The targets have a thick layer of carbon contaminants

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Unformatted text preview: ea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . in the C spectrum (specifically C5þ ), suggesting that an ultrathin layer of graphite is formed as a result of phase changes of the carbon compounds in the contaminant and that all C5þ ions from the layer experience approximately the same accelerating field as theoretically predicted (Esirkepov et al., 2002; Albright et al., 2006). Figure 22 shows spectra for targets with and without contaminant removal, together with hybrid simulation predictions. 3. Staged acceleration and beam control Staged acceleration employing two laser pulses on two separate targets has also been investigated as a possible route to spectral manipulation of laser-driven ion beams. This idea relies on accelerating a TNSA beam from a first target and direct it through a second foil, which is irradiated by a second laser pulse at the time that a particular group of TNSA protons crosses the foil. These protons should thus experience an accelerating field as they transit through the rear surface of the second foil and gain additional energy. An experiment by Pfotenhauer et al. (2010), also carried out on the JETI laser, tested this idea. Peaks and dips in the spectrum were observed at energies of $1 MeV which correlated well with the time of flight of protons reaching the second target as it is irradiated, showing that the field on the second target slightly boosts protons in a given energy range resulting in the spectral modification. Burza et al. (2011) reported a two-stage approach employing spherical shell targets, irradiated by a single laser pulse, in which protons accelerated by TNSA at the front of the shell experience a second accelerating field while they transit through the opposite side of the shell, which modifies the highenergy end of the spectrum. The field is due to a hot electron charge wave spreading along the target surface from the interaction point, as reported in several experiments (Toncian et al., 2006; McKenna et al., 2007; Quinn et al., 2009a). A different type of two-stage approach was tested by Markey et al. (2010) on the VULCAN laser. Two pulses of sub-ps duration were sequentially focused, with controllable delay, on the same target in order to modify the temporal history of the hot electron source driving the TNSA, as suggested originally by Robinson et al. (2007). An optimal delay was seen to result in an increase of energy and conversion efficiency and, additionally, a modification of the slope of the spectral profile. In this case, besides an optimization of hot electron production by the main pulse in a front surface plasma gradient, similar to Kaluza et al. (2004) and McKenna et al. (2008), they suggest that an additional modification of the proton spectrum arises from the fact that proton acceleration by the main pulse takes place in an already expanding multispecies, plasma sheath at the target rear surface. Under these conditions...
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This document was uploaded on 09/28/2013.

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