Currents from bulanov and esirkepov 2007 780 andrea

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Unformatted text preview: aser pulse, leading to an increase of the intensity in the plasma, rather than contributing to ion acceleration via cluster explosions. The generation of collimated ions from underdense plasmas at ultrahigh intensities (> 1021 W cmÀ2 ) was investigated theoretically and with numerical simulations more than a decade ago (Esirkepov et al., 1999; Bulanov et al., 2000; Sentoku et al., 2000). In particular, in these papers it was predicted that, for a0 > ðmi =me Þ1=2 ’ 43A1=2 , the effective inertia of the highly relativistic electrons in the laser field becomes comparable to those of ions. As a consequence the ions closely follow the electron displacement due to the ponderomotive action and the acceleration process may become similar to what is observed in an overdense plasma. A few more recent simulation studies investigated a regime where a small ion target is placed in an underdense plasma (Shen et al., 2009; L.-L. Yu et al., 2010). The superintense laser pulse accelerates and overruns the ion target and then generates a wakefield in the underdense plasma, where ions may be trapped and accelerated in a way similar to the wellknown scheme for laser acceleration of electrons (Esarey, Schroeder, and Leemans, 2009). In those simulations GeV energies were reached, but the required laser pulses should have multipetawatt power and multi-kJ energy, still far beyond present-day laser technology. E. Resistively enhanced acceleration Already during the ‘‘front versus rear side acceleration’’ debate related to experiments reported in 2000 (see Sec. I), it was suggested that protons may also be accelerated in the target bulk through a mechanism depending on the target resistivity  (Davies, 2002). The electric field generated in the target bulk to provide the return current E ¼ jr = (see Sec. II.B.3) increases for low  reducing the penetration of hot electrons through the target and at the same time favoring acceleration in the front and bulk regions versus TNSA. The mechanism was theoretically investigated by Gibbon (2005a) using a collisional tree-code approach. Indications of dominant front side acceleration due to resistivity effects have been reported in solid plastic targets (Lee et al., 2008, 2011) and also in low-density foams (Li et al., 2005), where an anomalously high resistivity might be due to spatially localized fields in the locally inhomogenous material. V. CURRENT AND FUTURE APPLICATIONS A. Proton radiography The use of ion beams, and particularly proton beams, for radiographic applications was first proposed in the 1960s (Koehler, 1968). Quasimonochromatic beams of ions from conventional accelerators have been used for detecting aereal density variations in samples via modifications of the proton beam density cross section, caused by differential stopping of the ions, or by scattering. Radiography with very high-energy protons ($ 1–10 GeV) is being developed as a tool for weapon testing (King et al., 1999). Ion beams from acce...
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This document was uploaded on 09/28/2013.

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