A mesh is placed between the probe and the sheath

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Unformatted text preview: From Romagnani et al., 2005. Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . number of publications (Borghesi et al., 2006; Fuchs et al., 2006b; Zeil et al., 2010). A number of experimental studies have been devoted to the investigation of the spatial and angular characteristics of the emitted beams. These are closely dependent on the electron sheath spatial distribution, and consequently on the target properties (resistivity, surface roughness, etc.) affecting the electron propagation. It was observed early on that the use of conducting targets leads to smooth proton beam profiles with a sharp boundary, as detectable, for example, in RCF data (Snavely et al., 2000; Fuchs et al., 2003), while using dielectric targets creates nonhomogeneities in the proton density across the beam section. In the latter case, the transport of the electron current is prone to electromagnetic instabilities, which break the hot electron flow into filaments. This leads to an uneven electron sheath at the target rear (Manclossi et al., 2006) and consequently to a modulated proton beam cross section (Roth et al., 2002; Fuchs et al., 2003). The close correlation between proton beam properties and electron beam transport characteristics has indeed been exploited in a number of experiments, which have used the proton beam as a diagnostic for the electron beam behavior inside the target, revealing, beside the aforementioned differences related to the target conductivity (Fuchs et al., 2003), effects of magnetic collimation on the beam transport (Yuan et al., 2010; Gizzi et al., 2011) or the role of lattice structure in dielectric targets (McKenna et al., 2011). Other factors that can lead to structured beam profiles even in conducting targets are surface roughness at the target rear, resulting in a randomized local orientation of the protons (Roth et al., 2002), and intensity modulations in the focal spot which can be coupled to the protons via structured electron beams in medium-Z thin targets (Fuchs et al., 2003). The existence of a sharp angular boundary in the proton angular distribution (clearer in higher-Z and thicker targets) is consistent with a bell-shaped transverse distribution of hot electrons in the rear surface sheath due to the fact that the density will naturally be higher along the laser axis and decrease with transverse radius. Protons are accelerated normal to the local isodensity contour, and the presence of an inflection point in the sheath therefore results in a maximum angle of acceleration (Fuchs et al., 2003). Comparison of experimental data with simple electrostatic models indicates that the shape of the accelerating sheath is generally Gaussian (Fuchs et al., 2003; Carroll et al., 2007) as also observed directly in sheath imaging data (Romagnani et al., 2005); see Fig. 13. A modulation of the proton beam angular distribution can be introd...
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