Uniquely detailed information on nonlinear phenomena

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Unformatted text preview: anized field structures in counterstreaming plasmas (Kugland et al., 2012), the charge-displacement channel formation dynamics following relativistic self-focusing of laser pulses (Kar et al., 2007; Sarri et al., 2010c; Willingale et al., 2011b), and the evolution of remnants of coherent electromagnetic structures and instabilities of various types (Borghesi et al., 2002a, 2005; Romagnani et al., 2010; Sarri et al., 2010d, 2011b). Application to ns laser-produced plasmas of ICF interest has also allowed one to investigate laser filamentation in underdense plasmas (Lancia et al., 2011; Sarri et al., 2011a), plasma expansion inside hohlraums (Sarri et al., 2010a), and self-generation of magnetic fields (Nilson et al., 2006; Cecchetti et al., 2009; Willingale et al., 2010; Sarri et al., 2011a). As an example of the use of a time-resolved proton diagnostic, Fig. 13 reports data from an experiment where the protons are used to probe the rear of a foil following ultraintense irradiation of the front of the foil (Romagnani et al., 2005). The probe proton pattern is modified by the fields Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 781 appearing at the target rear as a consequence of the interaction, and the technique effectively allows spatially and temporally resolved mapping of the electrostatic fields associated with TNSA acceleration from the foil (see Sec. III.A). Figure 13(a) shows the setup for both imaging and deflectometry measurements. Figures 13(b)–13(g) correspond to proton images at different times taken in a single shot, resolving the expansion of the plasma sheath and highlighting the multiframe capability of this diagnostic. It should be noted then on the basis of Eq. (37) it may not be possible in principle to unambiguously attribute the measured deflections to the sole action of either electric or magnetic fields. Confidence in the interpretation of observed patterns can be increased by supporting the analysis method of both imaging and deflectometry data with particle tracing codes. Such codes simulate the propagation of the protons through a given space- and time-dependent field configuration, which can be modified iteratively until the computational proton profile reproduces the experimental ones. State-of-the-art tracers allow realistic simulations including experimental proton spectrum and emission geometry, as well as detector response. Moreover, in some specific experiments it was possible to provide evidence of magnetic fields, discriminating their effect on probe protons from that due to electric fields, by using different probing directions (Cecchetti et al., 2009) or even exploiting the divergence of the probe beam (Romagnani et al., 2010). An example is given in Fig. 31 where the presence of an azimuthal B field FIG. 31. Proton probing of magnetic fields. (a), (c) Probing deflectogram of a laser-irradiated foil (ns pulse, 1015 W cmÀ2 on a 6 m Al foil) obtained with a 5.5 MeV proton beam facing...
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This document was uploaded on 09/28/2013.

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