• Ion acceleration
• Plasma
• Laser
• Thin target

In this thesis I will present a study of two ion acceleration techniques based on high-intensity
laser-plasma interaction that rely on radiation pressure (RP) effect, i.e momentum transfer from
photons to the target.
The framework of this work is the ultra-intense regime of laser-plasma interaction, in which
an extremely high intensity (I ≥ 10 20 W/cm 2 ) laser pulse interacts with an ultrathin (i.e.few
nanometers thick) solid target. Since the target transition to the plasma state occurs almost in-
stantaneously, the irradiated foil can be regarded as a solid-density plasma, whose microscopical
dynamics is dominated by the effects of the Ponderomotive Force.
In the first part I will focus on the study, through 1D numerical investigations, of the so-called
Light Sail regime, in which the plasma is regarded as a mirror accelerated by the laser pulse
and photons-target momentum transfer occurs during pulse reflection. In the LS regime a quasi-
equilibrium state between the RP and the electrostatic pressure is required for prevent the target
reflectivity to drop, and thus reach a stable and efficient acceleration.
The RP effects are investigated, with a particular attention on polarization effects on the overall
dynamics, focusing in the attempt to understand the transition threshold for which RP accelera-
tion (RPA) becomes the dominant acceleration mechanism when a linearly polarized laser pulse
is involved.
With the RPA scheme it is in principle possible to achieve a compact and collimated ion beam
and to reach ions energies up to the ≥ 100M eV threshold, with the possibility to obtain a peaked,
mono-energetic ion spectrum. Moreover, the aforementioned acceleration mechanism shows an
high mechanical efficiency.
While actual laser facilities can deliver laser pulses with a focused intensity I & 10 21 W/cm 2 ,
the technical requirements for the exploitation of radiation pressure for ion acceleration are very
demanding and the peak intensity of present day laser systems is not high enough to obtain
a purely RPA dominated regime. At such high intensities, indeed, other heating mechanisms
and strong non-linear effects occur, reducing the efficiency of RP. The pratical difficulties to
reproduce this dynamics in a laboratory have made numerical investigation an invaluable tool
for experimental campaigns, as fine structures can be more easily investigated in order to find
the best parameters configuration to be used in actual experiments.
The latest experimental and numerical RPA investigations have shown the development of
Rayleigh-Taylor (RT) instability, i.e. the classical process occurring when a light fluid accel-
erates an heavier one. This RT instability implies a reduction in the acceleration efficiency.
Recently a novel way to achieve ions acceleration through RP have been presented by Zhou et
al., with the aim to suppress the RT instabilities by reducing interaction time.
The main idea of the authors is that the Thin Film Compressor (TFC) novel laser pulse com-
pression technique may be used to obtain a single cycle ultra-intense laser pulse. In this regime,
named Single Cycle Laser Acceleration (SCLA), the interaction time is shorter than instabilities
growth rate, and thus it may lead to a favourable laser-ions energy transfer. Moreover, while
the LS configuration requires high values of target reflectivity, the SCLA regime strongly relies
on the exploitation of target transparency, as electrons are expelled from the target by the laser
ponderomotive force and ions are accelerated by the strong longitudinal electrostatic field in-
duced by charge separation.
The work presented in the second main part of the thesis, will mainly deal with a numerical
investigation of the SCLA regime, taking the work presented by Zhou et al. as a starting point.
Since for such short interactions the pulse shape can be a relevant parameter, polarization ef-
fects and pulse absolute phase variation on different plasma configuration have been studied in
this regime as well as the laser intensity-thickness relation for maximizing ion acceleration effi-
ciency. Once the best configuration have been obtained, the SCLA optimal regime is investigated.