Multi-species ion acceleration from sub-ps, PW interactions with ultra-thin foils

Multi-species ion acceleration from ultra-thin foils was studied experimentally, employing the Vulcan Petawatt laser at the Central Laser Facility, UK. Plastic (CH) foils with thicknesses in the range 10–340 nm were irradiated with intense, short (750 fs) laser pulses producing maximum energies of ∼65 MeV and 25 MeV/nucleon obtained for H+ and C6+ ions, respectively. Ion spectra obtained from high resolution spectrometers suggest differences in the acceleration dynamics for the two species. Comparisons are made with two-dimensional Particle in Cell simulations, which identify, for an optimal thickness, two main mechanisms contributing to the ion acceleration process, namely multi-species target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA). Ion energies are further enhanced by the onset of relativistically induced transparency. A final stage in the acceleration is caused by the formation of electron jets (as the target undergoes transparency), which accelerate the ions off-axis. By analysing the spatial and temporal evolution of the accelerating fields, we are able to infer the effect of the different mechanisms on each species and how this translates to the experimental observations. The two main mechanisms, TNSA and RPA, are seen to each produce a distinct population of high energy protons whereas a single population of carbon is accelerated by a summation of these effects. This species-specific analysis sheds new light on the complex dynamics in a multi-species target expansion and on the contribution of different acceleration processes to the acceleration of the most energetic ions in the spectrum.