For the first time, the properties of the phase invariant of sound to vector–scalar fields formed in a waveguide by directional multipole sources are studied. It is shown that, when even such complex sources are used, the phase invariant is valid for sound pressure and for projections of the vibrational velocity vector. It practically does not depend on the directivity and frequency of the source, the distance between the receiver and the source, or on the parameters of the waveguide.

The PEARL laser -plasma complex is set up on the basis of a petawatt laser and allows a wide range of experiments in laser -plasma interactions, including particle acceleration, X-ray generation, and research in the area of high energy density physics. Constant work to improve the system makes it possible not only to systematically expand experimental capabilities but also to develop new technologies for megascience class facilities. The paper outlines possible experimental scenarios for using the laser and provides a description of experiments already carried out with the laser in a broad range of areas.

A computational and theoretical study of the properties of the well-known Chuprov waveguide invariant (CI) was carried out in a plane-parallel Pekeris waveguide. In contrast to earlier works, in which predominantly omnidirectional (monopole) sources were used as a source and sound pressure fields (scalar fields) were studied, in this work not only scalar, but also vector fields formed in the waveguide by directional-combined multipole sources with directivity in both horizontal and vertical planes are investigated. A differential equation has been obtained that makes it possible to fairly accurately calculate the CI values under different conditions of signal propagation and different depths of the sources and receivers. This makes it possible, in a simpler way than “total computer simulation,” to predict the invariance (stability) of the CI when both the hydrophysical conditions in the waveguide and the geometry of the experiment are varied. It is shown that the directivity of sources in the horizontal plane has virtually no effect on the properties of the CI, and the directivity in the vertical plane leads to a shift in the fan structure of the signal amplitude fields, but has little effect on the CI values. The properties of the fan structure change similarly when using vertical projections of the vibrational velocity vector: despite the fact that another analytical relation different from scalar fields is used to calculate the CI, the CI value is close to (1) at all frequencies and distances, except for those at which new modes or dislocations appear. At these frequencies and in these zones, alternating emissions with different signs and magnitudes occur. It is concluded that the stability of the CI allows the application of signal processing algorithms developed for scalar fields and nondirectional sources to vector–scalar fields generated, including with the use of directional sources.

We carry out a numerical modeling of plasma injection with hot electrons into a thin layer of cold plasma in the presence of an external magnetic field. We show that the latter can significantly affect the emerging small-scale current filaments and sheets, even if it does not magnetize the particles and does not change the overall dynamics of the redistribution of the total plasma density in the process of injection. The effect observed depends on the orientation of the external magnetic field that is parallel to the plane that bounds the cold plasma layer, if the injection occurs from a narrow strip lying in this plane. In this situation, which corresponds to the ablation of a flat target by a femtosecond laser beam using cylindrical focusing, we study the evolution of the characteristic structures of the formed small-scale magnetic field. It is established that its generation is associated with instabilities of the anisotropic velocity distribution of electrons and that its value can be many times greater than the value of the external magnetic field.

Проведено экспериментальное исследование динамики разряда в постоянном квазиоднородном электрическом поле, инициированного плазменным филаментом, создаваемым фемтосекундным лазерным импульсом. Измерено время развития инициированного разряда (время задержки возникновения разряда относительно лазерного импульса) в зависимости от напряженности постоянного электрического поля в плазменном филаменте и проведено сравнение этой экспериментальной зависимости с результатами численного расчета разрядного процесса в филаменте.

By means of particle-in-cell numerical simulations, we find the possibility of the formation and long-term coexistence of orthogonal current structures in adjacent layers of an inhomogeneous cold plasma penetrated by a hot electron flow. The formationof these structures is shown to occur in a wide range of parameters specifying collisionless expansion of high-energy electrons out of a dense plasma into a rarefied plasma. These structures originate due to the development of Weibel instabilities of two different types that are associated with qualitatively different anisotropic electron velocity distributions. Experiments with a laser plasma produced in the course of target ablation by means of quasi-cylindrical focusing of a high-power femtosecond-laser radiation beam are proposed in order to observe the predicted phenomenon.

