RevModPhys.85.751

To address this last issue a target andrea macchi

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Unformatted text preview: thin (0:1 m) metallic targets [for parameters of shots 1–3, see the explanation in (b)]. (b) Peak energies for both ions with Z=A ¼ 1=2 (filled squares) and protons with Z=A ¼ 1 (empty squares) for seven shots with different polarization parameters [see, e.g., Eq. (11)]:  ¼ 0 (LP), 0.47 (EP), and 0.88 (CP). The peak energy is plotted as a function of the parameter a2 p =, which corresponds to ðmp =2me Þp in our 0 notations. The parameter set [a0 , target material, thickness (m), polarization] for the data points 17 is [15.5, Cu, 0.1, LP], [10, Cu, 0.05, CP], [13.8, Cu, 0.1, EP], [7.5, Al, 0.1, LP], [6.9, Al, 0.1, EP], [13.6, Al, 0.5, CP], and [14.1, Al, 0.8, LP], respectively. The circle is the data point from Henig et al. (2009c). The solid line is the LS scaling (35). From Kar et al., 2012. 777 fully ionized C6þ ions show a difference between linearly and circularly polarized pulses, with a broad peak at ’ 30 MeV appearing in the latter case, and reduced electron heating for CP. More recent experimental data by Dollar et al. (2012) using tightly focused (f=1) pulses with intensity up to 2 Â 1021 W cmÀ2 showed a weak difference between CP and linear polarization (LP), which was attributed to the early deformation of the thin targets causing excessive electron heating. Recently, preliminary indications of a transverse instability resulting in spatial modulations of the accelerated proton beam have been reported (Palmer et al., 2012). To summarize the experimental evidence, to date there is a confirmation of the expected LS scaling, but also indications of significant detrimental effects. The observed ion spectrum is relatively broad, suggesting that transverse inhomogeneity and heating effects need to be reduced. In perspective, the relatively slow growth of the ion energy with time (see Fig. 25) might pose the challenge to increase the acceleration length against the effect of, e.g., pulse diffraction and instabilities. B. Collisionless shock acceleration with modulated surface density was also proposed (M. Chen et al., 2009). In contrast to these studies Yan et al. (2009b) used a Gaussian intensity profile and found the formation of a narrow, high-energy ion bunch via a self-organization mechanism somewhat similar to that inferred by Bulanov et al. (2010b). Another open issue is the stability of the foil against transverse perturbations.23 Recent simulation studies characterized regimes of efficient LS-RPA for linearly polarized pulses at irradiances $1021 W cm2 m2 (Dover and Najmudin, 2012; Qiao et al., 2012). Possible indications of the onset of the LS regime have recently been provided in an experiment performed using 800 fs, 3 Â 1020 W cmÀ2 high-contrast (109 ) pulses from the VULCAN laser and very thin ($ 0:1 m) metallic targets (Kar et al., 2012). Narrow-band spectra with peak energies up to ’ 10 MeV=nucleon were observed for both proton and heavier Z=A ¼ 1=2 ions present as surface impurities [see Fig. 27(a...
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