Bunching of light ions driven by heavy-ion front in multispecies ion beam accelerated by laser.

Deeply modulated ion spectra from contaminants present on the target surface were measured at the interaction of ultraintense (2-5)×10^{20}W/cm^{2} and high-contrast laser pulses (≲10^{-10}) with thin (∼μm) and ultrathin (∼nm) targets. This phenomenon, observed over a wide range of laser and target parameters, suggests that it is a generic feature of multispecies ion acceleration at high laser pulse contrast. The modulation is ascribed to the acceleration of various ion species at the rear of the target with steplike density profiles which provide well-separated ion species in the accelerated beam. The observed coincidence of the velocity of the modulated region in the ion spectra with the maximum velocity of another ion with a lower mass-to-charge ratio is consistent with this model. The impact of heavy ions on light ions leads to a spectral "bunching" of light ions. Two-dimensional modeling has shown that high laser contrast prevents backside plasma expansion, which provides a well separated ion species with a steplike density profile that allows for the additional acceleration of "light" ions by the slower moving "heavy"-ion front. Spectral modulations can be controlled by tuning the ratio of heavy to light ions in future experiments with ultrathin rear coatings.