Electromagnetic pulse amplification in a Cherenkov laser

An investigation is made of the amplification of a Gaussian electromagnetic pulse in a Cherenkov waveguide laser for the cases of long and short waveguides. It is shown that in the first case, the concept of a characteristic pulse duration τ ₀ can be introduced. It is established that when the pulse duration is short (τ<τ ₀) the gain is determined only by its spectral width, and the amplification process leads to a change in the pulse envelope. It is shown that in a short waveguide pulse amplification can be achieved without any change in shape. Zh. Tekh. Fiz. 69, 79–83 (April 1999)     

Weak-pulse transparency enhancement in an optically dense three-level medium induced by a 2π pulse in a neighboring transition (V-scheme)

The coherent V-configuration interaction between an optically dense resonantly-absorbing three-level medium (neon) and two ultrashort superradiance pulses with converging wave fronts is investigated experimentally and theoretically. Both separate and combined propagation of pulses with wavelengths λ ₁=614.3 nm (strong field, θ ₁⩾π) and λ ₃=594.5 nm (weak field, θ ₃≈/20) are studied. For propagation of a separate strong-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at the difference frequency (Δν≈1700 MHz) and the dispersion-diffraction stabilization effect. Under these conditions a weak-field pulse is completely absorbed. Combined propagation of the pulses leads to novel effects. A below-threshold pulse (weak field) was observed to pass through the absorber while interacting coherently with a strong-field pulse at a neighboring transition. It is shown theoretically that absorption of the weak pulse is reduced for two reasons: first, as a result of incoherent transparency of the resonance transition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization between the upper levels via the two-photon processes. When the conditions for combined propagation are met, the latter mechanism ensures inversionless amplification of a weak pulse over a wide band of frequencies. In this case, the gain can even exceed the linear absorption coefficient in absolute value. A difference in propagation velocities of the weak and strong pulses was recorded experimentally, along with a shift in the carrier frequency of the weak field towards the red (≈600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed theoretically. Zh. éksp. Teor. Fiz. 113, 71–88 (January 1998)     

Effects of the diffusive acceleration of particles by shock waves in the primordial matter of the solar system

The effects of the shock wave diffusive acceleration of particles are considered in the case of formation of isotopic relations of the anomalous Xe-HL component of xenon in relic grains of nanodiamonds in chondrites. It is shown that this component could be formed and captured simultaneously with the nanodiamond synthesis in the conditions of the explosive shock wave propagation from supernova outbursts. The specificity of isotopic composition of Xe-HL is due to the high hardness of the spectrum of nuclear-active particles at the shock wave front and its enrichment with heavy isotopes. The spallogenic nature of both the anomalous and normal components of xenon is ascertained, and the role of the subsequent evolutionary processes in the change of their isotopic systems is shown. Experimental evidence of the formation of the power law spectrum of particles with the spectral index γ ∼ 1 by the supersonic turbulence during the carbon-detonation supernova SnIa explosion is obtained; this perhaps opens new perspectives in studying the problem of the origin of cosmic rays. It is shown that at the stage of free expansion of the explosive shock wave, the degree of compression of the matter at the wave front was σ = 31 (the corresponding Mach number M ∼ 97); this led to a 31-fold increase of the magnetic field as well as of the maximum energy of accelerated particles, so that even the energy of protons reached ∼ 3 × 10¹⁵ eV, i.e., the “knee” region.     

Mechanism for the influence of the geometric parameters on the electrical characteristics of Penning ion sources

A steady-state Penning ion source is studied experimentally. Depending on the geometric parameter l _a (the anode length to diameter ratio) and pressure, maxima are observed in the discharge current and in the ion beam current extracted from an aperture at the center of the cathode. It is shown that at pressures of the order of 10⁻⁴ Torr, two maxima appear in these currents: one, for short discharge gaps with l _a =1–1.5, corresponds to a diverging ion beam, and the other, for longer anodes with l _a =4–5, to a collimated ion beam. At pressures of the order of 10⁻⁵ Torr, only one maximum appears in these currents, for short anodes l _a =2–3 with a diverging ion beam. A physical explanation is proposed for these findings. Zh. Tekh. Fiz. 68, 29–32 (September 1998)     

