Prospects of target nanostructuring for laser proton acceleration

Abstract

In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser–plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.

Monitoring of Evolving Laser Induced Periodic Surface Structures

“…In the current work we investigated nanostructuring of 1 µm thick titanium and Si 3 N 4 foils within a proton acceleration experiment. Details for the laser acceleration experiments are described elsewhere [27]. Here, we concentrate only on the nanostructuring part.…”

“…Here, we present another monitoring method which is based on an interesting discovery in our recent experimental work. In that work, LIPSS on titanium and Si 3 N 4 samples were fabricated in-situ as targets for laser particle acceleration and X-ray generation [27,28]. In such experiments laser pulses with ultra-high intensity ∼(10 18 .…”

Protons and carbon ions acceleration in the target‐normal‐sheath‐acceleration regime using low‐contrast fs laser and metal‐graphene targets

“…The use of high laser intensity having high contrast, of nano‐structured foils with a composition enhancing the laser absorption, of the p‐polarized laser promoting plasma wave resonant absorption, and the choice of the optimal conditions of the laser focusing and target thickness permit to enhance the electron density transmitted to the rear side of the thin foils . Under such conditions, the forward ion acceleration produced in the TNSA regime is promoted by a quasi‐static high electric field E , developed in the rear side of the target having a duration comparable with the laser pulse (tens of fs ), driving the forward ion acceleration shown by the relation: where k B is the Boltzmann constant, T is the plasma temperature, n e is the electron density, and ϵ 0 is the vacuum permittivity.…”

Energy partitioning and electron momentum distributions in intense laser-solid interactions

“…Most notably, the energy source is the laser pulse, and there is of course a limit on how much energy one can transfer from the laser to the target (and, therefore, to the ions). Theoretical and experimental studies show that the energy absorption can be significantly increased by structures on the surface [32,[36][37][38][39][40][41], and the absorption can potentially be close to 100% [42,43]. As a natural continuation of these studies, we consider here how the structures affect the partitioning of the absorbed energy between the low and high energy electrons as well as between their normal and transverse motion.…”

“…Various studies have recently been performed of specially designed targets and laser pulse shapes [23][24][25][26][27][28][29][30][31][32][33][34][35]. However, it is well known that the TNSA scheme has several shortcomings, such as intrinsic angular and energy spread of the accelerated ions.…”