Ion Acceleration
In this chapter we discuss basic reference mechanisms for ion acceleration in laser-plasma interactions. The first two mechanisms, namely sheath acceleration and plasma expansion, are oriented to the modelization of experiments with solid targets in the s
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Ion Acceleration
Abstract In this chapter we discuss basic reference mechanisms for ion acceleration in laser-plasma interactions. The first two mechanisms, namely sheath acceleration and plasma expansion, are oriented to the modelization of experiments with solid targets in the so-called target normal sheath acceleration (TNSA) framework. The other mechanisms, namely shock acceleration, coulomb explosions and radiation pressure acceleration dominate over TNSA in particular conditions and may allow to develop advanced schemes of ion acceleration.
5.1 Ion Acceleration Scenario For all the phenomena described in the previous chapters, the dynamics of ions have been ignored because of their larger mass with respect to electrons, so that their contribution can be ignored on fast time scales such as ∼ω−1 or ∼ω−1 p with ω and ω p the laser and the (electron) plasma frequency, respectively. Ions typically respond on 1/2 ω−1 to slowly varying electric fields generated by a time scale ∼ ω−1 p pi ∼ (m p /m e ) large charge separations. The latter are generated by laser-plasma interactions either when large number of high energy “fast” electrons escape from the plasma or by the direct action of the ponderomotive force, as shown in example in the description of self-focusing (Sect. 3.3) and overdense penetration (Sect. 3.4.1). In this way, an indirect transfer of the laser energy to the ions occurs. The interest of ion acceleration in the superintense regime has been greatly boosted since the year 2000 when three experiments reported on the observation of collimated proton beams with multi-MeV energies from the rear (non-irradiated) side of solid targets (Clark et al. 2000; Maksimchuk et al. 2000; Snavely et al. 2000). The interpretation of these early observations, as well as of a great number of later experiments has stimulated a wide effort in the modeling of the acceleration process. Most of the experiments may be described in the framework of the Target Normal Sheath Acceleration (TNSA) model, which may be briefly described as follows. The fast
A. Macchi, A Superintense Laser-Plasma Interaction Theory Primer, SpringerBriefs in Physics, DOI: 10.1007/978-94-007-6125-4_5, © The Author(s) 2013
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5 Ion Acceleration
−Jf
Laser
+E + + + + + sheath surface layer
Fig. 5.1 Schematic of the TNSA mechanism. The flow of fast electrons generated at the front side crosses the target bulk and reaches the rear side, generating an electron sheath in vacuum. The electric field is almost perpendicular to the rear surface and accelerates ions, in particular protons from impurity layers on the surface
electrons produced in a high-intensity interaction with the front surface of a solid target propagate through the bulk and eventually reach the rear side, where they produce a negatively charged sheath (see Fig. 5.1). The electrostatic field generated in the sheath is almost normal to the surface and accelerates ions. The protons which are contained in impurity layers, ordinarily present on the surface of metal targets, are in a fa
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