Unformatted text preview: e sensitive to the incidence angle and the density
scale length, with the latter varying on the time scale of ion
motion (Gibbon, 1994) yielding a time-dependent absorption.
Experimental attempts (Flacco et al., 2008; McKenna et al.,
2008; Batani et al., 2010) have been made to vary the density
scale length in order to increase absorption in hot electrons
and consequently to enhance ion acceleration (see Sec. III.E).
Hot electron generation tends to become more efﬁcient for
lower plasma densities and, particularly, close to the critical
density nc , as it is observed that stronger coupling and
volumetric heating occurs near the transmission threshold.
A ‘‘near-critical’’ plasma may be produced either by the laser
prepulse or by using a special target material, e.g., a lowdensity foam (see Sec. IV.D).
2D simulations reveal additional effects, as for instance
the deformation of the plasma surface due to ‘‘hole boring’’
(HB) driven by radiation pressure (see also Sec. IV.A.1),
which changes the local incidence angle (Wilks et al.,
1992), leading to increased absorption and providing a dynamic ‘‘funnel’’ effect collimating the electron ﬂow inside the
target (Ruhl et al., 1999). A similar dynamics occurs in
microcone targets which have proved to be effective in
enhancing hot electron generation (Sentoku et al., 2004;
Nakamura et al., 2009; Gaillard et al., 2011, and references
Absorption is also sensitive to small-scale surface
deformations, either self-generated or preimposed, so that
the use of microstructures on the front target surface has
also been suggested as a way to enhance hot electron
generation; see, e.g., Klimo et al. (2011), and references
therein. Another possible approach is the use of grating
surfaces where the resonant excitation of surface plasma
waves can also lead to very high absorption (Raynaud
et al., 2007; Bigongiari et al., 2011).
The high sensitivity of hot electron generation to laser and
plasma parameters partly accounts for data scatter and
differences observed in the many experimental investigations
reported in the literature, with the above mentioned prepulse
effects bringing additional complexity. For these reasons,
absorption values and characteristics of the hot electron
distribution are often taken into account in a phenomenological way. It has often been considered acceptable to
assume the hot electron distribution to be Maxwellian with
a temperature Th given by Eq. (6) as a function of the laser
irradiance. Figure 6 presents a collection of temperature
measurements obtained for subpicosecond pulses up to the
year 2000 (Gibbon, 2005b); these data broadly support a
scaling of Th as ðI2 Þ1=2 . The total fractional absorption in
hot electrons h is usually estimated to be in the 10%–30%
range, with experimental indications of possibly quite higher
values at ultrarelativistic intensities (Ping et al., 2008).
Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 6 (color online). H...
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