Proton acceleration and high energy density generation by laser irradiation of micro-pillar arrays

Over the past decade, challenging applications (e.g. proton radiography or tumor therapy) have motivated intensive research effort in the field of laser-driven proton acceleration. Nano- and micro-structured targets have recently emerged as promising techniques for increasing the conversion efficiency from the laser to the ion beam. We report on the first measurements of proton beam production from metal foils coated with micro-pillar arrays. We have found that, compared to plain metal foils, such structured targets strongly enhance the protons' maximum energies and yields, at relatively moderate laser intensities (i.e. close to the relativistic threshold intensity). In addition, high-resolution x-ray spectroscopy brings evidences of the generation of high energy density conditions in the micro-pillar arrays. State-of-the-art particle-in-cell simulations reveal that this remarkable result stems from the greatly increased absorption of the laser pulse into energetic electrons. The micro-pillars indeed behave as a near-solid hollow material, in the interstices of which the laser is able to penetrate to large depths. The resulting specific geometry of the laser-pillar interaction leads to an efficient, direct electric-field acceleration of the electrons (instead of the ponderomotive acceleration that usually prevails in the laser-foil interaction). The pillar array then rapidly fills up with a dense cloud of energetic electrons, which subsequently act as a large energy reservoir efficiently driving ion acceleration at the substrate’s rear side.