Dependence and the steeper one to a scaling

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Unformatted text preview: ne to a scaling proportional to I. From Borghesi et al., 2008. (2007) at energies up to 400 J, pulse durations between 1 and 8 ps, and intensities up to 6 Â 1020 W cmÀ2 suggested a slower scaling. It is also of crucial importance to establish the most relevant scaling parameters as well as to improve or optimize beam emittance, brilliance, and monoenergeticity for specific applications. For instance, TNSA-generated proton beams are highly laminar and have very low emittance (see Sec. III.B) but the energy spectrum is ordinarily broad and thus not optimal for most applications. These issues motivate the search for other ion acceleration mechanisms. These include concepts which have been explored previously in different contexts (e.g., astrophysics), such as radiation pressure acceleration (RPA) and collisionless shock acceleration (CSA). Other proposed schemes exploit the potential of both advanced target engineering and nonlinear ‘‘relativistic’’ optical effects in plasmas, such as ion acceleration in ultrathin solid targets which become transparent to intense laser pulses [break-out afterburner (BOA)], or involving low-density targets. The basic physics of these mechanisms and the related experimental work, still in a preliminary stage with respect to TNSA, will be described in Sec. IV. The development of advanced acceleration schemes is sustained by the continuous trend toward laser pulses of higher intensity and energy. A detailed account of the many active projects and facilities under development based on optical and infrared lasers ( $ 1 m) is given in Sec. II of Di Piazza et al. (2012). Progress toward CO2 lasers ( $ 10 m) having multiterawatt power (Haberberger, Tochitsky, and Joshi, 2010) is also of growing interest for ion acceleration, as discussed in Secs. IV.A.1 and IV.B. On the theoretical side, the interpretation of experiments has revitalized classic and often controversial problems of plasma physics such as plasma expansion into vacuum and Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . 754 the formation of collisionless sheaths, at the basis of most of the TNSA models, as well as other general physics models such as the motion of relativistic moving mirrors, a concept already discussed in the original work on special relativity by Einstein (1905), which serves as a model for RPA. Simple, analytically affordable models are extremely useful to understand the basic acceleration mechanisms and, in particular, to provide scaling laws which may give directions for further developments. Reference models also highlight the several connections with other fields, such as the physics of discharges, of ultracold plasmas, and of particle acceleration in astrophysics. The theoretical discussions in Secs. III and IV are, to a significant extent, based on this approach. Beyond simple modeling a rich and complex dynamics of laser-plasma interaction and ion acceleration, involving co...
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This document was uploaded on 09/28/2013.

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