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out of the target for several Debye lengths, and giving rise to
an extremely intense electric ﬁeld, mostly directed along the
normal to the surface. A consequent distinctive feature is that
ions are accelerated perpendicularly to the surface, with high
beam collimation. The electric ﬁeld generated at the rear
surface depends on parameters of the electron distribution
(temperature, number, divergence) as well as of the surface
(mostly its density proﬁle as detailed below).
The acceleration is most effective on protons, which can be
present either in the form of surface contaminants or among
the constituents of the solid target as in plastic targets. The
heaviest ion populations provide a positive charge with much
more inertia, thus creating the charge separation which generates the accelerating ﬁeld. Part of the heavy population can
also be effectively accelerated, on a longer time scale, if the
proton number is not high enough to balance the charge of the
escaping hot electrons, and especially if impurity protons are
removed before the interaction, for example, by preheating
the target (Hegelich et al., 2002). In this way, ions of several
different species may be accelerated (Hegelich et al., 2005).
Several observations strongly supported the TNSA scenario taking place at the rear side. Already Snavely et al. FIG. 11 (color online). Proton emission from a wedge target
effectively having two rear surfaces. Two separate spots are produced on the detector, showing that most of the protons originate
from the rear side of the target. From Snavely et al., 2000. 762 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . FIG. 12 (color online). Effect of impurity removal on carbon ion
spectra. (a) and (b) show C ions traces (from CR-39 track detectors)
and spectra from Al foils coated with a C layer on the rear side, in
the presence of hydrocarbon contaminants on the surface. In (c) and
(d), the contaminants had been previously removed by resistive
heating. From Hegelich et al., 2002. (2000) gave clear evidence that the emission was normal to
the rear surface using wedge targets which effectively have
more than one rear surface. Two separate proton beams were
observed in the directions normal to the two rear surfaces of
the wedge (see Fig. 11).
Mackinnon et al. (2001) reported experimental observations of the interaction of ultraintense laser pulses using
targets with and without preformed plasmas on the rear
surface of the foil. The peak and mean energies of the proton
beam were found to strongly depend on the plasma scale
length at the rear of the target. While an energetic proton
beam was obtained with an unperturbed rear surface, no
evidence of high-energy protons was recorded when a large
local scale length in the ion density at the rear surface was
induced, consistently with the dependence of the accelerating
ﬁeld on the scale length in Eq. (13).
Hegelich et al. (2002) used Al and W foils a...
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