Physics of Wave Phenomena最新文献

Shock wave reflections in laterally colliding laser-produced aluminum plasmas at 10^–1 and 1 mbar argon ambient has been studied using fast imaging technique. In 10^–1 mbar pressure regime, the two shock fronts interact to form an intense interaction region. In the case of 1 mbar pressure regime, at early stages of plume expansion, individual shock fronts interact to form a collision front resulting in regular reflection of shock waves. At later times, the collision front split and a channel-like structure was observed and the width of the channel increases with time which is the cause of Mach reflection of shock waves. Incident angle and Mach stem length were measured from ICCD images, and it is found that Mach stem length increases with angle of incidence and time delay. Thus, the transition from regular reflection to Mach reflection of shock waves is reported in this paper.

The methods of direction finding for an immobile point source have been analyzed to obtain initial data for constructing acoustic distance measuring and tomography algorithms as applied to the deep sea. It is found that, to obtain reliable bearing estimates in the near- and far-field acoustic illumination zones (NFAIZ and FFAIZ), both in summer and in winter, it is sufficient to use the values of effective phase velocity (EPV) or effective group velocity (EGV) of sound, which are close to the measured speed of sound in water. However, in the shadow zone (SZ) under summer conditions, the effective velocities differ significantly from the speed of sound in water, and their values depend on distance, complicating additionally the solution of this problem. Therefore, to estimate the EPV and EGV, one must have information about the distance to the source. It is shown that application of vertically oriented antennas makes it possible to estimate the distance in the SZ and calculate independently the EPV and EGV values for each distance, which is necessary for direction finding. Thus, under summer conditions, conventional signal direction finding is performed in acoustic illumination zones, whereas in the SZ, in the case of simultaneous application of horizontal and vertical antennas, one must previously determine the distance to the source for calculating the bearing. The shadow zone is abscent in winter; thus, to phase a horizontal antenna on almost all distances, one can use the average speed of sound in water, but the antenna range must be determined.

The absorption cross-section of Cr²⁺ ions in a range of cubic Zn_1–xMn_xSe (x = 0–0.3) solid solutions was determined using nonlinear transmission measurements. The maximum absorption cross-section of about 1.04 × 10^–18 cm² was determined and shown to be practically independent of the Mn content (x) in the solid solution.

A lidar backscattering signal from an opaque target object, passed through a 9-m water layer with scattering meshes on the laser beam path, has been detected (for the first time, to the best of our knowledge) when sensing by pulses with eye-safe radiation energy density (~1 μJ/cm²). The new principle of laser sensing makes it possible to measure the position of meshes on the lidar path, in contrast to conventional laser rangefinders, which measure the distance to only the first target. The lidar has been developed based on a pulsed diode-pumped Nd³⁺:YAG laser (532 nm, 3 ns, 2 µJ/pulse, pulse repetition rate 4 kHz) and gated single-photon avalanche photodiode (SPAD) with a gain up to ~10⁶, serving as a detector. The large gain of the detector and suppression of its noise by gating ensured a signal-to-noise ratio of ≈35 for the target signal, which provides an estimate of underwater sensing range up to 30 m, according to the 3σ detection criterion. Compact lidars based on diode lasers (~1 µJ/pulse) with a radiation wavelength (~450 nm) in the spectral range of minimum losses in water and the increase in the safety of manned and unmanned underwater vehicles at early detection of nets (invisible for sonars) by a lidar are discussed.

Using the Sagdeev potential technique and fluid plasma equations, nonlinear ion-acoustic solitary waves in the presence of thermal fluid ions and two-temperature nonextensive electrons are studied. The existence domain of solitons with respect to electron concentrations and electron temperatures are determined, and it is found that opposite polarity solitons are exist in the given plasma system. Using these results, the effect of cold-to-hot electron temperatures on the characteristics of the large amplitude ion-acoustic solitary waves as well as ion-acoustic double-layers is investigated. Further, it is shown that both compressive and rarefactive solitons coexist in such a plasma system. Finally, the results are compared with the results of similar Maxwellian plasma.

