Angular distribution of ions accelerated by irradiation of nanowire arrays with laser pulses of relativistic intensity*

Laser-irradiated nanowire arrays act as micro-accelerators of high energy ions that can induce nuclear fusion reactions, producing high energy neutrons [1] and alpha particles [2].  Of interest is the energy spectrum and angular distribution of accelerated protons. We present the initial results of an experiment with polyethylene (CH₂) and nickel nanowire array targets irradiated with ultrahigh contrast laser femtosecond pulses from the Petawatt-class laser ALEPH.  Seven Thomson parabola spectrometers were place at different angles respect to the target normal. The results were compared with those obtained by irradiating flat solid targets from the same materials. Within the hemisphere facing the laser, ions from the flat targets are accelerated predominantly in the laser backward direction.  Fast ions from nanowire array targets are also accelerated in the laser forward direction, but also in other directions including at 90 degrees from the laser and nanowire axis, ie. in the target plane. The later result from radial proton acceleration in the irradiated nanowires where optical field ionization and electron acceleration create a charge sheet surrounding each of the nanowires, in agreement with the prediction of 3-D particle-in-cell simulations [3]. The results will be compared to simulations.