RevModPhys.85.751

Data romagnani et al 2005 see fig 13 a modulation of

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Unformatted text preview: uced purposefully by microstructuring the target surface. A technique based on micromachining shallow grooves on the rear surface of the target, introduced by Cowan et al. (2004), has successfully been used in several experiments for diagnosing the emission properties of the beam (Nuernberg et al., 2009). From these patterned targets, a periodic modulation of the beam angular envelope arises during TNSA due to the local perturbation of the target normal direction, which causes an initial beam microfocusing at the groove locations. As the sheath expands, the local modulations are added over the global divergence of the Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 763 beam (Ruhl, Cowan, and Fuchs, 2004) and are observable as a modulation of the proton dose on the detector (see Fig. 14). The modulations can be used as a spatial fiducial from which one can infer the dimensions of the area from where ions are accelerated, i.e., the proton or ion source size (Brambrink et al., 2006). Similar information has been obtained by considerations based on the projection by the ion beam of patterned objects, e.g., metal meshes (Borghesi et al., 2004) or knife edges (Schreiber et al., 2004). A crucial property of laser-driven ion beams is their laminarity. In an ideal laminar source, there is a correlation between the location within the source from where a particle is emitted and the angle of emission. The degree of laminarity of charged-particle beams is typically expressed in terms of their transverse emittance, a quantity which is proportional to the area of the bounding ellipsoid of the distribution of particles in phase space (Humphries, 1990). The highest quality ion beams have the lowest values of transverse and longitudinal emittance, indicating a low effective transverse ion temperature and a high degree of angle space and timeenergy correlation, respectively. Transverse emittance has been measured in a number of experiments. Methods based on mesh projection (which is broadly equivalent to the established ‘‘pepper-pot’’ method) indicate that the emittance is less than 0:1 mm mrad (Borghesi et al., 2004; Ceccotti et al., 2008; Nishiuchi et al., 2008). The above discussed groove imaging technique allows a full reconstruction of the transverse phase space, and possibly a more precise estimation of the transverse emittance (Cowan et al., 2004; Brambrink et al., 2006; Nuernberg et al., 2009) which, for protons of up to 10 MeV, has been estimated as 0:004 mm mrad, i.e., 100-fold better than typical rf accelerators and at a substantially higher ion current (kA range). It has also been found that the removal of the comoving electrons after 1 cm of the quasineutral beam expansion did not significantly increase the measured proton transverse FIG. 14. Top: Modulations in the proton distribution, for different energies, on a RCF detector from a target with microgrooves imprinted on the rear side. The target is a 18 m thick Al foil irradiated at 101...
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