Acceleration of collimated 45 MeV protons by collisionless shocks driven in low-density, large-scale gradient plasmas by a 1020W/cm2, 1 μm laser

Antici, P. and Boella, E. and Chen, Sophia and Andrews, D.S. and Barberio, M. and Böker, J. and Cardelli, F. and Feugeas, J.L. and Glesser, M. and Nicolaï, P. and Romagnani, L. and Scisciò, M. and Starodubtsev, M. and Willi, O. and Kieffer, J.C. and Tikhonchuk, V. and Pépin, H. and Silva, L.O. and Humières, E.D. and Fuchs, J. (2017) Acceleration of collimated 45 MeV protons by collisionless shocks driven in low-density, large-scale gradient plasmas by a 1020W/cm2, 1 μm laser. Scientific Reports, 7: 16463. ISSN 2045-2322

Abstract

A new type of proton acceleration stemming from large-scale gradients, low-density targets, irradiated by an intense near-infrared laser is observed. The produced protons are characterized by high-energies (with a broad spectrum), are emitted in a very directional manner, and the process is associated to relaxed laser (no need for high-contrast) and target (no need for ultra-thin or expensive targets) constraints. As such, this process appears quite effective compared to the standard and commonly used Target Normal Sheath Acceleration technique (TNSA), or more exploratory mechanisms like Radiation Pressure Acceleration (RPA). The data are underpinned by 3D numerical simulations which suggest that in these conditions a Low Density Collisionless Shock Acceleration (LDCSA) mechanism is at play, which combines an initial Collisionless Shock Acceleration (CSA) to a boost procured by a TNSA-like sheath field in the downward density ramp of the target, leading to an overall broad spectrum. Experiments performed at a laser intensity of 1020 W/cm2 show that LDCSA can accelerate, from ~1% critical density, mm-scale targets, up to 5 × 109 protons/MeV/sr/J with energies up to 45(±5) MeV in a collimated (~6° half-angle) manner.