Accurate modeling of the low-energy tail in laser-accelerated ion spectra

Develop numerical simulation methods that accurately reproduce the low-energy tail, including the observed spectral flattening, in proton energy spectra from laser-driven target normal sheath acceleration of narrow gold micro-bar targets irradiated by 27 fs, 120 mJ, p-polarized pulses focused to a 3.5 µm spot (a0 ≈ 4.6), while accounting for realistic plasma scale lengths under practical computational constraints.

Background

In the experiments, narrow micro-bar (µ-bar) targets produced proton spectra with a flattening at low energies that was not reproduced by the accompanying 3D PIC simulations. The authors note that capturing the low-energy tail of laser-accelerated ions is challenging due to computational limitations, such as the need for very high numbers of particles to represent realistic plasma scale lengths at limited spatial resolution.

They further suggest several possible physical contributors to the flattening—fast electron refluxing, non-TNSA acceleration mechanisms, and sensitivity to contaminant density profiles—highlighting the need for improved or augmented modeling approaches to accurately reflect these effects.

References

For narrow µ-bars, the measured proton energy spectra feature flattening in the low-energy region that is not captured by simulations. Simulating the low-energy tail of laser-accelerated ions remains a challenge, as it often requires extremely high numbers of computational particles to accurately represent a realistic plasma scale-length, when the spatial resolution is limited by computational constraints.

— Ion acceleration from micrometric targets immersed in an intense laser field  (2404.11135 - Elkind et al., 2024) in Results, paragraph discussing Fig. 2 (following panel f)