parent gas in which the main mechanism for ion scattering is resonant charge exchange. The ion scattering frequency is assumed to be constant.     

Investigation of plasma characteristics in an unbalanced magnetron sputtering system

Results are presented from experimental studies of a magnetron sputtering system for different configurations of the magnetic field above the cathode surface. The current-voltage characteristics of a magnetron discharge at different working gas pressures (0.08–0.3 Pa) and currents in the unbalancing coil were studied. The production and transport of charge carriers in a magnetron discharge with an unbalanced magnetic field was investigated by means of probe measurements of plasma characteristics and ion energies in the region between the substrate and the magnetic trap at the cathode surface. The radial distributions of the ion current density, plasma potential, and floating potential in the unbalanced operating mode are found to have pronounced extrema at the magnetron axis. It is shown that the plasma density near the substrate can be increased considerably when the axial magnetic field is high enough to efficiently confine plasma electrons and prevent their escape to the chamber wall.     

Low-frequency instability of an ion beam in the field of a high-current REB

A study is made of the linear and nonlinear stages of the low-frequency instability of an ensemble of ions that execute radial oscillations in the electric field of the space charge of an unneutralized high-current relativistic electron beam. Nonlinear mechanisms for stabilizing the low-frequency ion instability are considered. It is shown, in particular, that, under certain conditions, the development of the low-frequency instability can lead to the ejection of ions onto the walls of the drift chamber.     

Low-current moderate-pressure RF discharge with secondary electron photoemission

A model is proposed for a low-current RF discharge with secondary electron photoemission from the electrode surface caused by photons originating in the electrode sheath. The low-current state of RF discharges at moderate pressures is peculiar in that the electrons and ions produced during the preceding periods of the RF field promote the development of the discharge during subsequent periods. Since the ion space charge is induced during many periods of the RF field, even comparatively moderate fields in the electrode sheath are sufficient to ensure the conditions under which the current is self-sustaining, in which case the electron photoemission dominates over the remaining secondary processes at the electrode surface. In a low-current RF discharge, the ion-electron emission has essentially no impact on the formation of the electrode sheath because the half-period of the RF field is much shorter than the ion transit time through the sheath. The sheath results from the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface. The sheath parameters are determined by the conditions under which the current in the sheath is selfsustaining due to the secondary electron photoemission from the electrode surface. The capacitive susceptance of the electrode sheath is substantially higher than its electrical conductance. Low-current RF discharges can only exist when the time required for the ions to drift through the sheath and reach the electrode is much longer than the half-period of the RF field.     

Electrostatic potential and Born energy of charged molecules interacting with phospholipid membranes: calculation via 3-D numerical solution of the full Poisson equation.

Understanding the physicochemical basis of the interaction of molecules with lipid bilayers is fundamental to membrane biology. In this study, a new, three-dimensional numerical solution of the full Poisson equation including local dielectric variation is developed using finite difference techniques in order to model electrostatic interactions of charged molecules with a non-uniform dielectric. This solution is used to describe the electric field and electrostatic potential profile of a charged molecule interacting with a phospholipid bilayer in a manner consistent with the known composition and structure of the membrane. Furthermore, the Born interaction energy is then calculated by appropriate integration of the electric field over whole space. Numerical computations indicate that the electrostatic potential profile surrounding a charge molecule and its resultant Born interaction energy are a function of molecular position within the membrane and change most significantly within the polar region of the bilayer. The maximum interaction energy is observed when the charge is placed at the center of the hydrophobic core of the membrane and is strongly dependent on the size of the charge and on the thickness of the hydrocarbon core of the bilayer. The numerical results of this continuum model are compared with various analytical approximations for the Born energy including models established for discontinuous slab dielectrics. The calculated energies agree with the well-known Born analytical expression only when the charge is located near the center of a hydrocarbon core of greater than 60 A in thickness. The Born-image model shows excellent agreement with the numerical results only when modified to include an appropriate effective thickness of the low dielectric region. In addition, a newly derived approximation which considers the local mean dielectric provides a simple and continuous solution that also agrees well with the numerical results.     

Effect of space charge on the energy spectrum of a multispecies ion beam

The parameters of a multispecies metal ion beam extracted with the help of a set of grids from a plasma jet of a pulsed vacuum arc are studied experimentally. It is shown that the beam contains ions with energies that are both significantly lower and higher than the expected energy E _Z = \(\bar Z\) eU _acc, where \(\bar Z\) is the average ion charge number and U _acc is the extracting voltage. As a result, the mean ion energy is lower than E _Z and the ion energy spectrum is substantially wider than that in the plasma jet. It is found that this effect weakens with decreasing discharge current amplitude and that the shape of the spectrum depends on the accelerating voltage. Probe measurements show that, at accelerating voltages higher than 1 kV, a positive space charge forms in the drift gap, due to which the electric potential in the drift gap increases to a few hundred electronvolts. Analysis of experimental data indicates that the observed features of the ion spectrum can be attributed to the effect of the unsteady electric field of the space charge of the ion beam transported through the drift gap.     

Effect of the external magnetic field on the critical current for the onset of a virtual cathode in an electron beam

The critical current at which an unsteady oscillating virtual cathode forms in an electron beam is studied as a function of the external magnetic field guiding the beam electrons. It is shown that the critical beam current decreases with external magnetic field and that there is an optimum magnetic induction at which the critical current for the onset of an oscillating virtual cathode in the beam is minimum. For a strong guiding magnetic field, the critical beam current is described by relationships derived under the assumption that the motion of the beam electrons is one-dimensional. Such behavior is explained by the characteristic features of the dynamics of the beam electrons in longitudinal and radial directions in the interaction space at different inductions of the external magnetic field.     

Plasma dynamics near an earth satellite and neutralization of its electric charge during electron beam injection into the ionosphere

A study is made of the dynamics of the ionospheric plasma in the vicinity of an earth satellite injecting an electron beam. The time evolution of the electric charge of the satellite is determined. The electric potential of the satellite is found to be well below the beam-cutoff potential. It is shown that, under conditions typical of active experiments in space, the plasma electrons are capable of neutralizing the satellite’s charge.     

Observations on charging effects of non-conductive and uncoated materials by ion beam pre-bombardment in scanning electron microscopy

The specimen charging defects of non-conductive materials in scanning electron microscopy are discussed with reference to the surface electric field generated by the illuminating electron beam dose. If the charge density depends on the relaxation time constant as defined by a product of the permittivity and resistivity when known or available, the electric field can be evaluated by the incident dose stored when illuminated by an electron scanning beam.It was found by observation that uncoated or non-conductive materials pre-bombarded by a positive ion beam, which contributes to the generated negative field, together with the charging effects, could be eliminated at the optimum time of neutralization.In the normal process of double fixation and staining of biological specimens, the local electric field produces increased contrast due to polarization effects. The dark and bright images of secondary and backscattered electrons, respectively, can be analysed by taking into account local polarization, in addition to voltage contrast.     

Formation and nonlinear dynamics of the squeezed state of a helical electron beam with additional deceleration

Results of numerical simulations and analysis of the formation and nonlinear dynamics of the squeezed state of a helical electron beam in a vircator with a magnetron injection gun as an electron source and with additional electron deceleration are presented. The ranges of control parameters where the squeezed state can form in such a system are revealed, and specific features of the system dynamics are analyzed. It is shown that the formation of a squeezed state of a nonrelativistic helical electron beam in a system with electron deceleration is accompanied by low-frequency longitudinal dynamics of the space charge.