Has also allowed extended multiparametric studies see

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Unformatted text preview: ling laws and to provide 760 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . FIG. 8 (color online). Results from multiparametric 2D simulations for a double layer target (Esirkepov et al., 2002; Esirkepov, Yamagiwa, and Tajima, 2006). The maximum energy of protons accelerated from the rear layer is shown as a function of laser pulse energy E L and for different values of the intensity I , pulse length Lp , and focal spot diameter D. The target density and thickness are ne ¼ 100nc and ‘ ¼ , respectively. From Esirkepov, Yamagiwa, and Tajima, 2006. evidence for a transition from TNSA to RPA dominance at high intensities (Esirkepov, Yamagiwa, and Tajima, 2006). These and other numerical results will be discussed in Secs. III and IV. FIG. 9 (color online). A typical RCF stack obtained in an experiment with the TARANIS laser at Queen’s University, Belfast. Higher numbers correspond to higher energy protons with layer 1 corresponding to $1 MeV and layer 9 to $12 MeV. From Dzelzainis et al., 2010. E. Ion diagnostics The specific properties of laser-driven ion beams (e.g., broad spectrum, high flux, significant divergence) have required either modifications of established diagnostics techniques or the development of new ones. Radiochromic film (RCF) (McLaughlin et al., 1996) is a detector which is favored by many experimenters, since the early work by Snavely et al. (2000), mainly due to simplicity of use. This is a high-dose, high-dynamic range film, widely used in medical context for x-ray dosimetry (Niroomand-Rad et al., 1998). The films consist of one or more active layers containing a microcrystalline monomeric dispersion buried in a clear plastic substrate. Typical examples are the MDV3 and EBT2 Gafchromic varieties. After interaction with ionizing radiation, the active material undergoes polymerization and the film changes its color from nearly transparent to blue. The consequent change in optical density can be calibrated against the dose released in the film, and therefore provide information on the flux of particles directed at the layer. Typically, RCFs are used in a stack arrangement, so that each layer acts as a filter for the following ones in the stack. The signal in a given layer will be due only to ions having energy E ! EB , where EB is the energy of the ions which reach their Bragg peak within the layer (see Fig. 9). In the first approximation, for an exponential-like spectrum such as those typically produced by TNSA, the dose deposited in a layer can be taken as proportional to the number of protons with E $ EB , allowing a rough spectral characterization of the beam. Various, more refined procedures have been developed for deconvolving the spectral information (either integrated across the beam or angularly resolved) in multilayer RCF data (Breschi et al., 2004; Hey et al., 2008; Nuernberg et al., 2009; Kirby et al., 2011). Rev. Mod. Phys., Vol. 85, No. 2, April...
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This document was uploaded on 09/28/2013.

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