Simulation of the generation of characteristic X radiation under vacuum heating of electrons by a femtosecond laser puls
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Simulation of the Generation of Characteristic X Radiation under Vacuum Heating of Electrons by a Femtosecond Laser Pulse O. F. Kostenko and N. E. Andreev Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 132, Moscow, 125412 Russia Received July 7, 2010; in final form, September 30, 2010
Abstract—An analytic model of the generation of characteristic X radiation under vacuum heating of elec trons at the surface of a massive target by a ppolarized nonrelativistic femtosecond laser pulse is considered. The results of calculations satisfactorily describe the measured data on the output of Kα radiation generated by laser pulses with a wavelength of 1.24 µm and peak intensities of 5 × 1016–2 × 1017 W/cm2, incident at an angle of 45°. DOI: 10.1134/S1063780X11040064
1. INTRODUCTION Collisionless absorption plays an important role in the interaction of short laser pulses with intensities such that IL λ2 ≥ 1015 (W/cm2) μm2 (where λ is the laser wavelength) with solid targets and leads to the production of fast electrons in a surface plasma (see, e.g., [1]). These electrons penetrate into a solid and ionize the K shell to generate a narrowband X radia tion pulse. The characteristic X radiation generated in the 2p–1s transitions is used, e.g., to diagnose high density plasmas with picosecond time resolution [2]. The efficiency of conversion of laser energy into Kα radiation depends on the mechanism by which fast electrons are produced and which is, in turn, governed by the parameters of the interaction of a laser pulse with a target. During vacuum heating of electrons, the conversion efficiency can be increased, in particular, by controlling the shape and duration of a highcon trast laser pulse [3] or by optimizing the size of the clusters on the target surface [4, 5]. In the present paper, we study an analytic model of the generation of characteristic X radiation under vac uum heating of electrons at the surface of a massive metal target by a nonrelativistic femtosecond laser pulse via the Brunel mechanism [6]. We compare our results with the data measured in [7] in experiments with a 1.24μm femtosecond infrared (IR) laser. We also analyze how the assumption that the energy dis tribution of fast electrons is Maxwellian influences the resulting X radiation output. 2. MODEL In order to describe how the energy of an intense electromagnetic wave incident obliquely on the sharp boundary of a plasma with an overcritical density is
absorbed by fast electrons, Brunel [6] considered a onedimensional capacitor model. In this model, the uniform electric field Eext = E0sin(ωt) applied perpen dicularly to the surface of a perfect conductor draws electrons from it during the time interval 0 < t < π/(2ω); at later times, t > π/(2ω), the electrons return to the target with a velocity on the order of vos = eE0/(mω), where e is the absolute value of the charge of an electron and m is its mass. The electron motion is governed by the field Eext and by their own selfcon sistent field, unde
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