Active optimization of sheath field accelerated proton beams from intense, high-repetition-rate laser-solid interactions*

Active feedback control of high-repetition-rate (~Hz), high-intensity laser systems for optimization of high energy density (HED) science applications is a rapidly-evolving area of research, drawing on advancements in machine learning and high performance computing to accelerate the pace of discovery. A recent experiment at the ELI-Beamlines facility employed the L3-HAPLS laser in conjunction with a multivariate Bayesian optimizer. This optimizer, trained on data from a Proton Beam Imaging Energy Spectrometer (PROBIES), generated control directives for the laser's spectral dispersion, which were adjusted via an Acousto-Optic Programmable Dispersive Filter (AOPDF), demonstrating the potential of these advanced capabilities.

 

During this experiment over 3500 shots were fired onto 10 µm thick copper foils, irradiated at >10²¹ W/cm² with 30 fs, 8 J pulses of 800 nm light at a shot rate of 0.2 Hz, generating relativistic electrons, a broad spectrum of (predominantly bremsstrahlung) x-rays and an intense electric sheath field which accelerated protons to peak energies of ~20 MeV. In addition to the spectrally-resolved proton beam spatial profile provided by PROBIES, proton time-of-flight and Thomson parabola spectrometer diagnostics were run for cross-calibration, a Rep-rated Electron Positron Proton Spectrometer (REPPS) was used for correlation against the fast electron output and the x-ray radiographic properties of the source were tested utilizing a micro-scale test object fabricated through focused ion beam milling. We present some early results of the analysis of this experiment and discuss options for future development of the technique.