This preview shows page 1. Sign up to view the full content.
Unformatted text preview: s
VI. Conclusions and Outlook
787 I. INTRODUCTION More than half a century ago, Veksler (1957) introduced
the concept of ‘‘coherent acceleration’’ of particles as a
mechanism in which the accelerating ﬁeld on each particle
is proportional to the number of particles being accelerated, *http://www.df.unipi.it/~macchi
0034-6861= 2013 =85(2)=751(43) 770
778 751 Ó 2013 American Physical Society 752 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . in contrast to traditional techniques. Additional elements in
Veksler’s vision of future accelerators included the automatic
synchrony between the particles and the accelerating ﬁeld,
the localization of the latter inside the region where the
particles are, and the production of quasineutral groups
with large numbers of energetic particles.
These features are realized in the acceleration of ions
from plasmas produced by intense laser pulses. In this
case, as a very general description, strong electric ﬁelds
are generated by a collective displacement of a large number
of electrons, and such electric ﬁelds accelerate ions until
charge neutrality is restored and ions move together with
electrons in a ballistic way.
Before the year 2000, ions having energies up to several
MeVs had been observed in several high-intensity lasermatter interaction experiments and for different targets, including thick solid foils (Gitomer et al., 1986; Fews et al.,
1994; Beg et al., 1997, and references therein), gas jets
(Krushelnick et al., 1999; Sarkisov et al., 1999, and references therein) and submicrometric clusters (Ditmire et al.,
1997, 1999, and references therein). Common to these experiments was the rather isotropic ion emission and the
resulting low brilliance, making these conﬁgurations unattractive as ion accelerators for applications.
In 2000 three experiments (Clark et al., 2000a; Maksimchuk
et al., 2000; Snavely et al., 2000) independently reported the
observation of an intense emission of multi-MeV protons from
solid targets, either metallic or plastic (CH), of several microns
thickness irradiated by high-intensity laser pulses. The basic
setup of these experiments is shown in Fig. 1. The laser
intensity, number of protons, and maximum ion energy observed for the three experiments were 3 Â 1018 W cmÀ2 ,
* 109 and 1.5 MeV (Maksimchuk et al., 2000),
5 Â 1019 W cmÀ2 , $1012 and 18 MeV (Clark et al., 2000a),
and 3 Â 1020 W cmÀ2 , $2 Â 1013 , and 58 MeV (Snavely
et al., 2000), respectively. Figure 2 shows the spectrum of
protons observed by Snavely et al. (2000). The protons were
detected at the rear side of the target, opposite to the laserirradiated surface and were emitted, as a rath...
View Full Document