Relativistic Cherenkov microwave oscillator without a guiding magnetic field

A relativistic Cherenkov microwave oscillator without a guiding magnetic field has been designed, constructed, and tested in which a continuous cylindrical electron beam propagates in a short (L ≈ 3λ, λ being the radiation wavelength) resonant slow-wave structure. The electron beam is energy-modulated at the input of the interaction space, which provides conditions for the energy exchange at a wave phase velocity exceeding the particle velocity. The effective beam-wave coupling is provided by a nearly homogeneous profile of the longitudinal electric field component of the synchronous wave in the interaction space cross section. The efficiency of power conversion from high-current electron beam to electromagnetic radiation at E₀₁ mode is about 8% at a maximum output pulse radiation power of 1.2 ± 0.3 GW and a working frequency of 4.05 GHz.     

Induction self-acceleration of a high-current electron beam

Computer modeling using the particle-in-cell method confirmed that self-acceleration by induction of a high-current relativistic electron beam predicted by G. A. Askar’yan [At. énerg. 6, 658 (1959)] may occur when this beam passes through a thick diaphragm containing an aperture. Pis’ma Zh. Tekh. Fiz. 25, 52–56 (July 26, 1999)     

Magnetoplasmon modes in the response of a mesoscopic particle of a semimetal or a nonmagnetic insulator

The macroscopic features of the behavior of a mesoscopic particle in a magnetic field are investigated for mesoscopic particles possessing the properties of a semimetal or a nonmagnetic insulator. It is shown that for a fixed value of the magnetic field the frequency of the Alfve’n magnetoplasma mode falls off monotonically (as 1/R) as the radius of the particle increases. Pis’ma Zh. Tekh. Fiz. 23, 16–21 (December 12, 1997)     

Interaction of charged dust particles. Debye quasimolecule

An analysis is made of an electrically neutral system of two small positive charges surrounded by a common cloud of Boltzmann-distributed electrons. This system, which simulates macroparticle interaction in a dusty plasma, is called a Debye quasimolecule. Calculations are made of the force and potential interaction energy of shielded dust particles as a function of the distance between them. It is shown that the electron shells are polarized so that the forces of attraction of the dust particles in a Debye quasimolecule predominate over the forces of repulsion of their like charges. This may occur up to distances equal to the size of the dust particles. It is concluded that metastable dusty liquids and crystals cannot exist when the electron clouds of the charged particles obey a Boltzmann distribution. Pis’ma Zh. Tekh. Fiz. 25, 83–89 (August 26, 1999)     

Characteristics of microwave beam breakdown in nitrogen and oxygen

The time evolution of the electron density in microwave discharges in nitrogen and oxygen has been investigated. It is shown that the kinetic processes of electron detachment from negative ions or associative ionization of electronically excited gas molecules play a significant role in the interaction between above-threshold microwave radiation and a discharge plasma. Pis’ma Zh. Tekh. Fiz. 23, 74–80 (May 26, 1997)     

Rotation of the wavefront of an optical vortex in free space

It is shown experimentally and theoretically that when an optical vortex propagates in free space, its wavefront rotates through an angle numerically equal to the Gouy phase. It is found that both the energy maximum of the optical vortex light flux and the amplitude zero of the perturbed optical vortex field propagate along the ray surface. It is shown that the ray surface, which is a consequence of the relativistic constraints on the beam group velocity, forms an unparted hyperboloid of revolution and has various properties: 1) the circulation of the Poynting vector on the surface does not depend on the longitudinal coordinate z; 2) the evolution of the light flux and a pure screw dislocation takes place along straight lines of this surface; 3) the Poynting vector on the ray surface is always perpendicular to the wavefront surface. Pis’ma Zh. Tekh. Fiz. 25, 87–94 (March 12, 1999)     

Theory of the undulator superradiance of an electron beam pulse in the group synchronism regime

The properties of the superradiance of a short electron beam pulse moving in an undulator field and exciting a waveguide mode field under group synchronism conditions are analyzed. This regime is shown to be characterized by the maximum growth rate of superradiance instability. The development of such instability results in bunching of the particles and the coherent emission of a short electromagnetic pulse by the beam pulse. Pis’ma Zh. Tekh. Fiz. 25, 8–16 (April 26, 1999)     

Influence of feedback on the complex dynamics of an electron beam with a virtual cathode in a virtode

Results are presented of a numerical simulation of the nonlinear dynamics of a relativistic electron beam with a virtual cathode in a drift relativistic-electron-beam vircator system with and without external feedback. Pis’ma Zh. Tekh. Fiz. 24, 51–57 (March 12, 1998)     

A new class of electrostatic systems which keep plane homogeneous charged-particle beams exactly parallel

An analysis is made of electrostatic systems whose field is formed by two superposed two-dimensional fields with a common plane of symmetry (midplane). It is assumed that these fields overlap in the region where a charged particle beam propagates. The main property of these systems is that they conserve ideally (without angular aberrations) the parallelism of a charged particle beam of uniform energy-to-charge ratio, propagating in the midplane of the field. This new class of electrostatic system includes the four-electrode system given as an example in which each electrode consists of four plates positioned symmetrically relative to the midplane. Pis’ma Zh. Tekh. Fiz. 24, 57–61 (October 12, 1998)     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)     

Motion of a “drift island” in a toroidal magnetic confinement device with Coulomb scattering of the particles

