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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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