This paper presents the results of an experimental study of the spatial structure of a microwave discharge maintained in an argon flow by gyrotron radiation in a continuous mode with a frequency of 24 GHz at atmospheric pressure. In the structure of the plasma plume, stationary filamentary channels are observed, elongated along the direction of the argon flow, regardless of the orientation of the external electric field of the wave, surrounded by a diffusion halo. Measurements of the electron density, vibrational and rotational temperatures of gas molecules in plasma filaments have been carried out. The role of gas-dynamic mechanisms responsible for the formation of the inhomogeneous static structure of the plasma torch and the maintenance of a substantially nonequilibrium distribution of temperature characteristics in the discharge is discussed. Keywords: high-pressure microwave discharge, plasma torch, argon, filamentous plasma channels, filaments.

This paper presents the results of an experimental study of the spatial structure of a microwave discharge maintained in an argon flow by gyrotron radiation in a continuous mode with a frequency of 24 GHz at atmospheric pressure. In the structure of the plasma plume, stationary filamentary channels are observed, elongated along the direction of the argon flow, regardless of the orientation of the external electric field of the wave, surrounded by a diffusion halo. Measurements of the electron density, vibrational and rotational temperatures of gas molecules in plasma filaments have been carried out. The role of gas-dynamic mechanisms responsible for the formation of the inhomogeneous static structure of the plasma torch and the maintenance of a substantially nonequilibrium distribution of temperature characteristics in the discharge is discussed.

The concept of a phase invariant (PI), introduced in our previous studies, is used to describe the sound-field phase distribution in an underwater waveguide in the distance–frequency plane. This concept is similar to the classical waveguide Chuprov invariant (ChI), which specifies the slopes of constant-field intensity lines in the same plane. The PIs of vector–scalar fields excited by multipole sources are investigated for the first time in this paper. It is shown that, as well as in the case of a monopole source, PI is a stable characteristic of a sound field. It is demonstrated by numerical simulation that the PI value in shallow water barely depends on the source directional pattern, the path length, the frequency of emitted signal, and the waveguide characteristics. Significant deviations of PI from its basic value (–1) are observed only in the vicinity of interference minima (in the phase-dislocation zones).

The potential of KD2PO4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼200−300 μJ energy were converted to broadband (∼4 THz bandwidth) terahertz transients with the efficiency of ∼8×10−6. The transients produced electro-optic signals of a ∼0.15 modulation depth and ∼103 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d36≈3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation.

The photoemission of electrons from a metal needle under the action of radiation from a femtose-cond IR laser with a wavelength of 1240 nm is studied. Diagnostics of electron bunches using the Faraday cup and the time-of-flight method show the possibility of obtaining electron pulses with a charge of 40 pC at a laser pulse energy of 45 μJ. The dependence of the electron pulse charge, which is proportional to the sixth power of the laser pulse energy, indicates a multiphoton mechanism of electron emission.

Dynamics of a discharge in a constant quasi-homogeneous electric field initiated by a plasma filament created by a femtosecond laser pulse is studied experimentally. The time of development of the initiated discharge (the time delay of discharge onset relative to the laser pulse) is measured as a function of magnitude of the constant electric field in the plasma filament. The obtained experimental dependence is compared with the results of numerical simulations of the discharge in the filament.

We present the first experimental results obtained with a setup created on the basis of the PEARL laser facility for studying the processes of generating terahertz radiation from laser wake fields which are formed during the propagation of a high-power femtosecond laser pulse in a rarefied plasma. In particular, the occurrence of terahertz generation in the case where the laser–plasma interaction region is located between a pair of dielectric prisms of total internal reflection is demonstrated. The dependence of the terahertz radiation energy on the energy of a femtosecond laser pulse and on the plasma density is studied.

