Some thermonuclear X-ray bursters exhibit a high-frequency (about 300 Hz or more) brightness modulation at the rising phase of some bursts. These oscillations are explained by inhomogeneous heating of the surface layer on a rapidly rotating neutron star due to the finite propagation speed of thermonuclear burning. We suggest and substantiate a mechanism of this propagation that is consistent with experimental data. Initially, thermonuclear ignition occurs in a small region of the neutron star surface layer. The burning products rapidly rise and spread in the upper atmospheric layers due to turbulent convection. The accumulation of additional matter leads to matter compression and ignition at the bottom of the layer. This determines the propagation of the burning front. To substantiate this mechanism, we use the simplifying assumptions about a helium composition of the neutron star atmosphere and its initial adiabatic structure with a density of 1.75 × 108 g cm−3 at the bottom. 2D numerical simulations have been performed using a modified particle method in the adiabatic approximation.

The Boltzmann kinetic equation for nonhydrodynamic weakly ionized plasma in the presence of both electric and magnetic fields is considered. The charged particles distribution function is decomposed in terms of spherical harmonics in momentum space. After substituting the expansion into the Boltzmann equation an infinite hierarchy of differential equations for the distribution function expansion coefficients is derived. The cases of Cartesian, cylindrical and spherical coordinates in configuration space are studied. We applied obtained equations to the description of electron transport in nitrogen at high values of rf electric field intensity to number density ratio E/N.

The PM2D code is used to simulate relativistic electron-positron plasma generated in the target irradiated by laser light of relativistic intensities. The paper discusses results of PM2D simulations performed with the aim to increase mean energy and brightness of positron jets that could be formed near the rear surface of the target. The calculations demonstrate the possibility to significantly improve positron jet acceleration by target reconfiguring.

K-shell emission spectra from laser-exploded aluminum foils and mylar foils with aluminum dots are recorded with a high spectral and spatial resolution using vertical-geometry Johann spectrometer. The experiments are modelled using cylindrical version of two-dimensional hydrocode "ATLANT". We describe our novel atomic physics post-processor "XEPAP" that is used here for the synthesis of the emission spectra. The predictions of the simulations are compared with the experimental spectra and the parameters of the emitting plasmas are deduced.

The parameters of a plasma produced upon the interaction of ultrashort laser pulses with cluster targets are measured by the methods of X-ray spectroscopy. The dependence of the plasma parameters on the initial properties of a cluster target (the design of a supersonic nozzle, the average size of clusters, the spatial inhomogeneity) and the laser pulse properties (its duration and contrast) is studied. The plasma diagnostics is performed using the model of formation of emission spectra, which was proposed earlier and includes a number of fitting parameters, which provide good agreement with experimental spectra. The systematic experimental studies performed by us showed that our model of cluster heating by ultrashort pulses is indeed a physical model, and the fitting parameters represent the average values of plasma parameters in the corresponding space-time regions.

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n 1 P 1−1 1 S 0 ( n =5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ∼10 5 J/cm 2 . The independent modeling of the relative intensities of the transitions from the n =5,..., 10 levels, as well as of the 2 1 P 1 − 1 2 S 0 and 2 3 P 1 −1 2 S 0 lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.

The paper considers designs of moderators where fast positron stopping medium consists of very fine tungsten strips separated by vacuum gaps and the strips are arranged into Venetian blinds- or honeycomb-type structures. Moderator efficiency is evaluated through Monte-Carlo simulations. According to the maximal estimate, the efficiency of conversion of fast positrons into slow ones in the Venetian blinds and honeycomb-type moderators is ∼5×10−3 for the reasonable thickness of the tungsten foil. If such moderator is used, the intensity of slow positron source on the hard synchrotron of SPring-8 storage ring can reach the level of ∼5×1010e+/s.

A density-matrix theoretical model to calculate spectral line profiles of general multielectron ions in plasmas is described. The line-profile calculation involves electron collisional and radiative relaxation of ionic states, the emitter's motion (Doppler effect) and its interaction with a quasi-static ion microfield. Using the LineDM computer package implementing this model, line-profile calculations of the K- and L-shell transitions in Al XII, Ar XVII, Ar XVI, Cu XX, and Xe XLV ions have been performed in the context of plasma diagnostic issues of recent laboratory experiments. Comparisons of the calculated line profiles with experimental and other theoretical data show the applicability of the model and package for the detailed computational analysis of line radiation spectra from multielectron emitters in hot dense plasmas.

The possibility that fast electrons can escape in a direction close to the trajectory of a reflected ultrashort laser pulse at extremely high laser radiation fluxes is examined analytically and numerically. Analytic estimates are made of the feasibility of forming electron bursts in the plasma and of their subsequent motion. The self-consistent, collisionless motion of a plasma acted on by specified incident and reflected ultrashort laser pulses is modeled in two dimensions by the particle-in-cell method. It is shown that a substantial number of electrons located in the subcritical region are gathered into bunches by the resultant forces and escape to the vacuum in a direction different from the normal to the target surface within a narrow range of solid angles. This demonstrates the feasibility of laser acceleration of an electron burst during reflection of an ultrashort laser pulse from a solid target.

The paper provides results of numeric simulations of in-target positron production process, processes of moderation, thermalization, diffusion, and reemission of positrons in high-efficiency multi-wire moderator made of tungsten monocrystalline wire with regular wire spacing. The paper looks into dynamics of slow positrons in the moderator's vacuum gaps taking into account of external fields. The feasibility of using multi-wire moderator with non-regular structure - multi-layer “wire felt” moderator is discussed. According to maximal estimate the multi-wire moderators can reach very high efficiency of fast-slow positron transformation ∼10−2. Using such moderator the intensity of slow positron source on hard synchrotron radiation of SPring-8 can reach the level of ∼1011e+/s.

In result of numerical and analytical consideration is shown, that the essential part electrons, having been in area undercritical density, under action of arising forces is formed in clots and takes off for vacuum in a direction, different from normal to a normal of a surface in a narrow interval of corporal corners. The received results prove an opportunity of laser electrons acceleration at reflection USLP from a flat target.

A self-consistent solution is obtained for the problem of electron flux generation by interaction between an ultrashort laser pulse and a metal target.

The results of numerical simulation of fast electrons motion and generated electromagnetic fields at the picosecond pulse laser interaction with flat target are presented. The calculations were performed with PM2D code, where relativistic equation of electron motion joint with Maxwell equations is solved by particle method in cells. The efficiency of fast electrons energy conversion to the transverse electromagnetic wave of picosecond duration can reach the value 10(-4) for the intensity of ultrashort laser pulse at the target 10(16) - 10(17) W/cm(2).

The results of numerical simulations of fast electrons interaction with matter, calculated yield and angular distribution of bremsstrahlung and fluorescence K-α X-ray radiation and generation of electromagnetic fields, carried out by ERA, PM2D, and PRIZMA codes for the state of experiments on interaction of ultra-short laser pulses with high- Z targets at intensity of 10 16 −10 18 W/cm 2 are presented. Spherical targets with conical hollows and conical targets of high- Z matter are proposed for experiments with picosecond lasers to increase the brightness of hard X-ray sources.