High efficiency ion acceleration in foil plasma expansion driven by kJ petawatt lasers*

Kilojoule petawatt lasers with relativistic intensities such as LFEX and NIF-ARC have demonstrated efficient ion accelerations from thin foil plasmas. In the laser-plasma interaction, the foil plasma expands significantly to make a large-scale coronal plasma, in which fast electrons are accelerated much beyond the scaling for sub-ps laser-plasma interactions. Owing to the large laser spot, fast electrons are confined in the laser spot area [1]. In this new stage, the plasma expansion structure changes to a non-isothermal, fast expansion mode [2]. We here study the temporal evolution of energies of fast electrons and ions in the expanding plasma under a continuous laser energy input in 1-10 picosecond scale, to model the efficient proton acceleration seen in the kJ laser experiments. Particle-in-cell simulations show that fast electron energy density in the expanding foil plasma increases temporally, and a strong sheath electric field is maintained in the expansion. On a picosecond time scale, about one half of the absorbed laser energy is converted to the kinetic energy of over-MeV fast ions during the laser irradiation, resulting a high laser-to-ion energy coupling.