By superintense laser plasma heating of electrons as

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Unformatted text preview: al., 2007). 3D PIC simulations of BOA have been reported by Yin et al. (2011a). Modeling of BOA is not simple as the process appears to involve different stages. Analytical descriptions of BOA have been reported by Albright et al. (2010) and Yan et al. (2010) and a scaling of the maximum ion energy E max ’ ð1 þ 2 ÞZTe , with Te the electron temperature and a phenomenological parameter (estimated to be $3 from simulations), has been proposed. A fast growing relativistic Buneman instability, excited due to the relative drift between electron and ions, has been invoked as a mechanism enhancing the coupling with ions (Albright et al., 2007). Theoretical explanations for narrow C6þ spectra, based on an electromagnetic ‘‘ion-soliton’’ model (fundamentally different from electrostatic solitons described in Sec. IV.B), have been discussed by Yin et al. (2011b). D. Acceleration in near-critical and underdense plasmas A number of studies have been devoted to ion acceleration in near-critical plasmas with electron density close to the cutoff value (ne ’ nc ), in order to allow a more efficient generation of hot electrons to drive TNSA. Production of a low-density plasma by a laser prepulse has been investigated for laser and target parameters such that at the time of interaction with the main short pulse the preplasma was either underdense (Matsukado et al., 2003) or slightly overdense (Yogo et al., 2008); in the latter experiment, protons up to 3.8 MeV are observed at 1019 W cmÀ2 intensity and directed slightly off the normal to the target rear side. Analysis of these experiments gave indication of a regime where the pressure due to a self-generated magnetic field at the rear surface strongly contributes to charge separation. An alternative strategy to reduce the electron density is to use special target materials such as foams, which may be manufactured in order to have an average value of ne slightly larger, or even lower than ne (the average is meant over a length larger than the typical submicrometric scale of inhomogeneity). Experimentally, proton acceleration in low-density foams [ne ¼ ð0:9 À 30Þnc ] has been investigated by Willingale et al. (2009, 2011a) at intensities up to 1021 W cmÀ2 , showing that the proton energy is close to that obtained for solid foils and the same laser pulse for the lowest density value (ne ’ 0:9nc ). In this experiment, proton acceleration has mostly been investigated as an indication of the onset of relativistic transparency, leading to enhanced laser penetration and collimation of hot electrons and ions by self-generated magnetic fields. Recent simulation studies of ion acceleration in a solid target covered with foam layers (Zani et al., 2013) have also been reported (Nakamura et al., 2010; Sgattoni et al., 2012). Experimental investigations of ion acceleration using gas jet targets, with typical densities below 1020 cmÀ3 , have also been performed. These experiments include the already descri...
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