Influence of the beam-forming conditions on the development of space-charge oscillations in a long-pulse relativistic electron beam

The characteristics of the space-charge oscillations of a long-pulse relativistic electron beam in magnetically insulated diodes are determined for different geometries of the electron acceleration section and for explosive-emission cathodes of different materials. The important role of the stream of electrons having high transverse velocities in the evolution of the oscillations is demonstrated, and the laws governing the generation of this stream are determined. Possible mechanisms of the space-charge oscillations are described, taking into account the interaction of the electron stream in the beam halo with the main electron stream, the development of diocotron instability in the stream of electrons emitted by the outer lateral surface of the plasma emitter, and the instability of the space charge of “long-lived” electrons in the beam transport channel. Zh. Tekh. Fiz. 68, 102–106 (April 1998)     

Determination of the final equilibrium radius and the increase in the emittance of a nonequilibrium relativistic electron beam during transport along an external magnetic field in a gas-plasma scattering medium

An equation describing the evolution of the transverse energy of a segment of a paraxial axisymmetric relativistic electron beam (REB) propagating in a gas-plasma scattering medium along an external magnetic field is used to find the equation relating the final equilibrium radius of the beam to its initial nonequilibrium value. An analytical expression for the increase in the mean-square emittance of an REB during transport up to achievement of the equilibrium state is found for the case considered. The dependence of the final equilibrium radius and the corresponding increase in the mean-square emittance on the density of the scattering medium and the induction of the external magnetic field is investigated. Pis’ma Zh. Tekh. Fiz. 67, 108–111 (July 1997)     

Beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a medium-pressure rarefied gas

A theoretical model is given, along with a numerical analysis of the evolution of beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a rarefied gas at medium pressure. It is shown that the self-stabilization of beam-plasma discharge as a result of longitudinal inhomogeneity of the density of the discharge plasma makes it possible for the beam to traverse the beam chamber with relatively low total energy losses, including ionization losses and energy losses in the generation of oscillations. During the dissociative recombination of electrons and ions of the discharge-driven plasma, heat is released and spent in raising the temperature of the gas. The investigated collective-discharge mechanism underlying heating of the gas for a relativistic beam can be more efficient than the classical heating mechanism due to ionization losses of the beam in pair collisions of its electrons with gas particles. Zh. Tekh. Fiz. 67, 94–98 (May 1997)     

Calculation of the electrostatic tracking force in the transport of relativistic electron beams in Ohmic plasma channels of low conductivity

The interaction force between a paraxial relativistic electron beam and a preformed Ohmic plasma channel of low conductivity is calculated in the electrostatic limit. The dependence of this force on the channel conductivity and the distance from the beam front is found for concrete parameters of the relativistic electron beam and various values of the beam current rise rate. Zh. Tekh. Fiz. 67, 78–80 (December 1997)     

Determination of the dynamic characteristics of aerogels in the energy-release zone of a high-power electron beam

The dynamics of the interaction of a high-current electron beam with SiO₂ aerogels of different density and porosity are investigated by optical methods. A model for describing highly porous materials is developed on the basis of the information obtained regarding the unloading of aerogels in the energy-release zone of an electron beam, as well as the measured energy-release profiles. A corresponding nonlinear self-consistent equation of state is obtained which reflects the fractal properties of aerogels and permits determination of the thermodynamic characteristics of aerogels as the porosity varies tens of times. The influence of electric space charge on the energy-absorption profile of a high-power electron beam in aerogels of different density is discussed. Zh. Tekh. Fiz. 68, 112–120 (October 1998)     

Determination of the profile of energy release from a high-power electron beam in an aerogel

Optical methods are used to investigate the dynamics of the interaction of a high-current electron beam with an aerogel (a highly porous transparent dielectric with a low density ρ=0.36 g/cm³). The measured profile of the glow of the aerogel and the pattern of its expansion are compared with the results of a numerical simulation. The influence of the space charge on the profile of the energy absorption from the high-current relativistic electron beam is discussed. Zh. Tekh. Fiz. 67, 26–32 (November 1997)     

Theoretical investigation of the bunching of an electron beam in relativistic power amplifiers

The modern approach to designing an injector for the driver of a two-beam accelerator is based on the use of a bunched electron beam. The results of simulation and comparison of the processes leading to bunching of a relativistic electron beam in a free-electron laser and in a traveling-wave tube at low electron beam energies are discussed. The simulation and existing experimental results for bunching of an electron beam in a free-electron laser are compared. Zh. Tekh. Fiz. 69, 98–102 (February 1999)     

Electron-beam-induced fluorescence of carbon dioxide clusters. II. Molecular beam with clusters

