Breakdown limits on Gigavolt-per-meter electron-beam-driven wakefields in dielectric structures

First measurements of the breakdown threshold in a dielectric subjected to GV/m wakefields produced by short (30-330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of 100 microm inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to 27 GV/m to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches 13.8+/-0.7 GV/m. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.     

Generation and amplification of terahertz electromagnetic waves in a plasma waveguide with a dielectric rod and an annular degenerate plasma

The theoretical investigation of terahertz waves and their excitation by electron beam injection in a cylindrical metallic plasma waveguide is investigated. This metallic waveguide includes a cold collisionless unmagnetized annular degenerate plasma and a dielectric rod. The dispersion characteristics of these waves, the frequency spectra of fast and slow E-modes waves (transverse magnetic field), are studied. Also, the effects of the dielectric permittivity, the radius of the dielectric rod and the radius of the plasma on the frequency spectra and the time growth rate of waves are simulated. In addition, the electric field profiles in this waveguide are plotted.     

Nonlinear theory for relativistic plasma wakefields in the blowout regime

We present a theory for nonlinear, multidimensional plasma waves with phase velocities near the speed of light. It is appropriate for describing plasma waves excited when all electrons are expelled out from a finite region by either the space charge of a short electron beam or the radiation pressure of a short intense laser. It works very well for the first bucket before phase mixing occurs. We separate the plasma response into a cavity or blowout region void of all electrons and a sheath of electrons just beyond the cavity. This simple model permits the derivation of a single equation for the boundary of the cavity. It works particularly well for narrow electron bunches and for short lasers with spot sizes matched to the radius of the cavity. It is also used to describe the structure of both the accelerating and focusing fields in the wake.     

Nonlinear waves in a waveguide partially filled with a plasma

Nonlinear waves have been studied in a circular waveguide partially filled with a cold magnetized collisionless plasma. The solution obtained is periodical with the frequency increasing as a square root of the amplitude of the solution in the first approximation.The author is indebted to K.Jungwirth, R.Klíma and J.Preinhaelter for valuable discussions.     

Second-harmonic generation in a double-tuned microwave cavity partly filled with nonlinear dielectric crystals

The conditions for optimal second-harmonic generation using dielectric crystals in a microwave cavity are investigated. The general relations, obtained by a vector Green's theorem, are specified to a rectangular cavity, where the eigenfunctions can be calculated explicitely. A good agreement is found between theory and experiments performed at a fundamental frequency of 9 GHz using LiNbO₃ as the crystal.     

Coupling frequencies of rectangular waveguide-dielectric resonators partially filled with dielectric

The partial-region method is employed to solve the problem of quasinatural mode H_n0(p+) of coupled waveguide-dielectric resonators with axially and centrally symmetric placement of inserts in an overmoded waveguide. Good agreement is obtained between the calculated and experimental results. The dynamics of coupling-frequency behavior is studied.Khar'kov State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, Nos. 3, 4, pp. 356–362, March–April, 1992.     

Generation of uniform atmospheric pressure argon glow plasma by dielectric barrier discharge

In this paper, atmospheric pressure glow discharges (APGD) in argon generated in parallel plate dielectric barrier discharge system is investigated by means of electrical and optical measurements. Using a high voltage (0–20 kV) power supply operating at 10–30 kHz, homogeneous and steady APGD has been observed between the electrodes with gap spacing from 0.5 mm to 2 mm and with a dielectric barrier of thickness 2 mm while argon gas is fed at a controlled flow rate of 1 l/min. The electron temperature and electron density of the plasma are determined by means of optical emission spectroscopy. Our results show that the electron density of the discharge obtained is of the order of 10¹⁶ cm^???3 while the electron temperature is estimated to be 0.65 eV. The important result is that electron density determined from the line intensity ratio method and stark broadening method are in very good agreement. The Lissajous figure is used to estimate the energy deposited to the glow discharge. It is found that the energy deposited to the discharge is in the range of 20 to 25 μJ with a discharge voltage of 1.85 kV. The energy deposited to the discharge is observed to be higher at smaller gas spacing. The glow discharge plasma is tested to be effective in reducing the hydrophobicity of polyethylene film significantly.     

Propagation of azimuthal waves in magnetoactive waveguides filled with a current-carrying plasma

It is shown that a cylindrical metallic waveguide fully filled with a magnetoactive plasma can propagate coupled ordinarily polarized bulk and extraordinarily polarized surface waves along its azimuthal direction. Interaction between these waves is studied in the case when an external magnetic field has axial and azimuthal components. For the case of a regular profile, a practically convenient analytical expression for a correction to the eigenfrequency of the waves that is due to the external azimuthal magnetic field is derived.     

