Developed in secs iic and iiia related experimental

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Unformatted text preview: found in the literature (Spencer et al., 2003; Fuchs et al., 2006b). Effective suppression of the laser prepulse level, that is, the adoption of ultrahigh laser contrast, can significantly alter the physical picture, since ultrathin targets, down to the nm level, can maintain their integrity until the interaction with the main pulse. With these conditions a more effective acceleration process can be expected because the refluxing and concentration of hot electrons in a smaller volume may lead to the establishment of a stronger electric field and, consequently, to higher ion energies. These ideas have been successfully tested by Neely et al. (2006), using Al target with thicknesses as low as 20 nm in combination with 33 fs pulses with ASE intensity contrast reaching 1010 . A significant increase of both maximum proton energy and laser-to-proton energy conversion efficiency was found at an optimum thickness of 100 nm. Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 20. Maximum detectable proton energy as a function of target thickness for high-contrast (HC) and low-contrast (LC) conditions. Data are shown for both backward (BWD) and forward (FWD) directed ions, respectively, showing the symmetrical behavior of TNSA for HC and ultrathin targets. The LC results show the existence of an ‘‘optimal’’ thickness determined by the laser prepulse causing early target disruption, similar to Kaluza et al. (2004). The laser pulse had 65 fs duration, ð0:5–1Þ Â 1019 W cmÀ2 intensity, 45 incidence, and P polarization. From Ceccotti et al., 2007. Similar results have been obtained by Antici et al. (2007) and Ceccotti et al. (2007). As a further interesting feature of this latter experiment, a symmetrical TNSA on both front and rear sides has been demonstrated, as shown in Fig. 20, when a sufficiently high ( > 1010 ) laser contrast is used. This result confirms the universality of the TNSA process, which may also occur at the front side (accelerating ions in the backward direction) if the density profile is sharp enough. Recently, using a laser pulse with similar contrast, 40 fs duration, 1021 W cmÀ2 and irradiating targets of 800 nm thickness, Ogura et al. (2012) reported proton energies up to 40 MeV, the highest value reported so far for pulse energies below 10 J. Another possible strategy to exploit the effectiveness in the formation of the accelerating field in mass-limited targets is to reduce the lateral dimensions. Numerical investigations (Psikal et al., 2008) have shown that a reduced surface leads to higher densities of hot electrons at the rear side of the target and, thus, to higher accelerating electric fields. Buffechoux et al. (2010) experimentally confirmed these findings showing that in targets having limited transverse extent, down to tens of m, the laser-generated hot electrons moving with a component of the velocity along the lateral direction can be reflected from the target edges during time scales of the same order o...
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This document was uploaded on 09/28/2013.

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