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Unformatted text preview: annular beam around the high-intensity interaction region, which modifies the properties of hot electrons refluxing through the target (Carroll et al., 2007). The high degree of beam laminarity, and the fact that ion emission is substantially normal to the target surface, led early on to the suggestion that by appropriately shaping the surface it should be possible to ballistically focus down the protons to a tight spot (Ruhl et al., 2001; Wilks et al., 2001), ideally recovering the properties of the virtual source. The idea is consistent with (and complementary to) observations of TNSA ions from wire targets, where the curvature of the target leads to a highly diverging beam ion with the form of an expanding disk (Beg et al., 2004). An indirect experimental demonstration of focusing was obtained via enhanced, localized heating of a secondary target, as discussed in Sec. V.B (Patel et al., 2003; Snavely et al., 2007). Recently, a more direct demonstration of proton beam focusing was obtained by mesh projection methods in experiments where, employing targets with hemicylindrical (Kar et al., 2011) or hemispherical shape (Bartal et al., 2012), beam focusing (down to 20–25 m spots) over the whole spectrum (up to 25 MeV) was demonstrated. The data highlighted the achromatic nature of the focusing at the different energies, consistent with the energy dependent variations in divergence from a planar foil. Several groups implemented conventional accelerator techniques for energy selection or transport of laseraccelerated protons, in view of possible downstream applications of the proton beam (see Sec. V). Besides simple energy selection with bending magnets, the range of options explored includes the use of pairs of quadrupole magnets for refocusing protons at distances in the 5–60 cm range and in $100 m spots (Schollmeier et al., 2008; Nishiuchi et al., 2009) or to collimate (Ter-Avetisyan et al., 2008) protons within a given spectral band up to 14 MeV as found by Schollmeier et al. (2008). A crucial parameter in this approach is the acceptance angle of the quadrupole system, which may limit the number of particles that can be focused. Large acceptance pulsed solenoids ($ 9 T) were also used (Roth et al., 2009; Harres et al., 2010) for collimation and transport of a large number of $1012 particles. The use of synchronous rf fields for phase rotation resulted in the appearance of multiple peaks across a broadband spectrum (Ikegami et al., 2009). Although demonstrated only at relatively low energy and over low-energy bands, this technique is in principle interesting as, rather than ‘‘slicing’’ a portion of the spectrum, which is effectively what is done in several of the methods above, it can concentrate in a narrow spectral band protons originally contained within a larger spectral region. The phase rotation can be accompanied under the right conditions by a collimation effect. Some of these techniques have already been implemented sequ...
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