Ion acceleration by femtosecond laser pulses in small multispecies targets

Abstract

Ion acceleration by ultrashort intense femtosecond laser pulses (∼4×1019W∕cm2, ∼30fs) in small targets of uniform chemical composition of two ion species (protons and carbon C4+ ions) is studied theoretically via a particle-in-cell code with two spatial and three velocity components. Energy spectra of accelerated ions, the number and divergence of fast protons, are compared for various target shapes (cylinder, flat foil, curved foil) and density profiles. Dips and peaks are observed in proton energy spectra due to mutual interaction between two ion species. The simulations demonstrate that maximum energy of fast protons depends on the efficiency of laser absorption and the cross section of the hot electron cloud behind the target. A rear-side plasma density ramp can substantially decrease the energy of fast ions and simultaneously enhance their number. These results are compared with analytical estimates and with previously published experiments.

Effect of a nanometer scale plasma on laser-accelerated ion beams

“…This discrepancy may originate in the electrostatic field screening by the laser-accelerated protons, which is not described within the isothermal model. These two-species results reveal that the ion cut-off energy is sensitive to the species considered in simulations and their relative densities [21,22]. The discrepancy between numerical and experimental results certainly originates from the somewhat arbitrary choice of the C 4+ density and the frozen carbon ionization level.…”

Simulations of femtosecond laser pulse interaction with spray target

“…Thus, the spray consisting of pre-expanded droplets is to some extent similar to an inhomogeneous "foil section." The modulations (peaks) in proton energy distribution were also observed for foil section targets in previous numerical simulations (see Brantov et al, 2006;Psikal et al, 2008). The peaks were explained by mutual interaction and spatial separation of two ion species.…”

“…The advantage of clouds of droplets is their large surface to volume ratio leading to a higher absorption and their inhomogeneity in space leading to relatively high number of protons accelerated near to the cut-off energy. The main drawback of cylindrical targets in comparison to foil targets is almost isotropic acceleration of protons (Kemp & Ruhl, 2005;Psikal et al, 2008).…”

Targets for high repetition rate laser facilities: needs, challenges and perspectives

“…While most of these targets were produced using standard coating techniques, others required extensive research and development activities, as in the case of low density coatings: carbon foams produced by pulsed laser deposition (PLD) and characterized by energy dispersive X-ray spectroscopy (EDS) [86,87] and carbon nanotube coatings deposited by CVD [88] . Robust enhancement of proton energy was observed with reduced mass targets (thin foils with limited transverse size) produced by lithographic techniques [89][90][91] .…”