In several experiments bellei et al 2010 nilson et al

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Unformatted text preview: Quinn et al., 2011, and references therein). Filamentation instabilities and dependence on the target material have also been extensively studied (Fuchs et al., 2003; Manclossi et al., 2006; McKenna et al., 2011, and references therein). Simulation models accounting for both collisional effects and self-consistent generation of quasistatic fields are needed for quantitative investigations.8 Finally, it is noticeable that at least a fraction of hot electrons propagates coherently through the target conserving the temporal periodicity of the driving force, i.e., as bunches with ! or 2! rate depending on incidence angle and polarization, as inferred by optical transition radiation measurements (Popescu et al., 2005). C. Ion acceleration mechanisms In this section we give an overview of ion acceleration mechanisms including both those proposed to explain early experimental results in solid targets and those investigated later, either following inspiration from theoretical work or testing novel target designs. Some of the mechanisms described and the target regions where they are active are indicated in Fig. 7. Ion acceleration models will be described more in detail in Secs. III and IV along with the most relevant experiments. 1. Rear surface acceleration As outlined in Sec. II.B, a very intense current of highenergy hot electrons may be generated at the front side of the target and eventually reach the rear side. There, as the hot electrons cross the rear side boundary and attempt to escape in vacuum, the charge unbalance generates a sheath field Es , normal to the rear surface. Since Es must backhold electrons with a typical ‘‘temperature’’ Th , the typical spatial extension of the sheath Ls will be related to Es by eEs $ Th : Ls (13) From dimensional arguments, assuming a steep interface and nh and Th as the only parameters, Ls may be roughly estimated as the Debye length of hot electrons, Ls $ Dh ¼ ðTh =4e2 nh Þ1=2 . Assuming the simple scalings of Sec. II.B for Th , taking a laser irradiance I2 ¼ 1020 W cmÀ2 m2 and a fractional absorption h ¼ 0:1, we find Th ’ 5:1me c2 ¼ 2:6 MeV, nh $ 8 Â 1020 cmÀ3 , Dh ¼ 4:2 Â 10À5 cm, and Es $ 6 Â 1010 V cmÀ1 . This large field will backhold most of the escaping electrons, ionize atoms at the rear surface, and 8 See, e.g., Gremillet, Bonnaud, and Amiranoff (2002), Bell et al. (2006), Evans (2007), Klimo, Tikhonchuk, and Debayle (2007), Solodov et al. (2009), and Kemp, Cohen, and Divol (2010), and references therein. Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 7 (color online). Cartoon showing some of the possible acceleration mechanisms in the interaction with a thick solid target, including TNSA at the rear side (see Sec. II.C.1), hole boring RPA at the front side (see Sec. II.C.2), and backward acceleration in the plasma blowoff [see, e.g., Clark et al. (2000b)]. Also shown are the hot electron flow leading to sheath formation and expansion at the rear side and the...
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