From the fact that proton acceleration by the main

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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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This document was uploaded on 09/28/2013.

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