RevModPhys.85.751

Presence of signicant preplasma these results were

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Unformatted text preview: t al. (2008) reported on heating of soliddensity matter due to laser-driven density profile sweeping FIG. 24 (color online). Hole boring acceleration by a CO2 laser pulse in a gas jet. The left frame shows ion spectra for various values of the intensity I15 (in units of 1015 W cmÀ2 ) and the electron density n ¼ ne =nc : (a) I15 ¼ 6:4, n ¼ 6:1; (b) I15 ¼ 5:5, n ¼ 6:1; (c) I15 ¼ 5:9, n ¼ 7:6; (d) I15 ¼ 5:7, n ¼ 8:0. The right frame shows the observed scaling of ion energies with the ratio 4I=nc. From Palmer et al., 2011. Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . and shock formation at intensities up to 5 Â 1020 W cmÀ2 , and Henig et al. (2009a) reported on ion acceleration by a converging shock in spherical targets irradiated at 1 Â 1020 W cmÀ2 . For both these experiments, the analysis of data and supporting PIC seems also compatible with HB-RPA occurring at the front surface, although the electron heating due to the use of linear polarization complicates the picture. Indications of strong radiation pressure effects were also obtained from the modeling of collimated, high-density plasma jets at the rear side of targets with a few micron thickness, at intensities up to 2 Â 1020 W cmÀ2 (Kar et al., 2008a). It should be noted that, although the scaling of Eq. (30) leads to relatively modest energies in solid-density targets, the foreseeable values are of interest for applications requiring large numbers of ions at energies of only a few MeV (see Sec. V). 2. Thin targets: Light sail regime As discussed above, hole boring RPA applies to a ‘‘thick’’ target, i.e., much thicker than the skin layer in which ion acceleration by the space-charge field occurs. The laser pulse penetrates into the target by pushing adjacent surface layers via a repeated cycle of ion bunch acceleration. The situation changes when a target is thin enough that all the ions are accelerated before the end of the laser pulse, i.e., a complete hole boring occurs. In such a case, the laser pulse is able to further accelerate ions to higher energies since the ions are not screened by a background plasma anymore. The thin target regime of RPA has been named light sail as the term is appropriate to refer to a thin object of finite inertia, having large surface and low mass, so that it can receive a significant boost from radiation pressure. The invention of the laser soon stimulated possible applications of the LS concept, including visionary ones such as laser-driven spacecraft propulsion (Forward, 1984). To support this idea Marx (1966) used calculations based on the simple model of a flat, perfect mirror boosted by a plane wave. The analytical solution and scaling laws provided by such basic model (Simmons and McInnes, 1993) are useful to illustrate the most appealing features of LS-RPA, such as high conversion efficiency in the relativistic limit and the possibility to reach very high energies with foreseeab...
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This document was uploaded on 09/28/2013.

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