Generation of plasma Eields in the interaction between two oppositely propagating short laser pulses in an underdense plasma

A study is made of the interaction (“collision”) between two identical laser pulses with lengths much shorter than the diffraction length, propagating in a plasma toward one another. It is shown that the plasma response to the pulses depends essentially on the value of the parameter ω_pτ, where ω_p is the plasma frequency and τ is the pulse duration. Short laser pulses (such that \(\omega _p \tau \leqslant \sqrt 2 \)) efficiently generate plasma waves on two characteristic scale lengths. Large-scale wake waves with a wavelength of about c/ω_p are generated over the entire path of the pulses and form a two-dimensional standing plasma wave in the region between the pulses after their interaction. In the interaction region, the pulses excite small-scale plasma oscillations with a wavelength equal to half the laser wavelength, which remain in the plasma after the interaction. Long laser pulses (such that \(\omega _p \tau \leqslant \sqrt 2 \)) also generate quasistatic plasma perturbations on two scale lengths. Perturbations generated on large scales of about the pulse length accompany the propagating pulses and are somewhat amplified in the interaction between them. Small-scale plasma fields are generated only during the interaction between the pulses, and they disappear after the interaction. The influence of the generation of plasma fields on the energy of the laser pulses and on their shape, as well as the possible applications of the effects under consideration, is 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.     

Emission of low-frequency electromagnetic waves by a short laser pulse propagating in a plasma with density fluctuations

It is shown that a short laser pulse propagating in a plasma with electron density fluctuations can emit electromagnetic waves with frequencies much lower than the laser carrier frequency. Emissions with frequencies close to the plasma frequency and the doubled plasma frequency in a nonisothermal plasma, as well as emission generated in a turbulent plasma, are examined. The effects in question are related to the transformation of the laser pulse wakefield into electromagnetic radiation by electron density fluctuations. The phenomenon under study opens new possibilities for diagnostics of both plasma fields excited by laser pulses and electron density fluctuations in a plasma.     

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.     

Generation of a wakefield during gas ionization

One-dimensional equations are derived that describe the hydrodynamic and electrodynamic properties of a plasma created through gas ionization by a short intense laser pulse. Different approaches (in particular, the particle-in-cell method) are used to show that, with ionization processes included, the excitation of a wakefield by an intense laser pulse can be described by the method of slowly varying amplitudes. It is shown that ionization processes enhance the wakefield excited by a moderate-intensity laser by about 10% in the case of a linearly polarized laser and by about 50% in the case of a circularly polarized laser. Ionization processes in light gases irradiated with high-intensity laser pulses have essentially no effect on the wakefield during the resonant excitation of a plasma wave by the ponderomotive force and play a governing role far from the resonance.     

Exciton Annihilation in the Two Photosystems in Chloroplasts at 100°K

The fluorescence yield (F) of spinach chloroplasts at 100°K measured at 735 nm (photosystem I fluorescence—F 735) and at 685 nm (photosystem II fluorescence—F 685) has been determined with different modes of laser excitation. The modes of excitation included a single picosecond pulse, sequences of picosecond pulses (4, 22, and 300 pulses spaced 5 ns apart) and a single nonmode-locked 2-μs pulse (MP mode). The F 735/F 685 intensity ratios decrease from 1.62 to 0.61 when a single picosecond pulse (or low-power continuous helium-neon laser) is replaced by excitation with the 300-ps pulse train (PPT mode) or MP mode. In the PPT mode of excitation, the 735-nm fluorescence band is quenched by a factor of 45 as the intensity is increased from 10¹⁵ to 10¹⁸ photons/cm² per pulse train and the 685-nm fluorescence is quenched by a factor of 10. In the MP mode, the quenching factors are 25 and 7, respectively, in the same intensity range. Fluorescence quantum yield measurements with different picosecond pulse sequences indicate that relatively long-lived quenching species are operative, which survive from one picosecond pulse to another within the pulse train. The excitonic processes possible in the photosynthetic units are discussed in detail. The differences in the quenching factors between the MP and PPT modes of excitation are attributed to singlet-singlet annihilation, possible when picosecond pulses are utilized, but minimized in the MP mode of excitation. The long-lived quenchers are identified as triplets and/or bulk chlorophyll ions formed by singlet-singlet annihilation. The preferential quenching in photosystem I is attributed to triplet excitons. The influence of heating effects, photochemistry, bleaching, and two-photon processes is also considered and is shown to be negligible.     

Electron bunch acceleration in the wake wave breaking regime

Results are presented from theoretical analysis and 2D PIC simulations of electron acceleration in a breaking wake plasma wave generated by a short intense laser pulse during its interaction with a finite-length underdense plasma layer. The high energy electron energy spectrum and transverse emittance are obtained. It is shown that, for laser pulse lengths above the plasma wake wavelength, the wakefield-accelerated electrons are further accelerated by the electromagnetic wave. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 4, pp. 291–310. The text was submitted by the authors in English.     

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.     

Nonlinear scattering of two counterpropagating laser pulses in an underdense plasma

A study is made of the interaction (“collision”) of two laser pulses with the same frequency but different durations, propagating toward one another in a low-density plasma. It is found that, in the interaction region, the excitation of small-scale plasma fields localized within a distance on the order of the length of the longer pulse is accompanied by the backscattering of each of the pulses. The frequency shift of the backscattered radiation and its duration depend strongly on the lengths of the interacting pulses. It is shown that the spectrum of the long backscattered radiation “tail” that arises behind the shorter pulse as a result of its interaction with the longer pulse contains satellites shifted from the laser frequency by the plasma frequency.     

