Excitation of Langmuir oscillations by a laser pulse in a semi-infinite dense plasma

A study is made of the nonlinear mechanism for the excitation of Langmuir waves in a dense plasma by an intense laser pulse with the frequency ω = ω_p/2 (where ω_p is the electron plasma frequency).     

Excitation of surface waves at a plasma boundary by a short laser pulse

The excitation of surface waves by a laser pulse as it crosses a vacuum-plasma interface is considered. Surface waves are excited by a vortex electric current that is generated at the plasma boundary by the ponderomotive force of the pulse. The question is considered of how the duration and transverse dimensions of the pulse affect the spatiotemporal distribution and the spectral and energy parameters of the excited surface waves.     

In vitro exposure parameters with linearly and circularly polarized ELF magnetic fields.

A comparison is made of induced current densities, electric fields, and rates of energy deposition during in vitro studies with linearly and circularly polarized, extremely low frequency magnetic fields for a cylindrical volume of culture medium.     

Analysis of induced currents in a rat exposed to 50 Hz linearly and circularly polarized magnetic fields

This paper presents a numerical analysis of currents induced in a rat by linearly and circularly polarized magnetic fields of 50 Hz. Special focus was placed on pineal gland and retina of rats since these organs were often associated with the changes of melatonin synthesis and concentration. Induced currents in two MRI-based rat models with resolutions of up to 0.125 mm(3) were calculated by using the impedance method. We characterized the induced currents by amplitude and polarization. Calculated induced current densities were extremely small, i.e., < 30 microA/m(2) for both linearly and circularly polarized magnetic fields of 1.41 microT (peak). There were no significant differences in amplitude nor polarization of induced currents in the pineal gland between the linearly and the circularly polarized magnetic fields when the polarization was in a vertical plane. In contrast, the magnetic fields rotating in the horizontal plane produced most circularly polarized currents both in the pineal gland and in the retina.     

Generation of strong magnetic fields by counterpropagating moderate-intensity laser pulses in a low-density plasma

A study is made of the generation of strong quasistatic magnetic fields by counterpropagating moderate-intensity laser pulses of different frequencies in a low-density plasma. Strong magnetic fields are generated by small-scale large-amplitude plasma waves excited at different frequencies by ponderomotive forces in the interaction region of laser pulses. It is shown that magnetic fields are generated most efficiently under resonance conditions such that the frequency difference between laser pulses coincides with the plasma frequency. The spatial distribution of quasistatic magnetic fields is investigated, and the pattern of the contour lines of the electric current is calculated.     

Excitation of the isomeric states of silver isotope nuclei in the plasma produced by a subrelativistic picosecond laser pulse

Arrangement and results of experiments on the excitation of the (E _m = 93.125 keV, J ^p = 7/2⁺, T _1/2 = 44.3 s) and (E _m = 88.034 keV, J ^p = 7/2⁺, T _1/2 = 39.6 s) isomeric states of Ag¹⁰⁷ and Ag¹⁰⁹ nuclei under the action of X-ray emission in a hot (T _e ∼0.5 keV) dense plasma produced by a laser pulse with the energy ∼9 J, intensity ∼1.2 × 10¹⁸ W/cm², and duration 0.82 ps on the SOKOL-P facility are described. The experimentally determined half-life of the isomeric states agrees satisfactorily with the half-life of the Ag^107m and Ag^109m isomers, and their number N _m ∼ 6.9 × 10⁴ agrees with the qualitative estimate N _m ∼ 2.8 × 10⁴, obtained within the refined model of the physical processes in laser plasma.     

Auditory response in rats exposed to 2,450 MHz electromagnetic fields in a circularly polarized waveguide

Rats were exposed to 2,450-MHz pulsed microwave fields in a circularly polarized waveguide. The threshold incident energy density per pulse was about 1.5 to 3 microJ/cm2 over the range 1-10 microseconds. The corresponding whole-body averaged specific absorption of energy was 0.9 to 1.8 mJ/kg per pulse. The same response was evoked when the incident energy density or absorbed energy density per pulse was the same, regardless of the pulse widths.     

