Vol 85 no 2 apriljune 2013 fig 6 color online hot

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Unformatted text preview: ot electron temperature as a function of irradiance from experiments of sub-ps laser-solid interaction. See Table 5.2 in Gibbon (2005b) for details on experimental parameters, diagnostic methods, and references. The lines give scaling laws derived from different models [FKL: Forslund, Kindel, and Lee (1977); W: Wilks et al. (1992); GB: Gibbon and Bell (1992); B: Brunel (1987)]. From Gibbon, 2005b. An energy flux balance condition such as h I ’ nh vh Th (with vh ’ c at ultrahigh intensities) may then be used to estimate the ‘‘initial’’ density of hot electrons nh , which usually is not larger than nc , consistently with the argument that nh cannot exceed the density of the region where hot electrons are generated. Inside the target, the effective density may become different from the above estimate for nh under particular conditions due to, e.g., the angular divergence of the electron flow or to electron refluxing effects depending on the electron time of flight and recirculation and thus on the target thickness (Mackinnon et al., 2002). Still one can roughly estimate the total number of hot electrons Nh by an energy balance relation Nh $ h UL =Th , where UL is the energy of the laser pulse. The angular divergence div is also estimated from experiments to range between 20 and 60 and to increase with irradiance (Green et al., 2008, and references therein), although such estimates might depend on the accuracy of the sheath field modeling (Ridgers et al., 2011). 3. Hot electron transport in solid matter Transport of hot electrons in solid matter has been extensively investigated also because of its relevance to the electron-driven fast ignition (FI) scheme in inertial confinement fusion (ICF) [see Freeman et al. (2006) for a survey]. Key issues characterizing this regime are the very high values of the currents and the effect of self-generated fields. From the above estimates it can be inferred that near the front surface of the target the current density jh ¼ Àenh vh associated to hot electrons may reach values up to jh $ enc c ’ 4:8 Â 1012 A cmÀ2 , corresponding to a total current of $15 MA over a circular spot of 10 m radius. This large current must be locally neutralized by a return current jr such that jh þ jr ’ 0; otherwise, either the electric field generated by the charge unbalance or the magnetic field generated by the free flowing current jh would be strong enough to stop the Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . 758 hot electrons (Davies et al., 1997; Passoni et al., 2004). The free, cold electrons contributing to the return current are either present as conduction electrons in metals or produced by field and collisional ionization in insulators (Tikhonchuk, 2002). Additional complexity is introduced by effects such as target heating and hot electron refluxing, which have been inferred in several experiments (Bellei et al., 2010; Nilson et al., 2011;...
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