Macroscopic and mesoscopic matter waves

It has been shown earlier [3,6] that matter waves which are known to lie typically in the range of a few angstrom, can also manifest in the macrodomain with a wave length of a few centimeters, for electrons propagating along a magnetic field. This followed from the predictions of a probability amplitude theory by the author [1,2] in the classical macrodomain of the dynamics of charged particles in a magnetic field. It is shown in this paper that this case constitutes only a special case of a generic situation whereby composite systems such as atoms and molecules in their highly excited internal states, can exhibit matter wave manifestation in macro and mesodomains, in one-dimensional scattering. The wave length of these waves is determined, not by the mass of the particle as in the case of the de Broglie wave, but by the frequency ω, of the classical orbital motion of the internal state in the correspondence limit, and is given by a nonquantal expression, λ = 2πv/ω, v being the velocity of the particle. For the electrons in a magnetic field the frequency corresponds to the gyrofrequency, Ω and the nonquantal wave length is given by λ = 2πv _|| /Ω; v _|| being the velocity of electrons along the magnetic field. Received 29 September 2001 / Received in final form 23 May 2002 Published online 19 July 2002     

Solid-echo in solids with molecular motions: Effects of nonzero pulse widths

The effects of nonzero pulse widths on the solid-echo signals in solids with molecular motions have been investigated. It has been shown that in the slow-motion region (M ₂τ_c² ≈ 1) the amplitude of the echo signal is reduced and the maximum of the echo signal is shifted to the end of the second pulse. Comparison of the developed theory with experimental results obtained on polycrystalline C₆H₆ and NH₄Cl demonstrates good agreement between them.     

Cascade processes in a plasma oscillator

A theoretical and numerical analysis is made of the dynamics of nonlinear electron-beam scattering of a wave reflected by the emitting device of a plasma oscillator. It is shown that a counterpropagating plasma wave can interact nonlinearly with other waveguide modes of the system and with charge-density beam waves, leading to changes in the operation of the oscillator. It is established by means of a numerical simulation that the generation efficiency is reduced as a result of scattering of the counterpropagating wave and stimulated emission of a strong-potential plasma wave with phase velocity v _ph=ω/k _z≪c. Zh. éksp. Teor. Fiz. 112, 1299–1311 (October 1997)     

Electromagnetic absorption of acoustic-wave packets in metals

Damping of quasimonochromatic sound-wave packets in metals caused by the electromagnetic interaction of lattice vibrations with resonant current carriers is studied. It is assumed that the acoustic wavelength λ is much shorter than the electron mean free path length l but much longer than the characteristic damping depth σ of an electromagnetic wave in metals under anomalous-skin-effect conditions. It is also assumed that the transit time of a resonant particle through the region of field nonuniformity (∼λ) is shorter than the electronic relaxation time , where is the characteristic velocity of the resonant electrons). The distribution of the potential of the eddy electromagnetic field accompanying an acoustic radiopulse with a prescribed shape is found by solving the kinetic equation and Maxwell’s equations. The force exerted on the lattice by the resonant electrons is investigated, and the equation from the theory of elasticity that describes the evolution of a sound wave is solved. It is shown that a weak damping (of the order of the small parameter a) at the extrema of the deformation on the pulse edges as well as the appearance of high-frequency precursors near the pulse boundaries are characterisic for the evolution of a powerful transverse radiopulse. Such precursors were observed earlier by Fil’ et al., as components of a noise burst arising on the leading edge of a powerful transverse radiopulse propagating in ultrapure gallium. Fiz. Tverd. Tela (St. Petersburg) 40, 966–973 (June 1998)     

