Electron-positron pairs' generation occuring in the interaction of 10 18 -10 20 W/cm 2 laer radiation with high-Z targets are examined. Computational results are presented for the pair production and the positron yield from the target with allowance for the contribution of pair production processes due to electrons and bremsstrahlung photons. Monte-Carlo simulations using the prizma code confirm the estimates obtained. The possible positron yield from high-Z targets irradiated by picosecond lasers of power 10 2 -10 3 TW is estimated to be 10 9 -10 11 .The possibility of electron-positron pair production by relativistic electrons accelerated by a laser field has been discussed since many years [1].It was estimated that the positron production efficiency can be high [2]. The papers cited considered the case of pair production during oscillations of electrons in an electromagnetic wave in the focal region of laser radiation. Here we examine a somewhat different pair production scenario.The interaction of high-power laser radiation with matter results in the production of fast, high-temperature electrons [3]. Relativistic temperatures of fast electrons T f ≈ 1 MeV have been observed in experiments with powerful picosecond lasers [4]. Self-consistent electric fields confine these electrons in the target. When the electrons interact with the matter in a high-Z target, electron-positron pairs are produced [5]. The annihilation photon spectrum can be used for diagnostics of the electron-positron plasma.In the present letter we make estimates of the positron and photon yield as function of the laser power. We have made an assessment of the possibility of using high-power (10 2 -10 3 TW) ultrashort-pulse lasers to produce a highluminosity positron source. Such sources are required for the production of slow (1-10 eV) positrons with an intensity of 10 8 positrons per second. Such positrons find wide applications for the study of Fermi surfaces, defects and surfaces of various materials [6].The interaction of relativistic electrons with matter can lead to electron-positron pair production in the following two processes:In Ref.[7] analytical and numerical calculations of the total cross section of the pair electroproduction process are performed using the differential cross section of Ref. [8]. According to this work the total cross section of the process (i) near the threshold equalswhere r e is the classical electron's radius; α = 1/137; mc 2 is the electron mass, and E 0 is the kinetic energy of the initial electron. At high energies the cross section grows as [9]The approximation formula describes both limits. Fig. 1 shows the points obtained by numerically integrating the exact formulas for the differential section [7], the asymptotic cross sections (2) and (1), and a plot of the approximating function (3).

No abstract

No abstract

Results of 2D numerical simulations of thermonuclear burning propagation in a helium layer on a neutron star surface using the Euler-Lagrange TIGR-3T code are presented. This process is crucial for the development of type I X-ray bursts. Ignition and thermonuclear burning propagation are fairly easy to obtain when simulating a layer with a density at the bottom of 1.75 × 10 8 g cm −3 . Such a simulation allows it to be compared with previous simulations based on other codes, including the simulation based on the MPM code described previously. However, this density is two orders of magnitude higher than can be obtained in observed bursters. The implementation of efficient numerical methods for the description of heat conduction and turbulence in the TIGR-3T code has allowed simulations to be performed for a layer with a density at the bottom of 1.8 × 10 7 g cm −3 . The fulfilment of certain conditions on the size and shape of the initial temperature perturbation is required for the propagation of thermonuclear burning. Simulations reveal a peculiar burning propagation mechanism through compression of the layer ahead of the burning front by the matter transferred above the free surface.

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 × 10 8 g cm −3 at the bottom. 2D numerical simulations have been performed using a modified particle method in the adiabatic approximation.