Advancing the Energy Frontier of Plasma-Based Proton Acceleration with Petawatt Lasers

Laser-driven plasma accelerators can produce pulsed multi-MeV ion beams with high peak currents by irradiating solid targets with ultra-intense laser pulses. This innovative concept has gained significant attention as a compact and energy-efficient alternative to conventional accelerators, with applications in radiotherapy, neutron generation, and fast ignition for inertial confinement fusion. However, maturing plasma accelerators from complex physics experiments into turnkey particle sources requires advances in beam quality, robustness, and high-repetition-rate scalability.

A promising path toward this goal is the relativistically induced transparency (RIT) regime, where ion acceleration is enhanced by precisely synchronizing the laser pulse arrival with the onset of target transparency. In a series of experiments using two state-of-the-art petawatt laser systems, DRACO-PW (Dresden, Germany) and J-KAREN-P (Kyoto, Japan), we systematically investigated laser and target parameters to identify optimal conditions for plasma acceleration [1]. Our results demonstrate the immense potential of the RIT regime by reaching record proton energies of up to 150 MeV with just 22 J of laser energy [2]. The generated proton beam featured a high-energy, low-divergence component that was both spectrally and spatially distinct. Target transparency emerged as a simple yet powerful control parameter, highly sensitive to subtle laser–target variations.

Start-to-end simulations validate these results, elucidating the role of preceding laser light in pre-expanding the target along with the detailed acceleration dynamics during the main pulse interaction. Rather than optimizing a single acceleration stage, we tailored the plasma density so that the laser triggers a cascade of acceleration mechanisms, greatly enhancing energy transfer and acceleration efficiency. The insights into the role of the ultrashort pulse duration and the temporal contrast of the laser represent a significant step forward in understanding and controlling plasma-based ion acceleration in this regime, as confirmed by the successful reproduction of experimental results at two independent facilities.

Enabled by innovative laser diagnostics, advanced operation techniques, and hybrid simulation approaches, these advances mark a significant step toward next-generation laser-driven plasma accelerators, paving the way for their integration into scientific, industrial, and medical applications.

[1]     N.P. Dover and T. Ziegler et al., Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense laser facilities. Light Sci. Appl. 12, 71 (2023).
[2]     T. Ziegler et al., Laser-driven high-energy proton beams from cascaded acceleration regimes. Nat. Phys. (2024).