High-current Relativistic Electron Beam Research Articles

High-current fast electron beam propagation in a gas

Propagation of high-current relativistic electron beam in a gas has been studied experimentally in [1] in connection with the fast ignition concept of the inertial confinement fusion. In this paper, our numerical code based on the Particle-in-Cell method is utilized to simulate the electron beam propagation under the conditions similar to those of the experiment [1]. Our results demonstrate generation of a strong electrostatic field at the surface of the target from which the beam enters into the gas. The strength of this field depends in particular on the density of the gas and the field may reflect a significant part of the beam back into the target and decelerate the propagating beam. Ionization of the gas is provided by the electric field created by a dense bunch of runaway electrons and the ionization front may propagate with the velocity close to the velocity of light until the runaway bunch is depleted.

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.

Dynamics of a coaxial back-wave ubitron

A theory of a coaxial back-wave ubitron pumped by a high-current relativistic electron beam is constructed. The start currents of an ubitron are determined. The nonlinear dynamics of microwave generation at different electron beam currents is investigated.

Spall fracture of coarse-grained and ultrafine-grained aluminum under nanosecond relativistic high-current electron beam

The results of investigations of fracture behavior in coarse-grained and ultrafine-grained aluminum under the action of a nanosecond relativistic high-current electron beam in a SINUS-7 accelerator and under conditions of quasi-static tensile loading are reported. It is shown that for both types of deformation, irrespective of the grain size, the fracture is ductile both in deformation and structural features. Based on the examination of the fracture surface, it is found out that under quasi-static loading decohesion of coarse-grained and ultrafine-grained aluminum occurs through shear, and under spalling condition by rupture. It is shown for both grain structures that the thickness of the separated layer increases with the irradiated specimen thickness.

Frequency stability in plasma relativistic microwave oscillators

Some features in the operation of microwave oscillators based on the interaction of high-current relativistic electron beams with plasma are considered. The frequency of radiation generated by such oscillators depends of the plasma density, which can vary both during a single microwave pulse and from one pulse to another. The magnitude of these variations can reach several percent. The dependence of the oscillator frequency on the plasma density has been analyzed and the possibility of ensuring stable generation has been studied in numerical experiments. It is established that microwave pulses can be generated with a frequency scatter, which does not exceed the natural spectral bandwidth determined by the pulse duration. The results of calculations and numerical simulations are compared to the available experimental data.

Target ionization by a high current relativistic monoenergetic electron beam

The propagation through an insulator of a high-current monoenergetic fast electron beam is investigated in a one-dimensional model. The target ionization provides the charge and current neutralization and enables the beam propagation. The ionization process consists of two stages: (i) the self-consistent electric field ionization of atoms in the beam front and (ii) the collisional ionization of atoms by the return current in the beam body. The ionization in the beam front defines the propagation velocity. The charge neutralization quickly suppresses the electric field behind the beam front and the plasma heating by the return current supports the collisional ionization in the beam body. This constitutes the main mechanism of the energy loss for high beam densities.

Theory of microwave amplification in a coaxial ubitron

Results are presented from a theoretical study of the microwave amplification process in a coaxial ubitron with an undulator based on permanent annular magnets. Such an undulator is used for both focusing of a high-current relativistic electron beam (REB) and excitation of microwave radiation. The spatial structure of the magnetic field in a coaxial undulator is obtained. It is shown that optimum transport of the REB is possible under certain conditions for the parameters of the coaxial undulator (the intensity of the magnetic field and the undulator period).

Normal Doppler effect in experiments on the interaction of relativistic electron beams with plasma: Plasma relativistic microwave amplifier

The Cherenkov interaction of a high-current relativistic electron beam with a spatially bounded plasma was studied experimentally. In the generation of electromagnetic radiation, an important role is played by the counterpropagating plasma wave produced due to the reflection from the end of the plasma column. It is shown that, at the resonant value of the magnetic field, the normal Doppler effect occurs and the amplitude of the counterpropagating wave decreases. This effect was used to design and create a plasma relativistic microwave amplifier in which 10% of the beam energy is converted into radiation. The radiation frequency is 9.1 GHz, and the radiation spectrum width (±0.17%) is determined by the microwave-pulse duration. The maximum radiation power is 100 MW, the gain factor being 32 dB.