Self-localization of whispering-gallery modes in a spherical microcavity filled with a cold electron plasma

We have studied the mechanisms and conditions for self-localization of laser radiation in a cold collisionless electron plasma for a spherical microcavity in the quasistatic approximation, taking into account the appearance of striction nonlinearity and also relativistic effects connected with forced oscillatory motion of the electrons. We have shown that when whispering-gallery modes interact nonlinearly with the electron plasma, a spherical plasmon is excited which is localized within the finite volume of the microsphere. We have established that realization of self-localized spherical structures, in the form of an electromagnetic field and spherical plasmons within a microcavity filled with a cold electron plasma, is primarily due to appearance of relativistic nonlinearity of the medium. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 59–64, January–February, 2006.     

Theoretical analysis of a relativistic travelling wave tube filled with plasma

A cold and uniform plasma-filled travelling wave tube with sinusoidally corrugated slow wave structure is driven by a finite thick annular intense relativistic electron beam with the entire system immersed in a strong longitudinal magnetic field. By means of the linear field theory, the dispersion relation for the relativistic travelling wave tube (RTWT) is derived. By numerical computation, the dispersion characteristics of the RTWT are analysed in different cases of various geometric parameters of the slow wave structure and plasma densities. Also the gain versus frequency for three different plasma densities and the peak gain of the tube versus plasma density are analysed. Some useful results are obtained on the basis of the discussion.     

Dispersion relation of acoustic waves in phononic crystals filled with dielectric or metal

The results of the theoretical study of acoustic properties of globular phononic crystals based on artificial opals consisting of closely packed SiO₂ globules ∼200 nm in diameter are presented. Dispersion characteristics of the samples under study were calculated, the group velocity and effective mass of phonons were determined.     

Generation of a strong magnetic field inside a dielectric tube

Experiments on compression of an axial magnetic flux by a plasma liner are conducted with the MIG high-current generator. It is shown that, when the current passing through the liner is about 1 MA, a magnetic field with a strength of up to 50 T and a duration of 300 ns (in the steady state) can be achieved in a dielectric tube 3–4 mm in diameter that is placed on the axis. The physics of magnetic field evolution in the tube is discussed.     

Excitation of accelerating wakefields in inhomogeneous plasmas

The excitation of the wakefields in an inhomogeneous plasma by a short laser pulse is investigated theoretically. A general equation for the wake excitation in transversely nonuniform plasma is derived. This equation is applied to the step-function density profile model of hollow channel laser wakefield accelerator. A more realistic model, in which the transition between the evacuated channel and the homogeneous surrounding plasma occurs over a finite radial extent, is then analyzed. It is shown that the excited channel made can interact resonantly with the plasma electrons inside the channel wall, leading to secular growth of the electric field. This eventually results in wavebreaking and the dissipation of the accelerating mode. We introduce an effective quality factor Q for the hollow channel laser wakefield geometry. This resonance limits the number of electron bunches that can be accelerated in the wake of single laser pulse     

Studies of nanoporous matrices filled with sodium nitrite by nonlinear dielectric spectroscopy

Variation of nanocrystalline NaNO₂ properties was studied under the conditions of confined geometry by nonlinear dielectric spectroscopy. It was shown that a decrease in the pore size of silicate matrices MCM-41 filled with NaNO₂ leads to an increase of nonlinearity and a change in the shape of the curves of the third harmonic amplitude as a function of temperature.     

Generation of higher-order optical vortices by a dielectric wedge

Diffraction of a Gaussian beam with a system of successively located optical wedges is considered. It is shown that the system is able to form higher-order optical vortices.     

Cutoff of circularly polarized electromagnetic waves in a waveguide filled with axially magnetized plasma

The paper deals with an experimental investigation of the cutoff conditions for RHCP and LHCP waves propagating in an axially magnetized plasma filling a circular waveguide. The results of an experiment carried out in the X-band are in a qualitative agreement with the linear theory.     

Resonant interaction of a magneto-hydrodynamic wave with its second harmonic in a planar plasma filled waveguide

To describe the nonlinear interactions of a magneto-hydrodynamic wave with its second harmonic propagating in a planar waveguide filled with cold magnetized plasma, coupled mode equations have been derived. Solution of these equations shows the possibility of energy transfer between the fundamental and the second harmonic.     

Generation of LRS Bianchi type I universes filled with perfect fluids

A generating technique is presented which converts known LRS Bianchi type I models into new models of the same type. Starting from the general Kasner solutions new classes of models are obtained which add to the rare perfect-fluid solutions not satisfying the equation of state. The physical and kinematical properties of cosmological models are studied.