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.
Highlights
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 experimentally prove that nanostructuring can significantly enhance laser absorption at highest intensities and at different plasma scale lengths, i.e. they remain fully functional even at intensities of ~1020 W/cm[2] and without a double plasma mirror (DPM)
With DPM, the prepulse-free peak-to-amplified spontaneous emission contrast is better than 1014, without DPM the contrast is better than 1010, which corresponds to ASE intensities of ≤(106 ...1010) W/cm[2]
Summary
In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. E.g. micro-particles[10,11], nano-particles[12,13,14], bacteria[15] or foam[14,16,17], and surface patterning using heat embossing[18] or laser structuring[19] have been applied to study the enhancement of ion energy and flux in laser-ion-acceleration In those works a beneficial effect of surface structures could be demonstrated: For example, Margarone et al observed an increase of the maximum proton kinetic energy from 25 MeV to 30 MeV and a significant increase of overall proton numbers by use of a nanosphere target[13], which is still far from the 93 MeV achieved at the very same laser at very similar intensities. The slower rising edge of the laser pulse without DPM leads to a smaller plasma gradient which improves the coupling of laser energy into the plasma as compared to steeper gradients This is the reason why for our laser system the DPM is needed for RPA and ultrathin targets but it is not advantageous for the proton acceleration from thicker targets
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