The laser-driven ion acceleration via the interaction of short, intense laser pulses with matter, known as laser-plasma acceleration, is featured by its high accelerating electric fields and short pulse length compared to the conventional RF accelerators. It has been one of the most active areas of research during the last several years, since this method has a potential to construct a compact “laser accelerator”, which can be employed in medical and industrial applications.

We present substantial enhancement of accelerated ion energies up to 10-20 MeV/u by utilizing the unique properties of the cluster-gas target irradiated with 40-fs laser pulse of 150 mJ energy [1], corresponding to approximately tenfold increase in ion energies compared to previous experiments using thin foil targets.

In order to elucidate the ion acceleration process, we formulate a simple model of the magnetic vortex acceleration [2], which explains a high-energy ion generation from near-critical density plasmas which can be created from the cluster-gas target. The energy scaling of ions generated by this magnetic vortex acceleration mechanism suggests that the magnetic vortex acceleration can enhance the ion energy by using near-critical density plasmas, and a 100 TW-class laser is capable to generate 200 MeV protons.

The latest results including ion acceleration experiments using cluster-gas targets with the J-KAREN laser, the first petawatt-class OPCPA/Ti:sapphire hybrid laser system [3], will be presented in the presentation.

[1] Y. Fukuda et al., Phys. Rev. Lett. 103, 165002 (2009).

[2] T. Nakamura et al., Phys. Rev. Lett. 105, 135002 (2010).

[3] H. Kiriyama et al., Opt. Lett. 35, 1497 (2010).