42nd EPS Conference on Plasma Physics
P4.220
Generation of protons from thin polymer and aluminium targets irradiated
by a femtosecond laser pulse
M. Rosinski1, J. Badziak1, L. Torrisi2, B. Kaczmarczyk3, J. Makowski1, P. Parys1, L. Ryc1,
A. Torrisi4, A. Zaras-Szydlowska1,
1
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
2
Department of Plasma Physics, Messina University, Messina, Italy
3
4
LOS Company, Warsaw, Poland
Institute of Optoelectronics, Military University of Technology, Warsaw, Poland
The generation of fast protons from thin (1.5 – 6 m) mylar and aluminium targets irradiated
with 45-fs laser pulses of energy of 400 mJ and intensity up to 1019 W/cm2 was investigated.
The laser beam was focused onto the target by an f/3 off-axis parabolic mirror along the target
normal. Characteristics of forward-accelerated protons were measured by the time-of-flight
method using SiC semiconductor detectors placed behind the target. In the measurements, a
special attention was paid to the dependence of proton beam parameters on the laser focus
position (FP) with regard to the target surface. To change the focus position, the target was
shifted along the beam axis x in the forward (FP = – x) or backward (FP = +x) direction
by x  150 m, which resulted in the change of laser intensity on the target within a factor
~ 10. It was observed that in case of using the mylar target, the dependence of both the
maximum (Epmax) and the mean proton energy on x is clearly non-symmetric with regard to
the point where FP = 0 (the focal plane on the target surface) and highest proton energies are
achieved when the focal plane is situated in front of the target. In particular, for the target with
the thickness of 3.5 m Epmax reached 2.2 MeV for FP = – 100 m while for FP = 0 and
FP = + 100 m
the maximum proton energies attained only 1.6 MeV and 1.3 MeV,
respectively. On the other hand, for the aluminium target of 2 m thickness the dependence
of Epmax on FP was very weak (Epmax changed only within an experimental error of ~10%) and
the proton energies were close to the highest energies obtained for the mylar target (2.2 MeV).
In the paper, this unexpected result will be discussed in detail and an attempt to explain the
observed effects will be made.