Ion acceleration by short, high intensity laser pulses in sets of small targets is treated by an analytical model developed here, and by two-dimensional particle-in-cell simulations. When an intense short laser pulse illuminates a thin foil target at normal incidence, electrons in the target are accelerated by the ponderomotive force. At the rear surface of the foil they generate a strong electric field that accelerates the ions, and generates an ion beam of small divergence. Using a mass-limited small target like a droplet enhances the ion energy, but increases divergence at the same time. In this paper, a combination of several-micron targets in a periodic structure (for example, a droplet chain) is proposed in order to increase the conversion efficiency from the incident laser beam to the emergent protons. Improvement of the energy flux conversion efficiency from the laser to the ion beam at optimal conditions is demonstrated.

Anomalous absorption of an intense short laser pulse in overdense plasmas is analyzed with a stochastic theory. A diffusion equation describing a time evolution of the electron distribution function is derived. From the equation it is shown that the electron distribution function becomes anisotropic in the momentum space, which gives rise to the absorption of the energy. The diffusion is not dominant in the pz direction, which is longitudinal to the vacuum-plasma boundary, rather it is dominant in the px direction, which is transverse to the boundary. However, the diffusion in pz enhances the absorption. Analytical expressions of the absorption coefficient and skin depth are obtained for the anomalous skin effect regime (ω02c2 << ωp2ve2), which evolves in time as the electron distribution becomes anisotropic. The asymptotic value of the absorption coefficient is proportional to [square root]I. The temperature and density dependence of the absorption coefficient is also discussed.