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Unformatted text preview: expanding rear sheath due to the effect of the plasma flow (Zhidkov et al., 2002). Additional simulation studies of shock and solitary wave acceleration have been reported by He et al. (2007), Liu et al. (2009), and Macchi, Nindrayog, and Pegoraro (2012). FIG. 28 (color online). Proton spectra from CO2 laser interaction with a hydrogen gas jet. (a) Different spectra for a smooth (long) pulse (lower line) and a pulse train of 3 ps spikes (upper line); in the latter case, a peak appears in the spectrum. (b) Narrow spectra obtained in different shots. See text for parameters. From Haberberger et al., 2012. Several related experiments were performed at the TRIDENT laser facility at LANL, using pulse durations in the 500–700 fs range. C6þ ions with energies up to 15 MeV per nucleon were observed by irradiating DLC foils with $40 J, $7 Â 1019 W cmÀ2 pulses, for an optimal thickness of 30 nm that is determined by the condition that relativistic transparency occurs at the pulse peak (Henig et al., 2009b). Figure 29 shows spectra for different polarizations. For more energetic and intense pulses ($ 80 J, 1021 W cmÀ2 ) and thicker targets (140 nm), broad C6þ spectra with higher cutoff energies beyond 40 MeV=nucleon were observed, and the inferred conversion efficiency was $10% (Hegelich et al., 2011). Narrower C6þ spectra (ÁE i =E i ’ 15–20%) at lower energies ($ 3–10 MeV) were observed using either loose focusing or circular polarization (Hegelich et al., 2011; Jung et al., 2011b). Recently, energies up 80MeV=nucleon for carbon and 120 MeV=protons have been communicated (Hegelich, 2011). The onset of relativistic transparency in these conditions was recently investigated in detail with ultrafast temporal resolution (Palaniyappan et al., 2012). Simulation studies of this regime show that the increase of the cutoff energy is related to enhanced and volumetric C. Transparency regime: Breakout afterburner If ultrathin foils are used as targets (which requires ultrahigh-contrast, prepulse-free conditions), the expansion of the foil may lead to the onset of transparency during the short-pulse interaction, when the electron density ne is further decreased down to the cutoff value (of the order of nc due to relativistic effects, see Sec. II.A). While this effect limits the energy attainable via RPA (see Sec. IV.A.2), it can lead to enhanced ion acceleration via different mechanisms. Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 FIG. 29 (color online). Spectra of C6þ ions from laser interaction with ultrathin targets in the regime of relativistic transparency as a function of target thickness and laser polarization. From Henig et al., 2009b. Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . heating of electrons as the target becomes transparent, leading to a stronger accelerating field for ions; the name ‘‘break-out afterburner’’ was proposed for such regime by the Los Alamos group (Yin et...
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This document was uploaded on 09/28/2013.

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