Ion energy already at relatively low laser

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Unformatted text preview: -covered targets (Zigler et al., 2011) for which surprisingly high energies up to 5.5–7.5 MeV for a 5 Â 1017 W cmÀ2 , 40 fs laser pulse were reported. 2. Source spectrum manipulation Various approaches have been proposed in order to manipulate the spectrum of TNSA protons and ions, in most cases with the intent of obtaining narrow-band peaks, in other Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 771 cases with the aim to enhance proton numbers throughout the whole spectrum or in some spectral bands as required by specific applications. We first review a number of approaches in which the proton spectrum is modified at the source, leaving approaches which act on the proton beam after the initial acceleration to Sec. III.E.3. Spectral peaks can appear as a consequence of multispecies plasma expansion (see Sec. III.C.3). This effect has been invoked to explain observations in proton beams from thin foils, where the peaks appear as modulation of a continuum exponential spectrum (Allen et al., 2003) and in experiments employing droplets of heavy water, where peaks are observed in the deuterium spectrum (Ter-Avetisyan et al., 2006). Spectral peaks have been observed in experiments employing high-Z metallic targets where a plastic layer [0:5 m polymethyl methacrylate (PMMA)] was coated as a dot on the rear surface of a 5 m Ti foil (Schwoerer et al., 2006; Pfotenhauer et al., 2008). These results, obtained on the 10 TW JETI laser in Jena, were explained on the basis of the proton depletion approach first suggested by Esirkepov et al. (2002). Robinson and Gibbon (2007) suggested instead that the proton density in the multispecies plastic layer is the important factor in determining the appearance of the spectral peak. Experimental implementation required the removal of the native contaminant layer present at the surface and resulted in peaks in the proton spectra at $2 MeV, with $10% spread and good reproducibility (Pfotenhauer et al., 2008). Another experiment also relied on the (partial) removal of hydrogen contaminants from the surface of a palladium target (Hegelich et al., 2006) so that protons did not appear in the spectrum. Instead, monoenergetic features appeared FIG. 22 (color online). Ion spectra from preheated Pd substrate targets from which hydrogen contaminants have been removed (Hegelich et al., 2006). Black curve: spectrum of C5þ ions. Blue curve: spectrum of the dominant substrate charge state Pd22þ . Green and red curves: simulated C5þ and Pd21þ spectra. Gray curve: spectrum of dominant C4þ ions from a heated W target. Magenta curve: C5þ signal from a cold Pd target. In the cases of black and blue curves, an ultrathin layer of graphite is present on the target surface, and a quasimonoenergetic spectrum appears. In the last two cases (gray and magenta curves) the targets have a thick layer of carbon contaminants and do not form a monolayer source, resulting in exponential-like spectra. From Hegelich et al., 2006. 772 Andr...
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This document was uploaded on 09/28/2013.

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