Yu. A. Malkov

Initiation and channelling of a microwave discharge by a plasma filament created in atmospheric air by an intense femtosecond laser pulse

We study the initiation of a pulsed microwave discharge in atmospheric air by a plasma channel induced by intense femtosecond laser pulses. It is shown that the electric field threshold for the initiated discharge is lowered compared with the self-discharge by about a factor of two, from 25 to 12 kV·cm-1. Channelling of the atmospheric-pressure microwave discharge in the direction of the plasma filament has been detected. The time of existence of the initiated discharge plasma was determined by the duration of the microwave pulse and amounted to 1-2 μs for the maximum electron density estimated as about 4 × 1015 cm-3. The developed theory of propagation of the microwave radiation along the plasma channel created by a femtosecond laser pulse predicts that the relatively low conductivity of the plasma and its rapid decay limit the characteristic scale of decay of the microwave fields confined by the plasma channel to a few centimeters.

Decay of femtosecond laser-induced plasma filaments in air, nitrogen, and argon for atmospheric and subatmospheric pressures.

The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% O_{2}), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3-4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50-760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

An eco-friendly technology for polysaccharide production from logging and sawing waste

A polysaccharide recovery technology was developed with intent to be used in integrated processing of larch biomass waste into practically significant arabinogalactan, pectin, and crystalline glucose suitable for medicinal, food-industry, and agricultural applications. Theoretical aspects were considered for arabinogalactan extraction from larch wood, in which procedure some of individual stages and the entire process cycle of arabinogalactan recovery on a pilot installation were optimized. The possibility of saccharification of larch wood-derived lignocellulosic residue into crystalline glucose was demonstrated. The results of a technological study on pectin polysaccharide isolation from larch bark were reported along with the findings concerning the membrane tropic activity of pectin and ability to form nanobiocomposites via interaction with transition and noble metal ions.

Laser acceleration of electrons in two-dimensionally inhomogeneous plasma at the boundary of a metal foil

The electron acceleration mechanism associated with the generation of a plasma wave due to self-modulation instability of laser radiation in a subcritical plasma produced by a laser prepulse coming 10 ns before the arrival of the main intense femtosecond pulse is considered. Three-dimensional particle-in-cell simulations of the interaction of laser radiation with two-dimensionally inhomogeneous subcritical plasma have shown that, for a sufficiently strong plasma inhomogeneity and a sharp front of the laser pulse, efficient plasma wave excitation, electron trapping, and generation of collimated electron beams with energies on the order of 0.2–0.5 MeV can occur. The simulation results agree with experiments on the generation of collimated beams of accelerated electrons from metal targets irradiated by intense femtosecond laser pulses.

Fabrication of microchannels in fused silica using femtosecond Bessel beams

Extended birefringent waveguiding microchannels up to 15 mm long were created inside fused silica by single-pulse irradiation with femtosecond Bessel beams. The birefringent refractive index change of 2–4 × 10−4 is attributed to residual mechanical stress. The microchannels were chemically etched in KOH solution to produce 15 mm long microcapillaries with smooth walls and a high aspect ratio of 1:250. Bessel beams provide higher speed of material processing compared to conventional multipulse femtosecond laser micromachining techniques and permit simple control of the optical axis direction of the birefringent waveguides, which is important for practical applications [Corrielli et al., “Rotated waveplates in integrated waveguide optics,” Nat. Commun. 5, 4249 (2014)].

Investigation of plasma filament decay in gases at different pressures

Properties of the plasma filaments have been studied in numerous publications. Of primary interest among them are the plasma density in a filament and its decay. Filaments were generally studied in ambient air, whereas filament plasma density decay in other gases and at different pressures has not been investigated yet. In this work, plasma filament decay in air, N2 and Ar for pressure range 1-760 Torr were considered. To measure plasma density decay in the filament, transverse optical interferometry and pulsed terahertz scattering techniques were used. To simulate the observed plasma filament decay at various pressures, a set of balance equations for electrons and ions simultaneously with the balance equation for electron temperature were solved numerically.The analysis of the results allowed us to determine the dominant mechanisms of filament plasma decay in air, N2 and Ar at various pressures.

Optically produced collimated quasimonoenergetic electron beams for laser-plasma acceleration

Plasma-based electron accelerators offer a promising way in the development of the particle acceleration techniques. Acceleration gradients achievable in a laser wakefield are several orders of magnitude higher than in conventional electron accelerators, paving the way to table top electron accelerators suitable for various applications in science, medicine, and diagnostics. One of the key problems in the laser-wakefield electron acceleration is the problem of injection of electrons in a plasma wave. At high laser intensity (I>1018 W/cm2) excited wakefield amplitude can be sufficient to trap electrons from cold background plasma, however in this regime the acceleration is a highly nonlinear process leading to high instability of resulting energy, charge and direction. Many attempts have been made to make the process more reliable, however the problem has not been solved yet. The trapping issue is especially crucial when the excitation of a plasma wake occurs in more stable and reproducible linear regime. In the present work we have investigated experimentally generation of collimated quasimonoenergetic bunches of electrons by focusing intense femtosecond laser radiation on the edge of aluminum foil which can be further used for injection into a laser produced plasma wake.