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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
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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