Characterization and automated optimization of laser-driven proton beams from converging liquid sheet jet targets

Compact, stable, and versatile laser-driven ion sources hold great promise for applications ranging from medicine to materials science and fundamental physics. While single-shot sources have demonstrated favorable beam properties, including the peak fluxes necessary for several applications, high repetition rate operation will be necessary to generate and sustain the high average flux needed for many of the most exciting applications of laser-driven ion sources. Further, to navigate through the high-dimensional space of laser and target parameters towards experimental optima, it is essential to develop ion acceleration platforms compatible with machine learning learning techniques and capable of autonomous real-time optimization. Here we present a multi-Hz ion acceleration platform employing a liquid sheet jet target. We characterize the laser-plasma interaction and the laser-driven proton beam across a variety of key parameters governing the interaction using an extensive suite of online diagnostics. We also demonstrate real-time, closed-loop optimization of the ion beam maximum energy by tuning the laser wavefront using a Bayesian optimization scheme. This approach increased the maximum proton energy by 11% compared to a manually-optimized wavefront by enhancing the energy concentration within the laser focal spot, demonstrating the potential for closed-loop optimization schemes to tune future ion accelerators for robust high repetition rate operation.

💡 Research Summary

The paper presents a high‑repetition‑rate (up to 5 Hz, data mostly at 1 Hz) laser‑driven proton source that uses a continuously flowing liquid water sheet jet as the target. A 200 mJ, 75 fs Ti:Sapphire pulse is focused to ≈3 × 10¹⁹ W cm⁻² at a 30° incidence angle onto a 600 ± 100 nm thick water sheet. The authors equipped the experiment with a suite of online diagnostics: a magnetic electron spectrometer (Lanex screen + CCD), a scintillator‑based proton beam profiler (sensitive to ≈1.1 MeV protons), a diamond‑detector time‑of‑flight (TOF) spectrometer placed 357 mm downstream, and an 800 nm, 40 fs probe beam for shadowgraphy up to 700 ps after the main interaction.

Shadowgraphy reveals that the over‑critical plasma generated at the interaction point expands at ≈9.4 × 10⁷ m s⁻¹ (≈0.3 c) during the first few picoseconds, driven by collisional ionization from hot electrons. After ≈30 ps the plasma recombines and contracts at ≈7.3 × 10⁴ m s⁻¹, and a shock‑wave front becomes visible at later times. These dynamics are consistent with models of hot‑electron‑driven expansion and provide a real‑time view of target recovery, indicating that repetition rates above 1 Hz are feasible.

Proton beam measurements show that laser polarization strongly influences performance: p‑polarized pulses produce a three‑fold increase in maximum proton energy and an order‑of‑magnitude rise in peak dose compared with s‑ or circular polarization, reaching up to 30 Gy per shot. Electron spectra measured simultaneously confirm hotter, more numerous electrons for p‑polarization, linking the effect to enhanced sheath fields in the target‑rear TNSA process.

Shot‑to‑shot stability is demonstrated over four bursts of 50 shots each (200 shots total). The relative standard deviation of the integrated proton flux is ≤50 %, and both electron and proton spectra remain reproducible across bursts, confirming the liquid sheet’s ability to deliver a stable, debris‑free target at multi‑Hz rates.

The most novel contribution is the implementation of a closed‑loop, Bayesian optimization of the laser wavefront. The wavefront is parameterized by ten Zernike coefficients; the optimizer queries the TOF‑measured maximum proton energy as the objective function and updates the coefficients at 1 Hz. After ~30 iterations the algorithm converges to a wavefront that yields an 11 % increase in maximum proton energy relative to the best manually tuned configuration. The improvement is attributed to tighter focal spot intensity, which raises hot‑electron temperature and consequently the sheath potential.

Overall, the work demonstrates that (i) liquid water sheet jets provide a high‑density, ultrathin, self‑refreshing target with negligible debris, (ii) comprehensive diagnostics enable real‑time monitoring of plasma and ion dynamics, and (iii) machine‑learning‑driven closed‑loop control can autonomously enhance beam performance. The authors argue that scaling to kHz lasers could deliver the µA‑level average proton currents and multi‑MeV energies required for applications such as radiotherapy, isotope production, and hybrid laser‑RF accelerators, moving laser‑driven ion sources closer to practical deployment.