Excitation of growing wake waves by an electron bunch in a plasma in an intense pump wave

A study is made of the excitation of wake waves by a one-dimensional electron bunch in an electron plasma in the presence of an intense monochromatic pump wave with circular polarization. In the main state (in the absence of a bunch), the interaction between a pump wave and a plasma is described by Maxwell's equations and the nonlinear relativistic hydrodynamic equations for a cold plasma. The excitation of linear waves by a one-dimensional bunch is investigated against a cold plasma background. It is shown that, in a certain range of parameter values of the bunch, pump wave, and plasma, the excitation is resonant in character and the amplitude of the excited wake waves increases with distance from the bunch.     

Collisional electron heating by an ultraintense ultrashort laser pulse focused in a gas

Collisional heating of plasma electrons in the field of an ultraintense ultrashort laser pulse is studied. The numerical results obtained by the method of molecular dynamics are compared with the well-known results from kinetic simulations. A model is proposed that provides a good agreement with the results of calculations for both linearly and circularly polarized high-intensity laser pulses.     

Large-angle stimulated Raman scattering of short laser pulses in plasma

The features of the large-angle stimulated Raman scattering of short laser pulses in a homogeneous underdense plasma are studied analytically. It is found that, for scattering angles that are not too close to zero, a steady-state regime of the convective amplification of unstable waves is established in the frame of reference comoving with the laser pulse. The problem of convective amplification in a two-dimensional region is solved in both weak-and strong-coupling regimes. It is shown that the steady-state envelopes of the scattered radiation and scattering plasma waves are two-dimensional in nature. It is found that, for a given scattering angle, the maximum possible spatial amplification at the trailing edge of the pulse is achieved if the ratio of the transverse to longitudinal size of the pulse is larger than the cotangent of one-half of the scattering angle.     

Excitation of magnetic fields by a circularly polarized laser pulse in a plasma channel

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.     

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).     

Photoresponses of Halobacterium salinarum to repetitive pulse stimuli.

Halobacterium salinarum cells from 3-day-old cultures have been stimulated with different patterns of repetitive pulse stimuli. A short train of 0.6-s orange light pulses with a 4-s period resulted in reversal peaks of increasing intensity. The reverse occurred when blue light pulses were delivered as a finite train: with a 3-s period, the response declined in sequence from the first to the last pulse. To evaluate the response of the system under steady-state conditions of stimulation, continuous trains of pulses were also applied; whereas blue light always produced a sharply peaked response immediately after each pulse, orange pulses resulted in a declining peak of reversals that lasted until the subsequent pulse. An attempt to account for these results in terms of current excitation/adaptation models shows that additional mechanisms appear to be at work in this transduction chain.     

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.     

Diagnostics of soft X-ray emission from the plasma of a fast capillary discharge

Results are presented from time-resolved measurements of the soft X-ray emission in the 10-to 40-eV spectral range from the plasma of a pulsed capillary discharge in argon at current pulse amplitudes of up to 26 kA and a current rise time of ～10¹² A/s. The experiments were carried out with 0.3-cm-diameter 15.7-cm-long ceramic capillaries filled with argon at a pressure of 0.25–0.5 Torr in the SIGNAL electrophysical facility. The experimental data are interpreted via computer simulations of the magnetohydrodynamics and level-by-level ion kinetics of an argon plasma. The results obtained indicate that soft X-ray laser pulses with a photon energy of 26.4 eV and duration of 1–2 ns are generated ≈33 ns after the beginning of the discharge current pulse.     

Resonance Absorption of a Train of Ultrawideband Electromagnetic Pulses in the Ionosphere

The problem of collisionless absorption of ultrawideband electromagnetic pulses in the ionosphere is studied analytically. It is shown that absorption has a resonance character if the repetition rate of a train of pulses is equal to the frequency of the excited plasma waves.     

Radial structure of the wakefield excited during the self-modulation of a laser pulse in a plasma

A study is made of the structure of the wakefield excited in the linear stage of the self-modulation of a high-power laser pulse in a homogeneous underdense plasma. It is shown that the fronts of the wake wave are curved and the profile of the wakefield amplitude differs strongly from the intensity profile of the laser pulse. The diffraction effects are found to play a key role in the formation of the transverse profile of the wakefield.     

Decay instability of a laser pulse propagating in a transverse direction in a magnetized plasma

The decay instability of a lser pulse propagating across an external magnetic field in a subscritical plasma is investigated analytically and numerically. It is shown that, when the relaxation of the pulse is taken into account, the hydrodynamic growth rate of the decay instability is slower than that obtained earlier in the constant-amplitude pump wave approximation. The results of numerical simulations by a particle-in-cell method demonstrate that an increase in the amplitude of the parametrically excited waves is accompanied by a decrease in their group velocity; in this case, up to 85% of the laser energy is converted into the energy of the plasma particles. It is found that, under resonance conditions, the magnetic field acts to increase the energy of the accelerated ions that escape from the plasma slab through its front boundary.