RevModPhys.85.751

Reaching the rear side of the target there a cloud of

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Unformatted text preview: g out of the target for several Debye lengths, and giving rise to an extremely intense electric field, mostly directed along the normal to the surface. A consequent distinctive feature is that ions are accelerated perpendicularly to the surface, with high beam collimation. The electric field generated at the rear surface depends on parameters of the electron distribution (temperature, number, divergence) as well as of the surface (mostly its density profile as detailed below). The acceleration is most effective on protons, which can be present either in the form of surface contaminants or among the constituents of the solid target as in plastic targets. The heaviest ion populations provide a positive charge with much more inertia, thus creating the charge separation which generates the accelerating field. Part of the heavy population can also be effectively accelerated, on a longer time scale, if the proton number is not high enough to balance the charge of the escaping hot electrons, and especially if impurity protons are removed before the interaction, for example, by preheating the target (Hegelich et al., 2002). In this way, ions of several different species may be accelerated (Hegelich et al., 2005). Several observations strongly supported the TNSA scenario taking place at the rear side. Already Snavely et al. FIG. 11 (color online). Proton emission from a wedge target effectively having two rear surfaces. Two separate spots are produced on the detector, showing that most of the protons originate from the rear side of the target. From Snavely et al., 2000. 762 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . FIG. 12 (color online). Effect of impurity removal on carbon ion spectra. (a) and (b) show C ions traces (from CR-39 track detectors) and spectra from Al foils coated with a C layer on the rear side, in the presence of hydrocarbon contaminants on the surface. In (c) and (d), the contaminants had been previously removed by resistive heating. From Hegelich et al., 2002. (2000) gave clear evidence that the emission was normal to the rear surface using wedge targets which effectively have more than one rear surface. Two separate proton beams were observed in the directions normal to the two rear surfaces of the wedge (see Fig. 11). Mackinnon et al. (2001) reported experimental observations of the interaction of ultraintense laser pulses using targets with and without preformed plasmas on the rear surface of the foil. The peak and mean energies of the proton beam were found to strongly depend on the plasma scale length at the rear of the target. While an energetic proton beam was obtained with an unperturbed rear surface, no evidence of high-energy protons was recorded when a large local scale length in the ion density at the rear surface was induced, consistently with the dependence of the accelerating field on the scale length in Eq. (13). Hegelich et al. (2002) used Al and W foils a...
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This document was uploaded on 09/28/2013.

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