Acceleration of protons to high energies by an ultra-intense femtosecond laser pulse

Laser-driven ion accelerators have a potential to be applied in various branches of science, technology and medicine and are considered to be a compact, flexible and cost-effective alternative to the conventional ion sources based on RF-driven accelerators. However, to produce ion beams of parameters required for applications petawatt (PW) or multi-PW short-pulse lasers have to be used as the accelerator drivers and mechanisms and properties of the ion acceleration process using such lasers should be deeply and fully understood.
Extreme Light Infrastructure (ELI) is a currently implemented large-scale European project that uses cutting-edge laser technologies to build multi-PW lasers generating femtosecond pulses of ultra-relativistic intensities ~ 10^22 – 10^23 W/cm2. Parameters of these laser pulses seem to be sufficient to generate ion beams demanded for various applications, however studies of ion acceleration in the ultra-relativistic intensity regime are in a very initial stage and needs to be intensely developed.
This contribution presents results of two-dimensional particle-in-cell simulations of ion beam acceleration at the interactions of a 130-fs laser pulse of intensity from the range 1021 – 1023 W/cm2, predicted for the ELI lasers, with a thin hydrocarbon (CH) target. A special attention is paid to the effect of the laser pulse intensity and polarization (linear - LP, circular - CP) as well as the target thickness on the proton energy spectrum, the proton beam spatial distribution and the proton pulse shape and intensity. It is shown that for the highest, ultra-relativistic intensities (~ 1023 W/cm2) the effect of laser polarization on the proton beam parameters is relatively weak and for both polarizations quasi-monoenergetic proton beams of the mean proton energy ~ 2 GeV and dE/E ~= 0.3 for LP and dE/E ~= 0.2 for CP are generated from the 0.1-um CH target. At short distances from the irradiated target (< 50 um), the proton pulse is very short (< 20 fs), and the proton beam intensities and the proton current densities reach extremely high values, > 10^21 W/cm2 and > 10^12 A/cm2, respectively, which are much higher than those attainable in conventional accelerators. Such proton beams can open the door for new areas of research in high energy-density physics and nuclear physics as well as can also prove useful for applications in materials research e.g. as a tool for high-resolution proton radiography.