Initiation of nuclear reactions in femtosecond laser plasma
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CLEI Theory
Initiation of Nuclear Reactions in Femtosecond Laser Plasma P. A. Golovinski1), 2)* and E. A. Mikhin2) Received January 26, 2012; in final form, March 19, 2012
Abstract—Mechanisms of electron interaction with a nucleus via a direct electron–nucleus collision and via bremsstrahlung generated in electron scattering on a nucleus in a femtosecond laser plasma are considered. The description of this interaction is simplified substantially by using the instantaneous-impact and equivalent-photon approximations. The yields of photons, electron–positron pairs, and products of nuclear reactions initiated by a laser pulse for some nuclei are calculated. In particular, this is done for 235 U, 236 U, 238 U, and 232 Th fission in a laser field of intensity 1020 to 1022 W/cm2 . DOI: 10.1134/S1063778812120034
1. INTRODUCTION
focused ultrashort laser pulse of energy several hundred millijoules may provide intensities in the range of 1020 –1021 W/cm2 . Upon the irradiation of the surface of a solid body with such focused laser radiation, experimentalist observed the yield of fast electrons and hard photons from the interaction region that are able to initiate the experimentally recorded photonuclear reaction of 238 U fission [18]. A theoretical limit on this laser system is 1024 W/cm2 at the maximal time compression of a pulse. Similar parameters of laser radiation that ensure the observation of photonuclear reactions of nuclear fission became attainable in systems of smaller dimensions as well [19]. In order to perform quantitative calculations for respective processes, one needs a theory that would describe electron–nucleus interaction directly at the laser focus. A nonrelativistic theory of the excitation of nuclei in laser plasmas was proposed in [20]. A number of calculations were performed for electrically stimulated nuclear reactions in a strong light field of high intensity [14, 15]. However, those calculations dealt primarily with the case of a monochromatic laser-radiation field. The present study is devoted to considering nuclear processes in the field of a femtosecond laser pulse that are initiated both by relativistic electrons and by photons generated by these electrons.
Modern experimental equipment makes it possible to obtain femtosecond laser pulses of relativistic intensity in the range of I = 1019 –1021 W/cm2 [1–5]. Fundamental investigations of elementary processes under the effect of such pulses on laser plasmas are aimed primarily at the ionization of atoms, the acceleration of electrons at the laser focus, and nuclear reactions induced by fast secondary neutrons, protons, and photons [6–9]. The effect of electrostatic fields arising upon the separation of electrons and plasma ions is a dominant mechanism of proton acceleration in a femtosecond laser plasma. Protons flying apart after the termination of a laser pulse and colliding with a target induce nuclear reactions observed experimentally [10]. Quite moderate laser intensities around 1019 W/cm2 are sufficient for experimentally implementing conditions under
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