Monoenergetic proton beams from mass-limited targets irradiated by ultrashort laser pulses
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Monoenergetic Proton Beams from MassLimited Targets Irradiated by Ultrashort Laser Pulses A. V. Brantov and V. Yu. Bychenkov Lebedev Physical Institute, Russian Academy of Sciences, Leninskiі pr. 53, Moscow, 119991 Russia Received June 16, 2009
Abstract—Results are presented from threedimensional numerical simulations carried out to study different regimes of proton acceleration from plane targets (such as double and singlelayer foils, homogeneous foils of light and heavy ions, and masslimited targets) irradiated by laser pulses of moderate intensity. It is shown that, in the interaction of a laser pulse having an energy of about 20 J with masslimited targets consisting of heavy ions and protons, it is possible to generate a monoenergetic proton beam with an energy of about 150 MeV. DOI: 10.1134/S1063780X10030074
1. INTRODUCTION The creation of laser sources of highenergy ion beams has attracted increased interest in connection with their application in controlled nuclear fusion, nuclear physics, radiography, and also in medicine for producing shortlived isotopes and hadron therapy [1, 2]. In the latter case, one of the most important requirements on accelerated proton beams is that they should be highly monoenergetic (the width of the energy spectrum should be on the order of one to two percent) and should have a high energy (~200 MeV). It is also desirable to reach these purposes at the mini mum possible laser energy in order for laser devices to be capable of operating in the repetive mode, on the one hand, and to be as inexpensive as possible, on the other. One of the main approaches to producing monoenergetic ion beams is based on the use of dou blelayer targets consisting of a thin foil of heavy ions and an ultrathin, transversely narrow layer of light ions on its back surface. This approach was first proposed by Bulanov and Khoroshkov [1], then confirmed by numerical simulations in [2, 3], and finally imple mented experimentally in [4, 5]. In order for acceler ation to be efficient in this case, the density of the accelerated layer of light ions should be much lower than the density of solids [6], a requirement that stems from the fact that an initially dense layer of light ions is subject to Coulomb repulsion and thereby loses its monoenergetic character. In practice, however, it is difficult to coat a foil with a thin lowdensity layer. In addition, microstructuring of thin foils is a nontrivial engineering challenge. Another way of generating monoenergetic ion beams is to use a homogeneous target consisting of heavy and light ions [6–9], a method that, in some cases, makes it possible to avoid the above difficulties.
A peak in the energy spectrum of light ions can form due to the onset of a shock wave during the expansion of a quasineutral plasma [6, 8, 10, 11]—the main mechanism in the case of irradiation of a target with light ions of low density by a laser pulse of moderate intensity. At the same time, at relativistic laser intensi ties, the Coulomb piston effect also generates
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