Enhanced laser absorption and ion acceleration by boron nitride nanotube targets and high-energy PW laser pulses

Enhancing laser energy absorption with energy transfer to fast electrons is crucial for efficient laser-driven ion acceleration. In this work, we present an experimental demonstration of volumetric laser absorption using boron nitride nanotube (BNNT) targets with an average density of 51 of the solid density. We use a PW laser system operating at a pulse duration of 1.2 ps and an energy of 1.3 kJ, reaching intensities of 2 x 1019 W cm-2 on target with moderate nanosecond contrast (109), to generate energetic ion streams from a 250 mu m thick BNNT target. To characterize laser-accelerated ions, Thomson parabola spectrometers, CR-39 nuclear track detectors, and an electron spectrometer are employed. The results are compared to those achieved using flat targets made of polystyrene (PS) of the same thickness. The comparison reveals a 1.5-fold increase in proton maximum energy and a 2.5-fold increase in the maximum energy of heavy ions (C and N) when comparing the BNNT to PS. Moreover, the high-energy ion flux recorded at CR-39 is orders of magnitude higher for the BNNT after cutting off low-energy ions with Al filters. The enhanced ion acceleration is the result of a 2.3-fold increase in the electron temperature for BNNT, as measured by the electron spectrometer. These experimental findings are further validated through two-dimensional particle-in-cell simulations, which confirm the increase in electron temperature due to enhanced laser absorption ascribable to the low density and nanostructure of the BNNT target compared to the flat foil.