Excitation of plasma waves by nonlinear currents induced by a high-frequency electromagnetic pulse

Excitation of plasma waves by nonlinear currents induced by a high-frequency electromagnetic pulse is analyzed within the kinetic approach. It is shown that the most efficient source of plasma waves is the nonlinear current arising due to the gradient of the energy density of the high-frequency field. Generation of plasma waves by the drag current is usually less efficient but not negligibly small at relatively high frequencies of electron–ion collisions. The influence of electron collisions on the excitation of plasma waves by pulses of different duration is described quantitatively.     

Propagation and amplification of microwave radiation in a plasma channel created in gas by a high-power femtosecond UV laser pulse

The time evolution of a nonequilibrium plasma channel created in a noble gas by a high-power femtosecond KrF laser pulse is investigated. It is shown that such a channel possesses specific electrodynamic properties and can be used as a waveguide for efficient transportation and amplification of microwave pulses. The propagation of microwave radiation in a plasma waveguide is analyzed by self-consistently solving (i) the Boltzmann kinetic equation for the electron energy distribution function at different spatial points and (ii) the wave equation in the parabolic approximation for a microwave pulse transported along the plasma channel.     

Thomson scattering in a plasma created by a short intense laser pulse

Thomson scattering spectra from a plasma created through ionization of a gas consisting of multielectron atoms by a laser pulse with an intensity of about 10¹⁶ W/cm² or higher and with a duration τ_imp≤100 fs are studied theoretically with allowance for electron groups with different temperatures.     

Scattering of polarized radio waves by Langmuir turbulence in a plasma in a magnetic field

A study is made of radio-wave scattering by Langmuir turbulent pulsations in a plasma in a magnetic field. The effect of this process on the polarization of radio waves at frequencies far above or close to the electron plasma frequency is investigated. The wave scattering by Langmuir turbulence is shown to strongly affect the polarization characteristics. When the optical thickness typical of the scattering process is on the order of unity, the degree of wave polarization can change by 30% both at high frequencies and at frequencies close to the plasma frequency, in which case the circular polarization can reverse direction. It is shown that, as a result of wave scattering by Langmuir turbulence, the degree of circular polarization of radio waves depends on the wavelength even in a uniform magnetic field.     

Generation of quasistatic magnetic fields in the interaction of counterpropagating laser pulses in a low-density plasma

The effect of generation of quasistatic magnetic fields in the interaction of counterpropagating moderate-intensity laser pulses in a low-density plasma is considered. It is shown that the onset of strong magnetic fields is attributed to small-scale large-amplitude plasma waves excited in the region of interaction of laser pulses under the action of the averaged ponderomotive forces. The spatial distribution of quasistatic magnetic fields is investigated, as well as the structure of the magnetic-field and current lines.     

Electron acceleration by Langmuir waves excited by a laser pulse in a semi-infinite plasma

Results are presented from a theoretical investigation of the acceleration of test electrons by a Langmuir wave excited by a short laser pulse at half the electron plasma frequency. Such a pulse penetrates into the plasma over a distance equal to the skin depth and efficiently excites Langmuir waves in the resonant interaction at the second harmonic of the laser frequency. It is shown that the beam of electrons accelerated by these waves is modulated into a train of electron bunches, but because of the initial thermal spread of the accelerated electrons, the bunches widen and begin to overlap, with the result that, at large distances, the electron beam becomes unmodulated.     

Optical properties of a plasma produced by the tunneling ionization of atoms of a matter in the field of a circularly polarized wave

General features of the absorption and reflection of a test wave by a nonequilibrium plasma produced in the tunneling ionization of atoms of a matter by a circularly polarized laser pulse are described. Because of the highly anisotropic distribution of photoelectrons, the optical properties of a nonequilibrium plasma differ considerably from those of a plasma with a Maxwellian electron velocity distribution. Physically, an anomalous behavior of the absorption coefficient and of the phase shift stems from the fact that electron kinetics in the skin layer is modified by the alternating magnetic field of the test wave.     

