By a petawatt laser pulse via the mechanisms

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Unformatted text preview: the latter issue, the energy deposition profile of electrons is a smooth function, making it difficult to produce a localized hot spot. Observation of efficient generation of multi-MeV proton beams in petawatt experiments (Hatchett et al., 2000; Snavely et al., 2000) soon stimulated the proposal of the use of such protons as the ignitor beam (Roth et al., 2001). The most promising features of proton beam ignition as claimed were the highly localized energy deposition profile (see Fig. 3), the low emittance of the beam, and its focusability, for instance, by parabolically shaping the rear side of the proton-producing target as suggested by numerical simulations (Ruhl et al., 2001; Wilks et al., 2001). Detailed calculations by Atzeni, Temporal, and Honrubia (2002) and Temporal, Honrubia, and Atzeni (2002) addressed, in particular, the effects of the quasithermal energy distribution typical of TNSA protons, and of the related temporal dispersion. The latter could be beneficial for energy deposition since the proton stopping range increases with plasma temperature. Hence, heating due to the more energetic protons favors energy deposition by the less energetic ones which arrive later in the dense fuel region. A fit of simulations for a proton temperature of 5 MeV provided the following estimate of the ignition energy26 E ig as a function of fuel density  and distance d between proton source and fuel core: E ig ’ 90ðd=mmÞ0:7 ð=100 g cmÀ3 ÞÀ1:3 kJ: (38) Integration of the foil inside the cone of conical ICF targets already designed for electron FI was then proposed in order to reduce d and thus E ig . This raised the issue of shielding the foil from preheating caused, e.g., by external radiation, which may jeopardize efficient TNSA (see Sec. III.E.1); a preliminary analysis was mentioned by Geissel et al. (2005). Figure 35 sketches the target and foil assembly and summarizes suitable parameters for proton FI with cone targets. Temporal (2006) and Temporal, Honrubia, and Atzeni (2008) investigated a similar scheme but used two proton beams with suitably shaped radial profiles, obtaining a 40% reduction of the ignition energy. 26 E ig includes only the energy of the proton beam. The total energy of the laser driver is E ig =p with p < 1 the conversion efficiency into protons. 784 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . facilities to be developed, equipped with petawatt-class laser systems. D. Biomedical applications FIG. 35. Concept of proton-driven fast ignition in the TNSAbased, cone guided scheme. Typical parameters required for the ion beam and optimization issues are also indicated. From Key, 2007. Independent from Roth et al. (2001), fast ignition by laseraccelerated light ion beams was proposed by Bychenkov, Rozmus et al. (2001). The use of deuterons and beryllium ions was investigated, and it was suggested that using ions with Z > 1 could be advantageous because o...
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This document was uploaded on 09/28/2013.

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