Theory of parametric excitation of acoustic waves

A theory of parametric excitation of acoustic waves is constructed. It is shown that nonlinear attenuation is the main restriction mechanism for a parametrically generated sound wave. The intensity of generated waves is directly proportional to the difference ε between the value of pumping and bare attenuation. The calculated proportionality coefficient depends on the shape of the generated sound wave. Why an ordinary pattern does not form for acoustic waves is explained. The structure of the spectrum of excited waves was studied. It is shown that this structure has exponential asymptotic behavior at the frequency. The width of the intensity distribution depends on the shape of a wave. For different cases it behaves as ε ^α with α=1, 8/7, and 4/3. The results are compared with the experimental data of Ref. 5. Zh. éksp. Teor. Fiz. 112, 1630–1648 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Superelasticity and the propagation of shock waves in crystals

The separation of a shock wave into an elastic precursor and a plastic wave is a characteristic phenomenon occurring only in solid media. The existence of the elastic shock wave at pressures p ≈ 10 GPa, which is one or two orders of magnitude higher than the dynamic elastic limit, has been detected in recent numerical calculations and a femtosecond laser experiment. The plastic shock wave has no time to be formed in these ultrashort waves at p ≈ 10 GPa. The processes of the formation and propagation of the elastic and plastic waves in aluminum at higher pressures obtained by means of femtosecond lasers have been analyzed in this work. It has been found that the elastic precursor survives even under the conditions when the pressure behind the plastic front reaches a giant value p ∼ 1 Mbar at which the melting of the metal begins. It has been shown that superelasticity should be taken into account to correctly interpret the preceding laser experiments.     

Decay of an electronic vortex of a collisionless shock wave as a possible mechanism of a “Coulomb explosion”

A new mechanism of a “Coulomb explosion,” where ions are accelerated by the electric field separating charges at the magnetic Debye radius r _B∼B/4πen _e, is proposed on the basis of a nonquasineutral model of electronic vortices in a magnetic field. It is shown by means of numerical calculations that in the process of acceleration of the ions a collisionless shock wave, whose front has an effective width of the order of δ∼r _B, determined by the breakdown of quasineutrality, is formed in a time of the order of ω _pi ⁻¹ , where ω_pi is the ion plasma frequency. The origin of such explosive dynamics is the formation of “holes” in the electron density at characteristic times of the order of ω _pe ⁻¹ (ω_pe is the electronic plasma frequency) as a result of the generation of electronic vorticity by the Weibel instability of an electromagnetic wave. Calculations for a laser pulse with intensity J∼6×10¹⁸ W/cm² show that the ions expand in the radial direction with velocities up to 3.5×10⁸ cm/s. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 669–674 (25 November 1999)     

Use of optical anisotropy for study of ultrafast phase transformations at solid surfaces

A new method of crystalline-order detection in highly absorbing anisotropic crystals is worked out and is demonstrated experimentally on a monocrystal Zn. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarizedreflected wave due to optical anisotropy. The method is applicable to anisotropic metals (for example, Zn, Ti, Cd) and makes it possible to follow changes of crystalline structure in thin (10–100 nm) surface layers. It must be emphasized, that the method permits the detection of changes of the long-range order, whereas most of the conventional methods provide information on changes of the short-range order, which need not be changed on melting and amorphization for certain crystals. Using picosecond laser pump pulse (time duration ≈1 ps) and streak camera “Agat”, surface melting and evaporation of Zn are studied. By means of measurement of time dependencies of s- and p-components of a reflected probe pulse (time duration ≈500 ps) the dynamics of melting and evaporation of a surface layer was studied at various flows of energy laser pump pulse. The characteristic time of disappearance of the long-range order is <3 ps. The crystal structure is restored through 100–300 ps after action of a pump pulse. The theoretical analysis of experimental results was performed. Estimations, based on the proposed model, are in satisfactory agreement with the experimental results. Pump-probe experiments with time resolution higher than 3 ps are in progress. Received: 27 November 1998 / Accepted: 21 April 1999 / Published online: 27 October 1999     

Propagation Characteristics of Infrared Pulse Waves through Windblown Sand and Dust Atmosphere

