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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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