RevModPhys.85.751

Et al 2008 reconstructed the eos of the heated

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Unformatted text preview: p for x-ray probing of a solid Al target heated by protons: (a) top view of the experiment, (b) schematic of the x-ray spectrometer; (c), (d) snapshots of temperature and density profiles of the heated 1:6 m thick Al foil as given by selfconsistent simulations, demonstrating isochoric heating up to ˇ´ 15 eV temperatures. From Mancic et al., 2010a. target, via streaked thermal emission and chirped pulse interferometry. Pump-probe arrangements have been used in recent, more sophisticated experiments, which have provided novel information on the transition phase between cold solid and plasmas in isochorically heated Al and C targets. ˇ´ Mancic et al. (2010a) investigated the short range disordering of warm Al at solid density by applying time-resolved x-ray absorption near-edge spectroscopy (see Fig. 34). Progressive smoothing of spectral features near the K edge allowed one to place an upper bound on the onset of ion lattice disorder within the heated solid-density medium of 10 ps. Pelka et al. (2010) recently diagnosed ultrafast melting of carbon samples, by x-ray scattering techniques, which allowed determination of the fraction of melted carbon in the heated sample. Comparison to predictions based on different theoretical descriptions of the EOS of carbon indicates a departure from existing models, with implications for planetary core studies. In all the experiments mentioned above the isochoric heating by the protons is volumetric, but not uniform (Brambrink et al., 2007); see, e.g., Fig. 34(c). Uniform heating requires some degree of proton energy selection, and choosing the sample thickness so that the Bragg peak of the selected protons does not fall within the sample, as suggested, for example, by Schollmeier et al. (2008). C. Fast ignition of fusion targets The traditional route to ICF (Atzeni and Meyer-ter-Vehn, 2004) relies on the driven implosion of a pellet of thermonuclear fuel (a DT mixture). Ignition occurs in a central ‘‘hot spot’’ following pulse compression. This approach requires an extremely high symmetry and is prone to Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 783 hydrodynamics instabilities, making ICF a historically difficult goal. In the fast ignition (FI) concept [see Key (2007) for a compact review] ignition is driven by an external trigger, creating the hot spot in a time much shorter than the typical fuel disassembly time. Hence, ignition is separated from pulse compression. The FI approach might relax symmetry and stability requirements, reduce the energy need for ignition, and allow fuel burn in a isochoric regime with high fusion gain. In the original FI proposal by Tabak et al. (1994), the ignitor beam consisted of multi-MeV electrons accelerated by a petawatt laser pulse via the mechanisms described in Sec. II.B. Subsequent research showed that, besides generating an electron beam with enough power to ignite, most problematic were the issues of energy transport and deposition in the core. Concerning...
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This document was uploaded on 09/28/2013.

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