The physics characteristics of the windblown sand and dust atmosphere at the sand bench of Yellow River in China are discussed. The pulse distortion and time delay of infrared nanosecond pulse propagating through the atmosphere having sand and dust particles are investigated at 1.06 and 3.8 μm, respectively. It is shown that the delay of 10 ns laser pulse propagating through 5 km windblown sand and dust atmosphere are over 1 ns and 10 ns at 1.06 and 3.8 μm, respectively. The pulse spread increases slightly with wavelength increase. The pulse spread of a 10 ns laser pulse is over 40 ns at 3.8 μm. The pulse delay and spread increase rapidly with the sand particle density increasing in atmosphere. The narrower the pulse width is, the more the pulse distortion is. Hence, at infrared band, for a laser pulse propagating in sand and dust atmosphere, the pulse delay and spread are quite severe and need be taken into account for a narrower pulse laser system.     

Hollow cathode glow discharge in long tubes

An analysis is made of some burning characteristics of a hollow-cathode glow discharge with a long tube (L≫D) used as the cathode. It is shown that, as in the case L∼D, the main factor imposing a lower limit on the range of operating voltages is the drift of fast electrons through the aperture in the cavity. Assuming that the electrons move along the cavity as a result of diffusion, it was possible to calculate the critical pressure at which the discharge can no longer burn and to determine the optimum ratio L/D for which the discharge can be sustained at the lowest voltage. The calculations showed satisfactory agreement with the experiment. Zh. Tekh. Fiz. 69, 36–39 (June 1999)     

Magnetization of optically polarized gas atoms by an optical pulse train

The properties of the density matrix and the multipole moments arising in oriented and aligned atoms with zero nuclear spin through the interaction with strong resonant ultrashort pulses with wave vector k ₀ and circular or linear polarization have been found. Calculations have been made for the time-dependent light-induced magnetization μ(t′) of a gas of pre-oriented and prealigned atoms following the passage of a weak resonant elliptically polarized pulse with frequency ω and wave vector k collinear with k ₀. It is shown that for oriented atoms, μ(t′) is an even function of the detuning from resonance, ω-ω _ba, and can be split into two terms whose directions are a consequence of symmetry and are determined by the vectors k ₀ and k as well as by the direction of rotation of the electric fields corresponding to the pulses. For aligned atoms the vector μ(t′) is collinear with k, and the first term is an even function of ω-ω _ba. However, the second term is an odd function of ω-ω _ba and reverses direction when the sign of ω-ω _ba changes, as well as when the orientation of the axes of the polarization ellipse is changed. It is shown that if a series of weak linearly polarized pulses pass through the gas, the light-induced magnetization of the oriented and aligned gas atoms can be decomposed into three factors: the first determines the direction and is a consequence of the symmetry; the second (with the dimensions of magnetic moment) depends on the characteristics of the resonant transitions; and the third is a universal function of t′ and ω-ω _ba that does not depend on the underlying characteristics of the resonant transition. These vector factors and the universal functions are in principle different for oriented and aligned atoms. Zh. éksp. Teor. Fiz. 111, 63–92 (January 1997)     

On Travelling Waves for the Stochastic Fisher–Kolmogorov–Petrovsky–Piscunov Equation

This paper is concerned with properties of the wave speed for the stochastically perturbed Fisher–Kolmogorov–Petrovsky–Piscunov (FKPP) equation. It was shown in the classical 1937 paper by Kolmogorov, Petrovsky and Piscunov that the large time behavior of the solution to the FKPP equation with Heaviside initial data is a travelling wave. In a seminal 1995 paper Mueller and Sowers proved that this also holds for a stochastically perturbed FKPP equation. The wave speed depends on the strength σ of the noise. In this paper bounds on the asymptotic behavior of the wave speed c(σ) as σ→0 and σ→∞ are obtained.     

A study of possibilities of development of GaAs detectors for high-power nanosecond X-ray pulses

It is proposed to use chromium-compensated semi-insulating GaAs detectors for detecting high-power nanosecond X-ray pulses. An X-ray facility based on a small direct-acting electron accelerator “Sinus-150” developed at the Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences, Tomsk was used as a source of pulsed X-ray radiation. The detectors are shown to detect 5 ns pulses under exposure to 30 mR radiation during one pulse without distortions. In so doing, one-ampere currents flow through the active detector area 0.2 cm². It is found that the physical processes limiting the working capacity of GaAs detectors are due to capture of nonequilibrium electrons and holes by deep centers. At the nonequilibrium charge-carrier concentrations higher than 5·10¹³ cm³, nonuniform distributions of the electric field are formed in the active region and these can result in distortion of the output detector signal shape. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 14–18, September, 2008.     

