Emitted as a rather collimated beam along the target

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Unformatted text preview: er collimated beam, along the target normal direction. The emission of protons from metallic targets whose chemical composition does not include hydrogen may sound surprising, but it was already clear from previous experiments that protons originated from impurities, i.e., thin layers of FIG. 1 (color online). Artist’s view of a typical experiment on proton emission from laser-irradiated solid targets. Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 2. Proton energy spectrum from the rear side of a 100 m solid target irradiated by a 423 J, 0.5 ps pulse at normal incidence, corresponding to an intensity of 3 Â 1020 W cmÀ2 . The integrated energy of protons indicates a conversion efficiency of ’ 10% for protons above 10 MeV. From Snavely et al., 2000. water or hydrocarbons which are ordinarily present on solid surfaces under standard experimental conditions. In experiments performed with both ‘‘long’’ nanosecond pulses (Gitomer et al., 1986, and references therein) and ‘‘short’’ (sub)picosecond, high-intensity pulses (Fews et al., 1994; Beg et al., 1997; Clark et al., 2000b), protons and heavier ions were commonly detected in the backward direction (i.e., toward the laser) with a broad angular distribution, and their origin was interpreted in terms of acceleration during the expansion of the hot laser-produced plasma at the front (laserirradiated) side of the target. The characteristics of the forward proton emission in the new experiments, such as the high degree of collimation and laminarity of the beam, were much more impressive. These findings generated an enormous interest both in fundamental research and in the possible applications. In an applicative perspective, the most relevant and peculiar feature of multi-MeV ions is the profile of energy deposition in dense matter. Different from electrons and x rays, protons and light ions deliver most of their energy at the end of their path (see Fig. 3), at the so-called Bragg peak (Ziegler, Biersack, and Ziegler, 2008; Knoll, 2010). The physical reason is that the energy loss is dominated by Coulomb collisions for which the cross section strongly grows with decreasing energy, so that the stopping process becomes progressively more and more efficient. This property makes protons and ions very suitable for highly localized energy deposition. The applications that were proposed immediately after the discovery of multi-MeV proton acceleration included ion beam cancer therapy, laser triggering and control of nuclear reactions, production of warm dense matter, ‘‘fast ignition’’ of inertial confinement fusion targets, and injectors for ion accelerators. These foreseen applications are reviewed in Sec. V. As a particularly innovative and successful application, ultrafast probing of plasmas by laser-driven proton beams is described in Sec. V.A. While the potential for applications was apparent, the details of the physics behind proton acceleration were not clear. A deb...
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