RevModPhys.85.751

Via either uid or kinetic equations it is mostly

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Unformatted text preview: heavy (H) population: in this way it is possible, e.g., to model the acceleration of light species present on the surfaces of a solid target made of heavy ions. 9 See, e.g., Mora (2003), Passoni et al. (2004), Albright et al. (2006), Nishiuchi et al. (2006), and Robinson, Bell, and Kingham (2006). Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 Depending on the description of the ion populations, two main categories of TNSA models, to be discussed in detail in Secs. III.C.1 and III.C.2, respectively, may be identified as follows. The first includes static models in which it is assumed that the light ions, or at least the most energetic ones, are accelerated in the early stage of the formation of the sheath, so that the latter may be assumed as stationary. With these conditions, the effects of the light ions on the electrostatic potential are usually neglected, while the heavy ion population of the target is considered immobile. The aim is thus to provide the most accurate description of the sheath depending on assumptions on the hot electron distributions. The second category includes dynamic models where the system is described as a neutral plasma in which the ions acquire kinetic energy in the course of the sheath evolution. In several cases a unique ion component is considered. This approach is therefore strongly connected to the classic problem of plasma expansion in vacuum, first considered by Gurevich, Pariiskaya, and Pitaevskii (1966). In a cold fluid description, neglecting relativistic effects the ions are described by @uj Zj e þ uj Á ruj ¼ À r; @t mj (16) @nj þ r Á ð nj u j Þ ¼ 0 @t (17) ðj ¼ L; H Þ; where uj ¼ uj ðr; tÞ is the fluid velocity. If the ions are described kinetically, their Vlasov equation for the phasespace distribution fj ¼ fj ðr; v; tÞ is @fj Zj e @fj þ v Á rfj À ¼ 0: r Á @t @v mj (18) Most of the general studies of plasma expansion and related ion acceleration developed both before and after TNSA experiments10 as well as more specific models of TNSA11 so far proposed in the literature can be considered as suitable simplifications of the previous equations, falling into one of the two above mentioned categories (or suitable combinations of them) and obtained adding further, physically motivated assumptions. Most of these models assume a 1D geometry, consistently with the electrostatic approximation, and planar in most cases. This latter assumption, when applied to TNSA modeling, requires the rear surface to be sufficiently flat and 10 General studies of plasma expansion in vacuum include Gurevich, Pariiskaya, and Pitaevskii (1966), Allen and Andrews (1970), Widner, Alexeff, and Jones (1971), Crow, Auer, and Allen (1975), Pearlman and Morse (1978), Denavit (1979), Mora and Pellat (1979), and Dorozhkina and Semenov (1998). Early papers focused on modeling ion acceleration in laser-produced plasmas include Pearlman and Morse (1978), Wickens, Allen, and Rumsby (1978), True, Albritton, and...
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