Implementation of the thermonuclear process in D<Superscript>3</Superscript>He-<Superscript>9</Superscript>Be plasma on the basis of a Z pinch with an ultrafast laser ignition

A new concept of inertial-magnetic confinement fusion is proposed. This concept is based on a high-current Z pinch combined with a femtosecond laser. The fusion target is composed of a D³He fuel contained under a high pressure inside a sealed cylindrical capsule made from metallic ⁹Be. An electric discharge along the capsule preheats the target and transforms it into a state of compressed liner. A subsequent TW femtosecond-laser pulse focused on a target end face causes ultrafast cold ignition of a small portion of the D³He fuel. This laser impact generates energetic electrons and ions, which trigger a nuclear-physics mechanism of a catalytic heating of the fuel and also creates a detonation shock wave capable of propagating along the plasma filament. It is shown that the self-sustaining fusion burn wave can appear in the D³He-⁹Be plasma, in which case the bulk of the energy release is carried by nonradioactive ions, with the energy gain being in excess of 50. The possibility of probing the fusion process by means of gamma-ray spectroscopy is also discussed. The radiative-capture reactions ³He(d, γ), D(d, γ), and ³He(³He, γ) naturally accompanying the burning of the D³He fuel are shown to serve as a convenient diagnostic tool. A nuclear “marker” of D³He fusion on the basis of the detection of monochromatic gamma rays produced in the reaction ⁹Be(α, γn), which is induced in the liner beryllium shell by energetic fusion alpha particles, is also examined.     

Laser on nitrogen-electronegative gas mixtures, pumped by inductive energy storage generator: Experiment and theoretical model

The advantages of inductive energy storage (IES) generators for increasing the pulse energy, power, and duration for nitrogen laser pumped by self-sustained transverse discharge have been experimentally demonstrated. A theoretical model is developed and the operation of IES-pumped laser on nitrogen-electronegative gas mixtures is numerically simulated. It is shown experimentally and theoretically that, adding electronegative gases, one can control the pulse shape of lasing on the C³II _u -B³II _g transition in nitrogen. The increase in the electric field strength in the laser gap in N₂-NF₃ and N₂-SF₆ mixtures produced 337.1-nm laser pulses consisting of two spaced peaks and 40–50-ns pulses close to rectangular. The increase in the laser active volume to 6 l (discharge cross section to 6×10 cm²) in N₂–SF₆ mixtures made it possible to obtain the maximum output energy (Q=110 mJ) and UV power (P _las =6 MW). In N₂-NF₃ mixtures, the laser pulse duration was up to ∼100 ns with an energy up to Q=30 mJ.     

Spectra of Stimulated Electromagnetic Emission of the Ionosphere Upon Sweeping of the Pump Wave Frequency Near Gyroharmonics. I. Experimental Results

We present the results of experimental studies of the spectra of the stimulated electromagnetic emission excited in the ionosphere by powerful radio waves during the pump wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron cyclotron frequency nf _ce. The frequency sweep was carried out for long (continuous) pumping in vertical and inclined directions (at 14° and 18° south of the zenith), as well as for the pulse diagnostic wave both with and without additional pumping far from the gyroharmonics. The dependences of the spectral features of the stimulated electromagnetic emission on the ratio between the pump-wave frequency f ₀ (or on the diagnostic-wave frequency f_DW) and nf _ce were analyzed. It is found that near the multiple gyroresonance, different spectral features of the stimulated emission are quenched at the same frequency for different pump-wave frequencies. For a sufficiently large inclination of the pump wave beam from the vertical direction, the intensity of the stimulated electromagnetic emission is notably decreased for f ₀ ≲ nf _ce as compared with f ₀ > nf _ce. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 461–476, June 2008.