Dynamic fracture of copper under the action of a relativistic high-current electron beam

This paper presents the results of investigations of the dynamic spall fracture of bulk (2–6 mm) copper targets under the action of a relativistic high-current electron beam (1.3 MeV electron energy, 50 ns pulse duration, ∼1010 W/cm2 power density) generated by the SINUS-7 accelerator. By numerical simulation with the use of the BETAIN1 software package it has been found that the amplitude of the stress wave formed is 6 GPa and the deformation rate is 5·105 s−1. As established experimentally, there is a practically linear relationship between the thickness of the target and the thickness of the spalled layer. Comparison of the experimental data and the simulation results has shown that the spall strength of copper under the given conditions is 1.3 GPa, which is in good agreement with the data available in the literature. Analysis performed with the use of fractographic teqniques has revealed that in the case of recrystallized copper the size of the spall pits formed inside the grains is four times greater than the size of the pits formed on the grain boundaries.

Nonlinear electromagnetic theory of low-frequency instability of a relativistic electron beam in plasma with strongly nonpotential plasma and beam waves

A substantially nonpotential nonlinear theory of instability of a straight high-current relativistic electron beam propagating in a plasma waveguide under the conditions of the collective stimulated Cherenkov effect is elaborated. In the most general statement of problem, relativistic nonlinear equations for the temporal dynamics of this instability are obtained. The case of a low-density plasma is considered when both the plasma and beam waves are essentially nonpotential. Under the assumption that weak resonance plasma-beam interaction and linearity of plasma electrons are present, the general equations are transformed (by expanding the electron trajectories and momenta) to cubically nonlinear relativistic equations, which describe nonlinear shifts in frequencies of the plasma and beam waves. Analytic solutions to these equations are obtained with allowance for the dependence of the electron mass on the electron velocity and for the nonpotentiality of the interacting waves. The results are in good agreement with the numerical solutions of the general nonlinear equations in a wide range of values for the parameters.

Nonlinear theory of the low-frequency interaction of ion beams with the virtual cathode of a relativistic electron beam

A study was made of the nonlinear low-frequency interaction of a longitudinal ion beam with a virtual cathode of a relativistic high-current electron beam injected into a cylindrical drift chamber. Cases are considered in which the electron and ion beams have the same radii and in which the radius an ion beam is greaterthan that of an electron beam.

Effect of the parameters of the plasma channel produced by a high-current relativistic electron beam in dense gaseous media on the beam transportation stability

Results are presented from experimental studies of the time evolution of the plasma channel produced by a high-current electron beam (with an electron energy of Ee = 1.1 MeV, a beam current of Ib = 24 kA, and a pulse duration of t = 60 ns) in helium, nitrogen, neon, air, argon, krypton, xenon, and humid air (air: H2O) at pressures from 1 to 760 Torr. It is shown that, in gases characterized by a small ratio of the collision frequency to the gas ionization rate ui, the electron beam produces a broad high-conductivity plasma channel, such that Rb/Rp < 1, where Rb and Rp are the beam and channel radii, respectively. As a result, large-scale resistive hose instability is suppressed.

Excitation of Low-Frequency Oscillations Via Interaction of a High-Current Relativistic Electron Beam with a Radial Ion Flux

The nonlinear excitation of low-frequency oscillations in the case when an ion flux is radially injected into the drift chamber where a tubular relativistic electron beam propagates is studied. A mechanism behind low-frequency ion oscillations is discussed.

Viscous interaction on a flat plate with a surface discharge in magnetic field

A method of producing compact long-lived plasma formations (plasmoids) is described. The method is based on the effect of capture of a high-current relativistic electron beam under conditions of significant overcompensation of space charge of the beam by positive background ions. Requirements are formulated which are placed on the parameters of a plasma-beam system for the realization of this method, and numerical estimates are given of the overall electric charge and energy content of the electron component of a plasmoid. The dynamics of forming a plasmoid are investigated experimentally.