Vacuum electron acceleration by a tightly focused,radially polarized,relativistically strong laser pulse

A test particle approach is used to solve the problem of direct electron acceleration by a short, intense, radially polarized laser pulse the focal spot diameter of which can be on the order of the laser wavelength. The fields of a tightly focused laser beam are described in terms of the Stratton-Chu integrals, with which to investigate electron acceleration when the paraxial approximation is inapplicable to laser fields. The dynamics of electron motion in a radially polarized, relativistically strong laser field is analyzed depending on the initial position of an electron in the focal region of the laser beam. The properties of the generated jets of accelerated electrons are investigated depending on the tightness of laser pulse focusing. Possible advantages of using radially polarized laser pulses for charged particle acceleration, as opposed to the use of linearly polarized ones, are discussed.     

Breaking of a wake wave excited by a narrow laser pulse in a low-density plasma

The spatial structure of a wake wave excited in a low-density plasma by a laser pulse with a small focal spot radius is studied both analytically and numerically. Numerical study shows that, in a small-amplitude laser field, a wake wave breaks after the formation of an off-axis density maximum, which grows in height away from the pulse to become infinitely high after several periods. Analytical and numerical calculations show that the singularity in the density arises from the intersection of the trajectories of neighboring particles. Numerical simulations demonstrate that, as the laser field amplitude increases, the breaking point of the wake wave rapidly approaches the pulse trailing edge. For weakly nonlinear conditions, an analytic dependence of the coordinate of the breaking point on the amplitude and transverse size of the laser pulse is obtained.     

Stochastic electron acceleration in plasma waves driven by a high-power subpicosecond laser pulse

The mechanism of stochastic electron acceleration and heating by a picosecond laser pulse in underdense plasma is studied using particle-in-cell simulations and theoretical models. The formation of wide electron energy spectra in the simultaneously acting laser and plasma fields is analyzed. It is shown that electron scattering by turbulent plasma fluctuations excited through stimulated forward Raman scattering plays a governing role in the formation of high-energy tails in the electron distribution function.     

Efficiency of ion acceleration by a relativistically strong laser pulse in an underdense plasma

A particle-in-cell simulation is used to investigate ion acceleration by a femtosecond laser pulse propagating in an underdense plasma slab. In plasma slabs with different thicknesses, the ions are found to be accelerated by different mechanisms. It is shown that, for laser pulse intensities in the range (5–10)×10¹⁹ W/cm², the ions are accelerated near the plasma-vacuum interface. __________ Translated from Fizika Plazmy, Vol. 27, No. 3, 2001, pp. 225–234. Original Russian Text Copyright ￠ 2001 by Kuznetsov, Esirkepov, Kamenets, Bulanov.     

Generation of terahertz radiation from a low-density plasma slab irradiated by a laser pulse

The generation of terahertz electromagnetic radiation when a laser pulse propagates through a low-density plasma slab is considered. It is shown that terahertz waves are excited because of the growth of a weakly damped, antisymmetric leaking mode of the plasma slab. The spectral, angular, and energy parameters of the terahertz radiation are investigated, as well as the spatiotemporal structure of the emitted waves. It is demonstrated that terahertz electromagnetic wave fields are generated most efficiently when the pulse length is comparable to the slab thickness.     

Initiation of high-voltage discharge in air by a plasma filament produced by an intense femtosecond laser pulse

High-voltage discharge initiated in air by a plasma filament produced by an intense femtosecond laser pulse was studied experimentally. It is found that the threshold of a laser-induced discharge decrease three-fold as compared to that of a discharge in undisturbed air. It is shown that the formation time of a laser-induced discharge decreases by almost three orders of magnitude as the applied voltage increases by a factor of 2. A numerical model of the discharge process is developed that adequately describes the experimental results. In particular, simulations reproduce the experimentally observed steep dependence of the formation time of a laser-induced discharge on the applied voltage, as well as typical values of the electric field required for such a breakdown.