Also been reported nakamura et al 2010 sgattoni et al

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Unformatted text preview: bed investigations of hole boring RPA (see Sec. IV.A.1) and shock acceleration (see Sec. IV.B) using CO2 lasers for which gas jets are near-critical targets. Using optical or Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 779 near-infrared lasers, several experiments of high-intensity laser interaction with underdense gas jets have reported observations of energetic ions accelerated in the radial direction with respect to the laser pulse propagation axis (Krushelnick et al., 1999; Sarkisov et al., 1999; Wei et al., 2004) by the electric field created by the electron displacement in the channel drilled by the ponderomotive force. The ion spectrum may provide information on the self-focusing and channeling dynamics of the laser pulse and the acceleration mechanism shows indeed some similarity with those active in the interaction with solid targets (Macchi, Ceccherini et al., 2009). However, radial acceleration of ions is of modest interest for applications since the ions are not collimated at all. A collimated emission in the forward direction from an underdense He gas jet was reported by Willingale et al. (2006). Using a laser pulse of 1 ps duration, energy up to 340 J, and intensity up to 6 Â 1020 W cmÀ2 , He ions up to 40 MeV were observed collimated in a beam with <10 aperture. The data were interpreted by assuming that a large electric field was generated at the rear side of the gas jet by escaping hot electrons. Different from TNSA in solid targets, the mechanism was considered to be effective despite the relatively long density scale length at the rear surface because a larger fraction of hot electrons was generated by electron acceleration in the underdense plasma. Further analysis of simulations of the experiment (Willingale et al., 2007) also showed a significant contribution due to the generation of a quasistatic magnetic field at the rear surface, which in turn enhances the accelerating electric field via magnetic pressure and induction effects according to the model by Bulanov and Esirkepov (2007) that was also used to explain the above mentioned experimental results by Yogo et al. (2008) in a near-critical plasma. Figure 30 shows a sketch of such a mechanism. Acceleration sustained by a magnetic field was also indicated as the dominant mechanism in an experiment by Fukuda et al. (2009), where ions in a 10–20 MeV range and collimated in a 3.4 aperture cone were observed in the interaction of a 7 Â 1017 W cmÀ2 , 40 fs laser pulse with an underdense gas jet where CO2 clusters were formed. FIG. 30 (color online). Sketch of magnetic field sustained acceleration of ions, showing the topology of the magnetic and electric fields and the flows of escaping and returning electron currents. From Bulanov and Esirkepov, 2007. 780 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . The role of the clusters was apparently that of enhancing the self-channeling and focusing of the l...
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