Multispecies Ion Acceleration from Laser-Driven Ultrathin Foils

There is growing interest in laser-based, all-optical technologies for particle acceleration, due to their intrinsically compact nature, and to the unique properties of the beams produced through these techniques. The prospect for future clinical application has been a strong motivation for research in acceleration of protons and heavier ions. In particular, the ultrashort nature of the ion pulses makes their application in in-vitro and in-vivo radiobiology particularly interesting, in light of the current interest in FLASH irradiations at very high dose rates [1].

Significant attention is currently being paid to mechanisms acting on the bulk target ions, and therefore suited to accelerate efficiently heavier ion species, such as carbon. Particularly promising are processes of Radiation Pressure Acceleration in the Light Sail mode acting on ultrathin targets. A series of experimental campaigns carried out by our group and collaborators on the 40 fs, 350 TW GEMINI laser system at the Central Laser Facility (RAL, UK) has investigated the acceleration of protons and carbon ions from ultrathin (nm-scale) carbon foils. This work has highlighted strong dependences of the ion energies on target thickness and laser polarization, with particularly noticeable effects on carbon ions [2, 3]. Supported by an extensive programme of Particle in Cell simulations, the data present a scenario in which Light Sail is the dominant mechanism behind the carbon acceleration, reaching the highest efficiency for circular polarization and a thickness such that the target remains opaque to radiation past the laser pulse peak. At the optimal thickness for carbon acceleration, carbon energies reach ~ 30 MeV/n, while we observe significantly lower proton energies, leading to the delivery of a “pure” high energy carbon beam [4].

Perspectives for scaling up these results to the multi-PW regimes (e.g. accessible on the ELI and Apollon facilities) will be discussed, as well as plans for future experiments.

We will also show how these ion beams can already be used in radiobiology experiments investigating novel regimes, where the ion dose is delivered at extreme dose rates above 10$^{9}$ Gy/s [5], exceeding by many order of magnitudes what typically possible with conventional accelerators.

[1] P. Chaudhary et al, Frontiers Phys., 9, 624963 (2021)
[2] C. Scullion et al, Phys. Rev. Lett., 119, 054801 (2017)
[3] A. McIlvenny et al, Plasma Phys. Control. Fusion, 62, 054001 (2020)
[4] A. McIlvenny et al, Phys. Rev. Lett., 127, 194801 (2021)
[5] P. Chaudhary et al, Phys Med. Biol., 68, 025015 (2023)