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Unformatted text preview: ly electrostatic so that the laser-plasma interaction may be modeled only phenomenologically and magnetic field generation is not included. D. Comparison between models and experiments TNSA has been thoroughly investigated in a large number of experiments, performed, in the past decade, in many laser Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . facilities all over the world. The maximum observed value of the ion energy E max has probably been the most characterizing parameter of such experiments. Another important feature, mostly in light of potential applications, is the profile of the energy spectrum. All this effort has resulted in an extensive collection of experimental data, against which the predictive capability of the TNSA theoretical models can be tested. Moreover, a new generation of laser facilities will soon be available, and it will then be possible to investigate a wider range of experimental parameters. Therefore the challenge of satisfactorily predicting the result of a TNSA experiment, providing sufficiently reliable scaling laws to extrapolate guidelines for the future experiments, is even more important. Experimentally, great effort has been devoted to properly addressing the correlation among the above mentioned ion properties and the main laser and target parameters. Because of the importance of the laser irradiance in establishing the regime of interaction (see Sec. II), in the literature it has become common to report the maximum ion (mainly proton) energy E max as a function of this parameter (see, e.g., Fig. 4). Collections of experimental data for E max have been reported in several papers.15 On the other hand, it is apparent that the irradiance is not the only laser parameter playing a role in determining E max . In particular, it has been established by many experiments that, at fixed irradiance, more energetic pulses lead to higher E max . Moreover, already from the early experiments the strong influence of the laser prepulse level and duration and of the target properties on E max has been evident. We devote Sec. III.E to a discussion of these topics, while here we point out the general difficulties which can be encountered when attempting to provide predictions of E max for comparison with experimental data. For all the models introduced in Sec. III.C, E max can be evaluated once the required parameters of the physical system are known. This is a very delicate and often controversial issue because the input parameters are different in number, nature, and reliability. Some models use experimental laser and target parameters, which are known or controlled with well-defined precision, such as, e.g., mean irradiance, spot radius, energy and duration of the laser pulse, target thickness and chemical composition, or surface density of impurity protons in the target. Other models use, as input parameters, physical quantities determined by interaction and transpor...
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This document was uploaded on 09/28/2013.

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