Wei et al 2004 or by the effect of long prepulses in

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Unformatted text preview: ’ vHB implies that ‘‘reflected’’ ions directed into the bulk will have a velocity $2vHB , i.e., twice the surface recession velocity, as the fastest ions generated by the piston action in HB acceleration (see Sec. IV.A.1). This similarity may explain why HB and CSA are often confused in the literature. 778 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . strongly relativistic limit a0 ) 1 the condition to obtain supersonic shocks driven by radiation pressure (M > 1) can pffiffiffi be written as 2a0 > ne =nc . The reflected ions may get further acceleration by the transient sheath field at the rear surface as in TNSA, eventually producing a plateau in the ion spectrum. A similar signature was observed experimentally by Zepf et al. (2003) and thus interpreted as evidence of the front side contribution to ion acceleration, in contrast to pure TNSA at the rear side of the target. Under particular conditions, the staged CSA-TNSA acceleration might produce the highest energy component in the ion spectrum as observed ` in simulation studies (d’Humieres et al., 2005; Chen et al., 2007) which, however, also suggest lower efficiency and brilliance with respect to pure TNSA. Recently, CSA has been indicated as the mechanism responsible for monoenergetic acceleration of protons up to 22 MeV (see Fig. 28) in the interaction of CO2 laser pulses with hydrogen gas jets at intensities up to 6:5 Â 1016 W cmÀ2 corresponding to a0 ¼ 2:5 (Haberberger et al., 2012). The particular temporal structure of the laser pulse, i.e., a 100 ps train of 3 ps pulses, was found to be essential for the acceleration mechanism, since no spectral peaks were observed for a smooth, not modulated pulse. Comparison with PIC simulations suggested that the multiple pulses lead to efficient generation of suprathermal electrons, and that this process (rather than radiation pressure) drives the shocks which eventually accelerate protons. Simulations also suggest that the process could scale in order to produce 200 MeV protons at 1018 W cmÀ2 that may be foreseeable with future CO2 laser development. Such a scheme based on gas lasers and gas jet target would have the advantage of high-repetition rate operation, but the efficiency per shot might be low with respect to other approaches: in the experiment of Haberberger et al. (2012) the number of ions ($ 2:5 Â 105 ) in the narrow spectral peak at ’ 22 MeV for a 60 J pulse energy implies a conversion efficiency of $10À8 . In addition to collisionless shocks, the standard fluid theory also predicts solitons (Tidman and Krall, 1971) propagating at the velocity vsol with 1 < vsol =cs & 1:6. These solitons are characterized by ZeÈmax < mi v2 =2 and are thus transparent sol to background ions ‘‘by construction.’’ However, the generation of electrostatic solitons may lead to ion acceleration in some circumstances, e.g., when the soliton breaks in the...
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This document was uploaded on 09/28/2013.

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