We carry out three-dimensional and two-dimensional particle-in-cell simulations of the expansion of a magnetized plasma that initially uniformly fills a half-space and contains a semicylindrical region of heated electrons elongated along the surface of the plasma boundary. This geometry is related, for instance, to ablation of a plane target by a femtosecond laser beam under quasi-cylindrical focusing. We find that a decay of the inhomogeneous plasma–vacuum discontinuity is strongly affected by an external magnetic field parallel to its boundary. We observe various transient phenomena, including an anisotropic scattering of electrons and an accompanying Weibel instability, and reveal various spatial structures of the arising magnetic field and current, including multiple flying-apart filaments of a Z-pinch type and slowly evolving current sheets with different orientations. The magnitude of the self-generated magnetic field can be of the order of, or significantly exceed that of, the external one. Such phenomena are expected in the laser and cosmic plasmas, including the explosive processes in the planetary magnetospheres and stellar coronal arches.

For the first time, the existence of equiphase lines in the frequency-spatial domain is shown and a differential equation is obtained for calculating them as applied to the complex spectra of sound signals in a waveguide. It is shown that such lines are associated with a phase invariant, which is similar to the well-known Chuprov interference invariant, but has a different physical meaning: these lines are calculated on the phase plane rather that from the intensity field. The equiphase lines are stable and weakly dependent on the signal propagation conditions and allow optimized processing of weak signals for detecting them in a clutter environment.

Based on two- and three-dimensional numerical simulations, a new physical phenomenon is predicted during the decay of an elongated plasma region with hot electrons formed as a result of target ablation in vacuum by a femtosecond laser pulse focused on its surface by a cylindrical lens into a stripe with a width of several to hundreds of microns. It is established that both in the absence and in the presence of external magnetic field (up to ~10 3 T), oriented along the target surface in the direction of the axis of the semicylinder with electrons heated to keV energies, the multiple formation of thin filaments of the electron current and their further expansion together with the cold plasma cloud’s expansion can occur. It is shown that an inhomogeneous system of such filaments with the transverse dimensions ranging from a few to tens of microns develops due to the Weibel-type instability resulting from the anisotropic cooling of an expanding electron cloud and exists at times ranging from picoseconds to nanoseconds, creating localized magnetic fields ranging in strength from a few to several hundreds of T.

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was investigated experimentally and explained theoretically. We found that emitted photons are polarized predominantly perpendicular to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. Comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate a momentum isotropization time for electrons in graphene to be ~25 fs and roughly reconstruct the distribution function evolution in k-space.

We study, both experimentally and theoretically, the second-harmonic (SH) generation (SHG) of optical radiation assisted by intense terahertz electromagnetic pulses from the surface of high-resistivity Si(111). The study was performed within the framework of the analysis of the superposition of different SH sources, including the surface-dipole, bulk-quadruple, and electric-field-induced (EFI) bulk-dipole (arising from internal built-in and external terahertz fields) contributions. Azimuthal rotation anisotropy of SHG for an arbitrary mutual polarization of the terahertz, fundamental, and SH optical fields was calculated and experimentally verified. The strongest impact of the terahertz field on SHG was observed in the case where the polarizations of the terahertz and SH fields coincide. In these polarization configurations, the terahertz field added mainly an isotropic term to the SH azimuthal dependences. It has been demonstrated that the application of the terahertz field can be helpful for distinguishing the SHG mechanism. For the sample under study, the contribution to the SH energy from the built-in field-induced source was found to be >4 times greater than the contribution from the surface and quadrupole nonlinear sources. The dominance of the EFISHG allowed us to find the ratio between the components of a silicon third-order nonlinear tensor chi (3)(2 omega; omega, omega, 0): chi 1122 = chi 1212 approximate to 0.51 chi 1111 and chi 1212 approximate to chi 1221 for the fundamental optical wavelength 790 nm.