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Unformatted text preview: , the electrostatic ﬁeld peaks at the front separating
protons from heavier species, and reaccelerates mainly the
lower energy part of the spectrum. Similarly, in a recent
experiment, Dollar et al. (2011) obtained spectral modiﬁcations, resulting in the appearance of narrow-band spectral
peaks at $2–3 MeV energies, by focusing a prepulse (10À5
of the 1021 W cmÀ2 peak intensity) on ultrathin foils a few
tens of ps before the peak of the main pulse.
A staged technique which acts on the protons after the
acceleration, but employing all-optical means, was demonstrated by Toncian et al. (2006, 2011). A transient electric
ﬁeld is excited at the inner surface of a metal cylinder (having
$mm diameter and length) irradiated on the outer surface by
a high-intensity laser pulse while a laser-driven proton beam
transits through it; see Figs. 23(a)–23(c). The ﬁeld acts on the
protons by modifying their divergence leading to a narrow,
collimated beamlet. As the ﬁeld is transient, typically lasting
for $10 ps, it affects only the proton component transiting
through the cylinder within this time window, i.e., within a
narrow energy band, leading to a spike in the energy spectrum, as visible in Fig. 23(f), showing a 0.2 MeV band at
$6 MeV. Further experiments have shown that the position
of the spectral peak can be controlled by varying the delay
between the two laser pulses (Toncian et al., 2011) and
conﬁrmed that the focusing is chromatic, i.e., the focal
position varies with proton energy.
A similar approach, but employing a single pulse, was
developed by Kar et al. (2008b) for reducing the proton
beam divergence. Also conceptually similar to the approach
described above by Burza et al. (2011), the scheme employs
specially designed targets in which a thin foil is inserted in a
thicker frame, so that the charge wave expanding outward
from the acceleration region at the rear of the foil generates on
the frames surface an electric ﬁeld transverse to the expanding
beam, which partially constrains its natural divergence.
Other proposed methods of optical control of proton beam
properties include beam steering triggered by shock waves FIG. 23 (color online). (a)–(c)Schematic of laser-driven electrostatic lens. (d), (e) RCF stack beam proﬁles for protons of 9 and 7.5 MeV,
respectively, showing that the 7.5 MeV protons are focused by the ﬁelds inside the cylinder and form a black spot on the RCF. (f): Proton
spectra. Green line: spectrum obtained under same triggering conditions as in (e). Black line: typical exponential spectrum obtained when
cylinder is not triggered. From Toncian et al., 2006.
Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . locally deforming the target surface (Lindau et al., 2005;
Lundh et al., 2007), an effect also reported by Zeil
et al. (2010), and control of the beam homogeneity and
cross section proﬁle by focusing an...
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