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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mstyle scriptlevel="0" displaystyle="false"><a:mfrac><a:mn>1</a:mn><a:mn>5</a:mn></a:mfrac></a:mstyle></a:math> 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 × <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msup><d:mrow><d:mn>10</d:mn></d:mrow><d:mn>19</d:mn></d:msup><d:mo> </d:mo><d:mrow><d:mi mathvariant="normal">W</d:mi><d:mspace width="0.16em"/><d:mi mathvariant="normal">c</d:mi><d:msup><d:mrow><d:mi mathvariant="normal">m</d:mi></d:mrow><d:mrow><d:mo>−</d:mo><d:mn>2</d:mn></d:mrow></d:msup></d:mrow></d:math> on target with moderate nanosecond contrast (<i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:msup><i:mrow><i:mn>10</i:mn></i:mrow><i:mn>9</i:mn></i:msup></i:math>), to generate energetic ion streams from a 250 µm 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.