The possibility of performing quantitative absorption measurements of particle concentrations using frequency-tunable lasers is investigated. At fast frequency scanning, when the recording time of spectrum is shorter or comparable with its formation time, well-known time-dependent interference interactions between the radiation incident on an absorbing medium and the radiation induced in it are observed. Under these conditions steady-state absorption spectra are distorted, and the classical relations lying in the basis of absorption measurements are violated. The character of the distortions depends on the type and density of particles, their absorption state, the mechanisms of spectra formation, and the laser beam power and geometry. In this paper, we report the results of studying the manifestations of Doppler profile narrowing caused by the Dicke effect in time-dependent spectra and their influence on the results of measuring the concentrations of absorbing particles. It is shown that the static spectrum can be reconstructed and quantitative measurements by integrated absorption spectroscopy can be performed under these conditions.

Nonplanar two-dimensional (2D) spherical dust acoustic solitary waves (DASWs) in unmagnetized, collisionless, Boltzmann distributed electrons, negatively charged dust fluid and trapped ions following vortex-like ion distribution, in a dusty plasma were investigated theoretically. Using standard reductive perturbation technique, which is valid for a small but finite amplitude limit condition, nonlinear spherical modified Korteweg–de Vries (K-dV) equation was achieved. Two motions are observed in the radial and angular directions, with transverse perturbations in the angular direction. It is found that the properties of the DASWs in a 2D spherical geometry differ from 1D spherical geometry where transverse perturbations and unidirectional waves are observed for 2D spherical geometry. The effects of dusty plasma parameters and vortex-like ion distribution on the properties (such as amplitude and width) of spherical DASWs were theoretically investigated. These numerical investigations show that under such specific conditions, only compressive DASWs can exist.

Propagation of the Airy–Gauss beam is analyzed in the paraxial approximation for the case where its waist associated with the Gaussian exponential is in an arbitrarily located plane perpendicular to the propagation direction. It is shown that from the viewpoint of the quality criteria for the approximation of the Airy function, the characteristic length of diffraction-free propagation, and the beam intensity and its derivatives with respect to transverse coordinates at different points of space, the Airy–Gauss beams are superior to the widely used Airy beams. The results of the analysis of how the Airy–Gauss beam characteristics affect the longitudinal and transverse density components of the orbital and spin constituents of the momentum and angular momentum, which is necessary information for problems of optical manipulation of micro- and nanoparticles, allows these beams to be considered as more promising for solving these problems.

The characteristics of spatial relaxation of the average electron energy in pure helium and in helium containing an admixture of water vapor in a constant electric field have been investigated by Monte Carlo simulation. The conditions under which the spatial relaxation in helium has a form of damped oscillations are considered. It is shown that even a small (0.1%) additive of water vapor leads to a significant decrease in the relaxation length, and the spatial oscillations almost disappear at 3% H₂O. Calculations were also performed for a mixture He : (H₂O : H₂ : O₂ : H), where the composition of atoms and molecules in the parentheses correspond to the composition formed from the initial water vapor in discharge plasma due to the dissociation of water molecules and subsequent plasma-chemical reactions.

This research involves a novel liquid concentration measurement of helical long-period fiber grating (H-LPFG) by dual-wavelength difference. The grating with a period of 782 μm is spirally processed via the commercial welding machine. The resonant peak appears around 1520 nm. Coupled mode theory is used to study the transmission strength as a function of liquid concentration. Theoretically, the transmission intensity depends only on the liquid concentration. Then, we experimentally researched the relationship between the transmission intensity and the concentration as a function of the wavelength at 1495 and 1518 nm. Transmission intensity at these two places is e-exponential with respect to the liquid concentration. Due to the transmission intensity measurement concentration is influenced by the fluctuation of the light source, since the fluctuation of the light source will affect the transmission intensity. Finally, a demodulation system for H-LPFG is proposed using only filters and light detector. This allows a new way to develop a high-precision, low-cost, high-potential liquid concentration sensor.