Formation of fast multicharged heavy ions under the action of a superintense femtosecond laser pulse on the cleaned surf
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, NONLINEAR, AND SOFT MATTER PHYSICS
Formation of Fast Multicharged Heavy Ions under the Action of a Superintense Femtosecond Laser Pulse on the Cleaned Surface of a Target R. V. Volkov, V. M. Gordienko, I. M. Lachko, A. A. Rusanov, A. B. Savel’ev, and D. S. Uryupina Moscow State University, Vorob’evy gory, Moscow, 119992 Russia International Laser Center, Faculty of Physics, Moscow State University, Moscow, 119992 Russia e-mail: [email protected] Received February 16, 2006
Abstract—It is found that the mean charge of tungsten ions in a solid tungsten target cleaned from the surface layer of hydrocarbon and oxide compounds and exposed to femtosecond laser radiation with an intensity exceeding 1016 W/cm2 attains 22+, while the maximum charge is 29+. The maximum energy of such ions approaches 1 MeV. The corresponding values obtained on a dirty target with the same laser pulse parameters constitute 3+, 5+, and 150 keV. The results of numerical simulation show that such a large maximum charge of ions can be attained owing to the emergence of an electrostatic ambipolar field at the sharp boundary between the plasma and vacuum. The main mechanism of ionization of ions with maximum charges is apparently impact ionization in the presence of an external quasi-static field. In addition, direct above-threshold ionization by this field can also play a significant role. It is also shown that heavy ions in a clean target are accelerated by hot electrons. This leads to the formation of high-energy ions. The effect of recombination on the charge of the ions being detected is analyzed in detail. PACS numbers: 52.38.Kd, 52.50.Jm DOI: 10.1134/S1063776106080139
1. INTRODUCTION Acceleration of ions as a result of interaction of femtosecond laser radiation with a hot high-density plasma constitutes one of the most promising trends in the physics of ultrahigh light fields [1–5]. Such ions can find wide application in solving a number of problems, from rapid initiation of fusion reaction [6–8] to simulation of astrophysical processes [9], formation of ionic clusters for ion accelerators, and ion implantation [10]. For subrelativistic intensities (approximately up to 1018 W/cm2), the acceleration of ions along the normal to the surface of the target occurs in a quasi-static electric field emerging as a result of charge separation at the plasma-vacuum interface [11, 12]. The energy spectrum of ions can be conditionally decomposed into two (“fast” and “slow”) components, which appear due to the existence of two (hot and thermal) electron components in the plasma [13, 14]. The mean energy of each spectral component is proportional to the mean energy of the corresponding electron component and to the mean charge of the ions [15]. It was shown in a number of publications [16–19] that an adsorbed layer of hydrocarbons is always present on the target surface even when the residual pressure in the reaction chamber is at a level of 10−5 Torr. This leads to predominant acceleration of
light ions (primarily protons) to high energies, while hea
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