Controlling the Time Interval between Pulses of Two High-Current Relativistic Electron Beams Injected into the Gaseous Volume of a Plasmachemical Reactor

The experimental results on the adjustment of the time pause in the two-pulse mode of generating and transporting high-current relativistic electron beams (REBs) in the gaseous medium of a plasmachemical reactor are presented. A modified circuit for shaping high-voltage pulses installed on the Tonus accelerator makes it possible to generate two successive high-current REBs with a duration of 60 ns. The pulse energies of the first and second REBs are 1.8 and 16 GW, respectively. The use of a short-circuited turn in the region of the anode–cathode gap, which creates a sharply inhomogeneous pulsed magnetic field, helps to extend the control range of the time interval between pulses to 80–300 μs. The transport characteristics of two REBs injected into a 1.4-m-long plasmachemical reactor filled with an N2 : O2 gaseous mixture are studied.

Influence of the electrons reflected from the collector on the parameters of a high-current relativistic electron beam

In plasma microwave oscillators, electrons fall onto the surface of a graphite collector, which leads to the generation of secondary electrons. The influence of the electrons reflected from the collector on the parameters of a high-current relativistic electron beam propagating in a strong longitudinal magnetic field was studied experimentally and by numerical simulations. It is shown that the penetration of the reflected electrons into the drift space can lead to a substantial increase in the depth of the potential well in the drift space, a decrease in the velocity of the beam electrons, and a broadening of the electron energy distribution function.

Classification of the regimes of Cherenkov beam instabilities in plasma waveguides

The regimes of the instabilities of an annular relativistic electron beam in a waveguide with an annular plasma are systematically analyzed and classified. The growth rates of the instabilities are calculated different limiting cases, and the resonance conditions for the development of the instabilities are determined. The fastest growing instability of a high-current relativistic electron beam in a waveguide with a dense plasma is considered. The possible onset of a low-frequency instability of a beam in a waveguide with a low-density plasma is investigated. Typical examples of how the growth rates depend on the perturbation wavenumbers are presented for systems with parameters close to the experimental ones.

Measurements of the transverse electron velocities in high-current microsecond relativistic electron beams in a strong magnetic field

An analyzer is created for time-resolved measurements of the electron pitch-angles in high-current microsecond relativistic electron beams in a strong magnetic field. The electron pitch-angles in a 500-keV relativistic electron beam with a current density of ∼1 kA/cm2 and a 1-µs flat-top current profile are measured. The diode proposed previously by the authors allows one to produce a high-current electron beam in which pitchangles vary only slightly with time and over the beam cross section.

Obtaining high pressures and metastable states in condensed media by using high-current relativistic electron beams

Experimental studies performed at the Russian Research Centre Kurchatov Institute on the interaction of high-current relativistic electron beams with various condensed media, including highly porous materials, are reviewed. The experiments on obtaining high pressures and accomplishing the structural and chemical conversions in the focal spot of a high-current beam are described. The principles of imitating an ultra-highspeed impact and other energetic actions on an obstacle with the help of high-current relativistic beams are discussed. The possibility of using such beams for surface modification is considered. Experimental data on the induced electric conductivity in highly porous SiO2 aerogels in the region of the beam energy deposition are presented.

Two-pulse generation of high-current relativistic electron beams and their transportation in the gaseous medium of a plasmochemical reactor

Experimental results on two-pulse generation and transportation of high-current relativistic electron beams (REBs) through the gaseous medium of a plasmochemical reactor (PR) are presented. The generation of two consecutive high-current REB pulses with a duration of 60 ns was achieved at the Tonus accelerator with modified schemes of high-voltage pulse formation. The first version of the formation scheme enabled pulse powers of 2 and 4.0–9.6 GW with a time interval between the pulses of 500 ns. The second version enabled one to generate pulses with powers of 1.8 and 16 GW and time interval between the pulses of 160 µs. The transportation parameters of an REB injected into a 1.4-m-long PR filled with an N2: O2 gas mixture are studied. The conductance of the plasma produced under the action of the electron beams is measured. It is shown that the schemes proposed provide more efficient (by 35–45%) transportation of the REBs in the reactor volume as compared to single-pulse high-current REBs of the same power and pulse duration.