Of the accelerating eld on the scale length in eq 13

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Unformatted text preview: s targets, resistively heated to remove hydrogen contaminants and coated on the rear side with thin C and CaF2 layers, respectively. The observation of high-energy C, Ca, and F ions from these prepared source layers proves the existence of an effective rear surface acceleration mechanism (see Fig. 12). Further evidence was provided by Allen et al. (2004) who showed that removing contamination from the back surface of Au foils strongly reduced the total yield of accelerated protons, while removing contamination from the front surface of the target had no observable effect on the proton beam. Further proof that ions are accelerated at the rear side was given by the observation that a structuring (i.e., grooving) of the rear surface produced modulations in the proton beam (Cowan et al., 2004). This effect also evidences the high laminarity of the beam and allows one to measure its emittance, as discussed in Sec. III.B. Direct experimental evidence of the generation of an initial intense sheath field at the rear surface and a late time field peaking at the beam front was provided by Romagnani et al. (2005) using the proton imaging technique (see Sec. V.A). In this case, TNSA itself provided a unique diagnostic which allowed direct experimental confirmation of the nature of the acceleration process. Figure 13 shows a temporal series of ‘‘proton images’’ in which the propagation of the bell-shaped front of ion expansion can be observed. More recent developments have shown the possibility to significantly control and optimize the TNSA process by acting on the detailed properties of both the laser pulse and the irradiated target. These developments have also highlighted the capability of achieving interesting and promising variations of the main scheme, also in light of possible specific applications. These topics will be presented in Sec. III.E. B. Characterization of beam properties Several experiments have investigated in detail the properties of the TNSA ion beams. The energy spectrum of the beams is typically broadband, up to a cutoff energy (see Fig. 2). The particle number per MeV can be roughly approximated by a quasithermal distribution with a sharp cutoff at a maximum energy (Kaluza et al., 2004; Fuchs et al., 2005) which scales with the laser parameters as discussed in Sec. III.D. Many experiments have reported spectral observations for a wide range of laser and target parameters, and partial surveys have been provided in a FIG. 13. Proton probing of the expanding sheath at the rear surface of a laser-irradiated target. (a) Setup for the experiment. A proton beam is used as a transverse probe of the sheath. (b)–(g) Temporal series of images produced by the deflection of probe protons in the fields, in a time-of-flight arrangement. The probing times are relative to the peak of the interaction. (h) A deflectometry image where a mesh is placed between the probe and the sheath plasma for a quantitative measure of proton deflections....
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