The fluorescence of CO₂ clusters excited by an electron beam in a molecular beam formed from a free jet expansion of carbon dioxide is investigated. Data on the fluorescence of the clusters are obtained by comparing the electron-beam-induced fluorescence with the signal appearing in an electronic-fluorescence detector as a result of reflection of the cluster beam from an obstacle placed after the electron beam, as well as with the intensity of the cluster beam. It is established that, unlike a jet expansion, for CO₂ clusters in a molecular beam the fluorescence yield from a cluster decreases significantly with increasing cluster size and is very small for clusters of large size. It is concluded on the basis of the results from an investigation of the fluorescence of CO₂ clusters that the electron-beam-induced fluorescence technique can be used for measurements in molecular beams with CO₂ clusters. Zh. Tekh. Fiz. 67, 63–71 (July 1997)     

Effect of an electron beam generated in an X-pinch plasma on the structure of the K spectra of multiply charged ions

The first experimental studies of an electron beam generated in an X pinch on the XP machine (Cornell University, USA) and the BIN machine (P. N. Lebedev Physical Institute, Russian Academy of Sciences) are reported. It is shown that it is possible in an X pinch to isolate the effect of a plasma-generated electron beam on the multiply charged ion radiation. The intensities of the satellite lines corresponding to Li-, Be-, B-, and C-like ions are calculated for the Al spectrum on the basis of a collisional-radiative model with a non-Maxwellian electron distribution in the plasma. The effect of an electron beam on the multiply charged light ion radiation in an X-pinch plasma is demonstrated. Comparing our calculations with the experimental spectra, we conclude that the present model can be used to estimate the electron beam intensity. Zh. éksp. Teor. Fiz. 112, 894–909 (September 1997)     

Formation of a shock wave in aerogel irradiated with a high-current pulsed electron beam

The propagation of pressure jumps excited by a high-current pulsed electron beam in SiO₂ aerogels with density ranging from 0.025 to 0.25 g/cm³ is investigated using a laser differential interferometer and optical methods. Spallation on the back side of the aerogel targets is observed and the velocity of the spallation fragments is measured. The expansion velocity of the aerogel in the direction of the electron beam is determined. The parameters of the shock adiabat are established in a wide range of aerogel porosities. The depth of the energy-deposition zone of the electron beam is determined experimentally as a function of the aerogel density in the range from 0.015 to 0.25 g/cm³. A model describing highly porous materials which reflects the fractal properties of highly porous aerogels is developed on the basis of the experimental data. Numerical calculations of the observed phenomena are performed. Zh. Tekh. Fiz. 69, 18–25 (December 1999)     

Dynamics of an electron beam from a high-current picosecond accelerator

The dynamics of a high-current (10²–10⁴ A) electron beam with energies of 10⁵–10⁶ eV and picosecond duration (10⁻¹⁰ s) at the output of the accelerator tube is investigated. The slowing of electrons by the residual positive charge on the surface of the tube is found to have a significant influence in the case of short pulse durations. The distance of the electron beam from the surface of the tube in vacuum is estimated on the basis of a one-dimensional model. It is shown that the electron radiation can travel to a distance of several centimeters from the surface at current densities below 20 A/mm², whereas at high current densities the beam is trapped near the surface. Zh. Tekh. Fiz. 69, 111–115 (May 1999)     

Physical mechanisms for transition radiation of electromagnetic pulses

The excitation of electromagnetic pulses is examined in various generation schemes. The physical mechanism for excitation of the radiation is studied and the field from transition radiation by high current relativistic electron beams is analyzed. An analysis of the efficiency of the various generation schemes shows that the electron efficiency of beam radiators based on transition radiation may be quite substantial for beam parameters that are easily realized experimentally. Zh. Tekh. Fiz. 69, 90–95 (October 1999)     

Theory of stimulated nonresonant emission from relativistic beams

A study is made of the radiative Pierce instability of a relativistic electron beam in a waveguide stabilized by an infinitely strong magnetic field. Analytical and computational methods are used to determine the growth rate of the instability, as well as the efficiency for conversion of the beam energy into electromagnetic field energy as a function of the beam current, how relativistic the beam is, and the geometry of the system. The physical nature of the instability is clarified and the mechanisms for its saturation are discussed. Zh. éksp. Teor. Fiz. 115, 2037–2050 (June 1999)     

