Physics vi conclusions and outlook acknowledgments

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Unformatted text preview: s VI. Conclusions and Outlook Acknowledgments References 751 754 754 755 755 757 757 758 758 758 759 759 760 761 761 762 764 765 766 767 768 768 779 780 780 780 782 783 784 785 786 787 787 I. INTRODUCTION More than half a century ago, Veksler (1957) introduced the concept of ‘‘coherent acceleration’’ of particles as a mechanism in which the accelerating field on each particle is proportional to the number of particles being accelerated, * † ‡ 0034-6861= 2013 =85(2)=751(43) 770 770 771 772 773 773 773 775 777 778 751 Ó 2013 American Physical Society 752 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . in contrast to traditional techniques. Additional elements in Veksler’s vision of future accelerators included the automatic synchrony between the particles and the accelerating field, the localization of the latter inside the region where the particles are, and the production of quasineutral groups with large numbers of energetic particles. These features are realized in the acceleration of ions from plasmas produced by intense laser pulses. In this case, as a very general description, strong electric fields are generated by a collective displacement of a large number of electrons, and such electric fields accelerate ions until charge neutrality is restored and ions move together with electrons in a ballistic way. Before the year 2000, ions having energies up to several MeVs had been observed in several high-intensity lasermatter interaction experiments and for different targets, including thick solid foils (Gitomer et al., 1986; Fews et al., 1994; Beg et al., 1997, and references therein), gas jets (Krushelnick et al., 1999; Sarkisov et al., 1999, and references therein) and submicrometric clusters (Ditmire et al., 1997, 1999, and references therein). Common to these experiments was the rather isotropic ion emission and the resulting low brilliance, making these configurations unattractive as ion accelerators for applications. In 2000 three experiments (Clark et al., 2000a; Maksimchuk et al., 2000; Snavely et al., 2000) independently reported the observation of an intense emission of multi-MeV protons from solid targets, either metallic or plastic (CH), of several microns thickness irradiated by high-intensity laser pulses. The basic setup of these experiments is shown in Fig. 1. The laser intensity, number of protons, and maximum ion energy observed for the three experiments were 3 Â 1018 W cmÀ2 , * 109 and 1.5 MeV (Maksimchuk et al., 2000), 5 Â 1019 W cmÀ2 , $1012 and 18 MeV (Clark et al., 2000a), and 3 Â 1020 W cmÀ2 , $2 Â 1013 , and 58 MeV (Snavely et al., 2000), respectively. Figure 2 shows the spectrum of protons observed by Snavely et al. (2000). The protons were detected at the rear side of the target, opposite to the laserirradiated surface and were emitted, as a rath...
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