A resonant drift trajectory of a charged-particle in a magnetic field (a “drift island”) can be used to remove high-energy impurities from a thermonuclear plasma and to introduce (inject) high-energy particles into the plasma. As a rule, these effects are studied neglecting the Coulomb scattering, i.e., in the collisionless approximation. In the present letter, the effect of Coulomb scattering of a particle with a resonant trajectory by plasma particles is studied. The conditions under which the drift resonance is maintained are found, i.e., the plasma densities and plasma density profiles for which the “drift island” can still move over the transverse section of the plasma are determined. Pis’ma Zh. Tekh. Fiz. 25, 19–27 (September 26, 1999)     

The Interaction between a Crack and a Particle Cluster

A numerical method has been developed to study the interaction between a crack and second phase particles in a discontinuously reinforced composite material. The simulation is achieved using a ‘dual’ boundary integral method, coupled with a maximum energy release rate criterion for determining the direction of crack propagation. The method has been applied to a composite material composed of components having the elastic properties of Aluminium (matrix) and Silicon Carbide (reinforcement). In particular, the method is used to investigate the crack trajectory and energetics as it interacts with a single particle and with clusters consisting of two particles or a random distribution of ten particles. It is found that although the energy release rate is affected by the particle(s) at relatively large distances, the crack trajectory is not substantially altered until the crack is very close to the particle(s). A pre-existing interface flaw is observed to attract the crack and substantially increase the energy release rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experimental study of the transient process in a pulsed relativistic backward-wave tube for the millimeter range

The influence of the rise time of the current pulse of a nanosecond high-current electron beam on the self-oscillation regime that is established in a relativistic backward-wave tube for the 38 GHz range is investigated experimentally. It is shown that a peak power of more than 50 MW is attained in a time of ∼300 ps. Pis’ma Zh. Tekh. Fiz. 25, 19–23 (May 26, 1999)     

Metal-insulator cathode in an rf electron gun

The possibility of using metal-insulator cathodes in rf guns of 10 cm linear resonant electron accelerators has been studied experimentally for the first time. Results of calculations and the development of a prototype cathode are described. Results of an experimental investigation of the operating regimes of an rf gun are presented. The gun delivered a beam having a particle energy greater than 300 keV, a pulse length of 40–50 ns, and a pulsed current of 3.5–4.5 A. Pis’ma Zh. Tekh. Fiz. 24, 36–39 (October 12, 1998)     

Possibility of using a large orbit regime for operation at bounce-frequency harmonics in a free-electron maser with a guiding magnetic field

It is suggested that a free-electron maser based on the LIU-3000 accelerator (Joint Institute for Nuclear Research, Dubna) can be made to emit in the short-wavelength part of the microwave range by using radiation at harmonics of the bounce frequency of an electron beam propagating along a helical trajectory with a large gyration radius on the transverse inhomogeneity scale of the rf field and selectively exciting a cylindrical waveguide mode whose azimuthal index is equal to the number of the harmonic. Pis’ma Zh. Tekh. Fiz. 25, 30–36 (January 12, 1999)     

Bulk changes in the microhardness of a solid WC-110G13 steel alloy exposed to a low-energy, high-current electron beam

Bulk changes in the microhardness of a solid WC-110G13 steel alloy are studied as a function of the energy density of a low-energy, high-current electron beam, the number of pulses, and the target thickness. It is established that the beam energy density has a threshold at which quasiperiodic changes in the microhardness occur in the bulk of the alloy. Pis’ma Zh. Tekh. Fiz. 25, 54–59 (October 26, 1999)     

Possible use of two-dimensional Bragg structures in an FEL amplifier powered by a sheet electron beam

It is shown that two-dimensional doubly-periodic Bragg structures can be used effectively in the amplification schemes of free electron lasers powered by sheet relativistic electron beams. The presence of such a structure ensures spatial coherence of radiation from different parts of the electron beam when the input signal propagates across the electron flux. Theoretical analysis shows that the gain in the regenerative scheme can reach 50 dB. Pis’ma Zh. Tekh. Fiz. 25, 87–95 (October 12, 1999)     

Probing of a random phase screen by a focused spatially modulated laser beam. Diffraction at a large number of inhomogeneities

The contrast of the Young’s interference fringes formed in the diffraction field when a dynamic random phase screen is illuminated by a focused, spatially modulated laser beam is obtained analytically as a function of the statistical parameters of the screen. A threshold relative bleaching effect is established for a highly dispersive medium when a low-divergence illuminating beam is used. Pis’ma Zh. Tekh. Fiz. 23, 47–53 (October 12, 1997)     

Instability induced in a relativistic plasma flux by the excitation of surface waves

An analysis is made of the instability of a plasma flux caused by the excitation of a new type of cylindrical surface electromagnetic waves at the interface between the flux and a stationary plasma. It is shown that, unlike the conventional case ε₁>0 and ε₂,<0, at the interface of a relativistic plasma beam there exist growing surface waves at frequencies corresponding to positive values of the permittivities on both sides of the discontinuity. For a given geometry and plasma density the critical parameter for the excitation of these waves is the wave radius of the flux. Pis’ma Zh. Tekh. Fiz. 23, 76–79 (March 12, 1997)     

Investigation of the propagation of microwave beams in an inhomogeneous plasma in a magnetic field

An analysis is made of the propagation of microwave beams in an inhomogeneous magnetized plasma. It is shown that the presence of a plasma increases the astigmatism of the beams, increases the amplitude of the beam compared with vacuum conditions, and reduces the curvature of their phase fronts compared with vacuum. Pis’ma Zh. Tekh. Fiz. 25, 67–71 (December 12, 1999)