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.
Highlights
Laser-driven proton acceleration[1,2], is a field of intense research due to the numerous fundamental[3,4], and applicative[5,6,7,8] prospects these beams offer
A second acceleration scheme that has been emerging is the so called Break Out Afterburner (BOA) in which an ultra-intense laser can relativistically penetrate through an ultra-thin target during the laser lifetime while the target is being exploded and in which an instability coupling the electrons accelerated by the transmitted laser and the target ions sets up, instability that accelerates the ions[29,30]
In contrary to the “pure” Collisionless Shock Acceleration (CSA) mechanism revealed in the refs[30,31], which requires the production of a shock in an overcritical density medium and is difficult to achieve homogeneously for a 1 μm wavelength laser, Low-Density Collisionless Shock Acceleration mechanism (LDCSA) combines shock front acceleration with the volumetric low density acceleration variant of Target Normal Sheath Acceleration (TNSA)
Summary
Laser-driven proton acceleration[1,2], is a field of intense research due to the numerous fundamental[3,4], and applicative[5,6,7,8] prospects these beams offer. In TNSA, high-energy electrons (MeV), generated at the front surface of the target by the laser, cross the target and create at the target rear surface an intense sheath[17] with a ~TV/m electrostatic field that accelerates protons and ions from hydro-vapor contaminants.
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