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).
Measurements and calculations of the resonance between the Ly-a line of magnesium and the 2s~3p transition in neonlike germanium are presented. This resonance is of possible use as part of a resonantly photo-pumped x-ray laser scheme recently proposed which would lase on several 2p~2 s transitions be-0 tween 64 and 89 A in neonlike germanium. Measurements of the resonance are made by three different approaches: one using the electron beam ion trap, the second using laser-produced plasma, and the third on a tokamak. These results are compared with calculations and other measurements.Certain discrepancies are observed with previous measurements which may be due to the partial overlap between oxygenlike and neonlike lines in germanium. The results do show a good resonance between magnesium and germanium, with the germanium line between the two Ly-a components of magnesium but nearer to the short-wavelength component.PACS number(s): 32.30. Rj, 42.60.By
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