Direct ion acceleration with variable-frequency lasers

Laser-based ion acceleration commonly relies on indirect schemes, in which the ions are accelerated by the space-charge field in laser-irradiated solid targets, either via plasma-expansion processes [1], or resorting to electrostatic shock structures [2]. Here, we propose the production of monoergetic ion beams via direct acceleration by the laser field (in vacuum or in tenuous plasmas) [3]. The method exploits two counterpropagating lasers with variable frequency to drive a beat-wave structure with variable phase velocity: the ions are trapped in the beat wave and accelerated to high energies. The physical mechanism is described with a 1D theory, providing the general conditions for trapping and scaling laws for the relevant ion-beam features. The validity and the robustness of the method are confirmed by 2D PIC simulations with OSIRIS [4]. \newline [1] J. Fuchs\textit{ et al.}, Nature Phys. \textbf{2}, 48 (2006); L. Robson \textit{et al.}, Nature Phys. \textbf{3}, 58 (2007); B.M. Hegelich \textit{et al.}, Nature \textbf{439}, 441 (2006). \newline [2] L.O. Silva \textit{et al.}, Phys. Rev. Lett. \textbf{92}, 015002 (2004). \newline [3] F. Peano \textit{et al.}, submitted for publication (2007). \newline [4] R. A. Fonseca \textit{et al.}, Lect. Notes Comp. Sci. \textbf{2331}, 342 (Springer-Verlag, Heidelberg, 2002).