The effects of laser pulse length and collisional ionization on the acceleration of titanium ions.

The interaction of a relativistic laser pulse (10¹⁸ W cm⁻²µm²) with foil targets can accelerate ions to energies of tens of MeV u⁻¹ with optimized laser and target parameters. We report the results on simulations of the interaction of a 6.0 × 10²⁰ W cm⁻² laser pulse incident on ultrathin (10–500 nm) titanium foils to investigate the roles of laser pulse duration and ionization mechanisms in the acceleration of titanium ions. While holding peak intensity constant, two laser pulse durations were investigated, 140 and 650 fs. The optimum thickness is dependent on pulse duration, as it requires the concurrence of target transparency with the incidence of the peak laser intensity. The collisional processes do not play a significant role in Ti ion beam generation from the 140 fs laser pulse duration at the optimum thickness (30 nm). However, for the 650 fs laser optimum, collisions improve the conversion efficiency of highly energetic (10 MeV u⁻¹), high charge titanium (Ti^{21 − 22 +}) by a factor of 20, and the titanium ion cutoff energy by ∼15%. This improvement is due to the fact that collisional ionization increases the electron density of the plasma, which delays the time of relativistic transparency, causing collisions to decrease the optimum foil thickness from 150 to 100 nm. Additionally, collisional ionization increases the charge-to-mass ratio of the titanium, and injects more electrons into the accelerating sheath field. At the optimum thickness, target normal sheath acceleration is the dominant mechanism of acceleration, with additional contributions from radiation pressure and shock wave acceleration. [ABSTRACT FROM AUTHOR]