M such waves are associated with the reection of ions

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Unformatted text preview: t, resulting in a velocity vi ¼ 2Mcs and an energy per nucleon E i ¼ 2mp M2 c2 ¼ 2ðZ=AÞM2 Th , being s qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cs ¼ ZTh =Amp the ion sound velocity. Such a CSA scenario and related experiments are discussed in Sec. IV.B. Finally, the possibility of front side (or bulk) acceleration being favored by resistivity effects is discussed in Sec. IV.E. 3. Acceleration schemes using innovative targetry Both TNSA and RPA can have different features in targets having peculiar geometrical and physical properties, if compared to the solid targets used in the 2000 experiments (Clark et al., 2000a; Maksimchuk et al., 2000; Snavely et al., 2000) which had thickness in the 1–100 m range and were much wider than the laser spot diameter. Experimental investigations of ‘‘ultrathin,’’ submicrometric targets require extremely ‘‘clean,’’ prepulse-free pulses to avoid early target evaporation and thus became possible only recently thanks to the development of advanced pulse cleaning techniques (see Sec. III.E). The use of ‘‘mass-limited’’ targets which also have limited lateral dimensions (in the sub-mm range) allows the refluxing and concentration of hot electrons in a small volume and may lead to higher ion energies via TNSA. These studies are presented in Sec. III.E. For RPA, a sufficiently thin foil target is expected to be accelerated as a whole. Assuming the foil to be a perfect mirror of thickness ‘, its nonrelativistic motion may be simply described by mi ni ‘dV=dt ¼ 2I=c from which we obtain an energy E i ¼ mi V 2 =2 ¼ ð2=mi ÞðF=ni ‘cÞ2 , where R F ¼ Idt is the laser pulse fluence. This is the basis of the ‘‘light sail’’ (LS) regime of RPA (see Sec. IV.A.2) which seems very promising in view of the foreseen fast scaling and the intrinsic monoenergeticity. For extremely thin (a few nm) targets, the breakthrough of the laser pulse through the foil due to relativistic transparency may stop LS-RPA, but at the same time lead to additional strong heating of electrons. This effect opens up a regime of possible enhanced acceleration (BOA), which will be discussed in Sec. IV.C. In general, reducing the effective size of the target allows for laser pulse penetration, volumetric heating, and energy confinement, which can lead to efficient ion acceleration even at low laser pulse intensities. As a well-known example, the interaction of ultrashort, moderate intensity (’ 1016 W cmÀ2 ) pulses with subwavelength clusters allowed acceleration of ions up to energies sufficient to produce nuclear fusion reactions (Ditmire et al., 1997, 1999). A limitation on the use of such clusters as ion Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 759 sources is the isotropic ion emission and the resulting low brilliance. ‘‘Droplet’’ targets with size of the order of one wavelength have been investigated as a trade-off approach, as discussed in Sec. III.E.2. As...
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This document was uploaded on 09/28/2013.

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