Energy exchange between a test charge and an unstable electron beam

An investigation is made of the interaction between a test charge and an electron beam which is convectively unstable against plasma oscillations. An analysis is made of the negative-mass instability of the longitudinal motion. Expressions are derived for the reaction force exerted by the electron plasma polarized by the moving charge and for the diffusion tensor arising from electric field fluctuations. The approximation of uniform rectilinear motion of a test particle is used to estimate the systematic and diffusion components of the rates of change in its energy. It is shown that in this case, interaction with an unstable electron beam leads to acceleration of the particle in respect to the longitudinal degree of freedom (antifriction) and under certain conditions enhances the friction effect (cooling) in respect to the transverse degrees of freedom. An assessment is made of the possibility of obtaining an overall positive friction effect by using a special “sweeping” procedure whereby the longitudinal velocity of the electron beam is changed in a specific manner consistent with the cooling process. Zh. Tekh. Fiz. 67, 58–61 (November 1997)     

Initiated subcritical microwave streamer discharge and the problem of global elimination of Freons from the Earth’s atmosphere

An analysis is made of an initiated streamer discharge in a microwave wave beam at subcritical field from the point of view of using this type of discharge for global elimination of Freons, which are damaging to the ozone layer, from the Earth’s atmosphere. The various stages of discharge evolution are illustrated with photographs and the physical factors determining its properties at these stages are indicated. Zh. Tekh. Fiz. 68, 15–24 (December 1998)     

Effect of electron beam on structural,linear and nonlinear properties of nanostructured Fluorine doped ZnO thin films

Electron beam induced effects on Fluorine doped ZnO thin films (FZO) grown by chemical spray pyrolysis deposition technique were studied. The samples were exposed to 8 MeV electron beam at different dose rate ranging from 1 kGy to 4 kGy. All films exhibit a polycrystalline nature which shows an increase in crystallanity with irradiation dosages. The electron beam irradiation effectively controls the films surface morphology and its linear optical characteristics. Z-Scan technique was employed to evaluate the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient using a continuous wave laser at 632.8 nm as light source. Enhancement in the third order nonlinear optical properties was were noted due to electron beam irradiation. Tailoring the physical and NLO properties by electron beam, the FZO thin films becomes a promising candidate for various optoelectronic applications such as phase change memory devices, optical pulse compression, optical switching and laser pulse narrowing.     

Nonuniform distribution of absorbed energy in high-resistance materials excited by an electron beam

Experimental results are presented on the changes in the optical characteristics of lithium fluoride induced by an electron beam with time-varying density and pulse energies close to the threshold for destruction of the material. The spatial distribution of color centers is investigated, especially near breakdown channels. Mechanisms for nonuniform accumulation of defects are discussed, along with the fundamental causes of the inhomogeneous energy distributions induced by the high-current electron beam. Concrete results of calculations of the field intensity distribution in LiF crystals during irradiation are presented, based on models of “uniform” and nonuniform charging of the sample. An abrupt increase in the electric field intensity is predicted near the breakdown channel. Zh. Tekh. Fiz. 68, 53–59 (April 1998)     

Influence of nonuniformities of a magnetic-mirror field on the space-time characteristics of a long-pulse relativistic electron beam

An experimental investigation is made of the influence of local nonuniformities of a mirror-configuration magnetic field on oscillations of the space charge and the structure of a long-pulse relativistic electron beam. It is found that the outcome depends on the axial configuration of the nonuniformity. A nonuniformity near the cathode can substantially reduce the amplitude of the oscillations and improve the beam transport. The creation of a nonuniformity far from the cathode leads to an accelerated increase in the oscillations and causes spreading of the transverse structure of the beam. A possible explanation is given for the mechanism responsible for the influence of these local magnetic field nonuniformities assuming reflection of the cathode plasma and electron flux from the magnetic mirror, and also allowing for a jump in the drift velocity. Zh. Tekh. Fiz. 67, 83–88 (August 1997)     

Strong Interaction of Low-Power Electron Beams with the Ionosphere

The late stages of the beam-plasma instability are not yet completely understood. A better knowledge of the evolution of the beam is necessary to explain the results of the electron beam experiments carried out in the ionosphere. An alternative to the complete stabilization by quasi-linear (QL) diffusion is the parametric "stabilization." In this case the beam remains unstable for very long distances, while retaining its "hot-beam" characteristics. A recent flight provides data that indicate the relevance of this mechanism in the evolution of the beam-plasma instability.     

Calculation of the interaction force between a relativistic electron beam and an Ohmic plasma channel

A formula for calculating the interaction force between a relativistic electron beam and a preformed Ohmic plasma channel with an arbitrary offset of the channel axis from the beam axis is obtained in the case of complete charge neutralization. It is shown that this force is repulsive for radial profiles of the conductivity with a peak on the channel axis. Zh. Tekh. Fiz. 67, 69–76 (June 1997)