From the vulcan laser and very thin 01 m metallic

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Unformatted text preview: )], while heavier bulk ions had a broad spectrum at lower energies. The peak energies scaled with the fluence parameter as $F 2 $ a4 t2 [see Fig. 27(b)], in 1 0p agreement with Eq. (35) for nonrelativistic ions, and differently from scalings as a0 , a2 , or a2 tp which have been 0 0 inferred for TNSA or for other mechanisms effective for ultrathin targets (see Sec. IV.C). The Z=A ¼ 1 peaks are at slightly higher energy than the Z=A ¼ 1=2 ones, suggesting that the LS stage is accompanied by a multispecies expansion (see Sec. III.C.3) in the sheath field where protons gain additional energy and the spectral peak separation may be further enforced. The scaling plot in Fig. 27(b) also contains data from Henig et al. (2009c) who investigated LS using 45 fs, CP pulses at ultrahigh contrast ( $ 1011 ) and $5 Â 1019 W cmÀ2 intensity, and few-nm DLC foils. Experimental spectra of 23 See, e.g., Pegoraro and Bulanov (2007), Klimo et al. (2008), Tikhonchuk (2010), T.-P. Yu et al. (2010), Chen et al. (2011), and Adusumilli, Goyal, and Tripathi (2012). Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 Acceleration of particles by shock waves (briefly, shocks) in plasmas is a problem of great interest in astrophysics (Martins et al., 2009). The existence of an ion component that is reflected by the shock front is actually integral to the formation of the collisionless, electrostatic shock waves in basic fluid theory, where the electrons are assumed to be in a Boltzmann equilibrium (Forslund and Shonk, 1970; Forslund and Freidberg, 1971; Tidman and Krall, 1971). In the frame moving at the shock velocity, ions are reflected by the shock if the height of the electrostatic potential barrier Èmax at the front is such that ZeÈmax > mi v2 =2, with v1 the velocity of the ion 1 component in the shock frame. Behind the shock front, the fields have an oscillatory behavior. Reflected ions initially at rest acquire a velocity in the laboratory frame equal to 2vsho , where vsho is the shock front velocity. CSA was proposed as an ion acceleration mechanism in superintense laser interaction with an overdense plasma on the basis of PIC simulations by Denavit (1992) and Silva et al. (2004).24 In the latter work simulations showed the generation of shocks with high Mach numbers M ¼ vsho =cs ¼ 2–3, where the sound speed is estimated using the hot electron energy as the temperature, i.e., Th ’ E p [Eq. (6)]. The shocks are generated at the front surface with a velocity close to vHB given by Eq. (29), consistently with the assumption that they are driven by the piston action of radiation pressure. By estimating vsho ’ v HB ,25 in the 24 In experiments on underdense plasmas created either by using gas jet targets (Wei et al., 2004) or by the effect of long prepulses in solid targets (Habara et al., 2004b), the observation of ion acceleration along the radial direction has been attributed to radial shock generation in a laser-driven channel. 25 Note that vsho...
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This document was uploaded on 09/28/2013.

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