Abstract
Using particle-in-cell simulations, we demonstrate an improvement of the target-normal-sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron generation, caused by a more complex laser–electron interaction geometry and increased effective interaction area and duration. We show that TNSA is optimized for a nanohole distribution of relatively low areal density and that is not required to be periodic, thus relaxing the manufacturing constraints.
Highlights
Laser-driven ion acceleration has become a well-established technique to produce compact, high-energy ion beams, owing to the ultra-strong accelerating fields that can be achieved at the surfaces of solid targets (Daido, Nishiuchi & Pirozhkov 2012; Macchi, Borghesi & Passoni 2013)
We investigate by means of particle-in-cell (PIC) simulations the potential of non-periodically structured targets to enhance target-normal-sheath acceleration (TNSA)
The sub-micron dense regions that make up the nanohole targets effectively behave as mass-limited targets (Psikal et al 2008; Buffechoux et al 2010), leading to efficient electrostatic confinement of the hot electrons
Summary
Laser-driven ion acceleration has become a well-established technique to produce compact, high-energy ion beams, owing to the ultra-strong accelerating fields that can be achieved at the surfaces of solid targets (Daido, Nishiuchi & Pirozhkov 2012; Macchi, Borghesi & Passoni 2013). Ferri and others whereby surface ions are driven outwards by the charge-separation field set up by the laser-accelerated relativistic electrons escaping into vacuum Because of their large charge-to-mass ratio, the protons that are naturally present due to hydrogen-containing contaminants at the target surfaces respond the fastest to the electric sheath field, and reach the highest velocities. Two target types are considered, consisting of a flat foil either coated on the front surface with randomly positioned nanocones (‘nanocone targets’) or perforated by nanoholes (‘nanohole targets’) In both cases, the proton cutoff energy is increased by up to a factor of two compared with flat foils. The nanohole target, displayed, consists of the reference foil pierced by holes of the same width and location as the above nanocones The surfaces of both types of structured targets are coated with a proton layer. We use 100 macro-particles per cell and per species in the bulk plasma, while the surface proton–electron layers are represented by 1000 macro-particles per cell and per species
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