The absorption degree of a p polarized laser pulse is

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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 therein). 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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