High-Repetition-Rate Two-Stage Ion Acceleration*

We propose a parameter-rich experiment uniquely possible at a high-repetition rate (> 1 Hz) laser-plasma science facility. A single laser is unequally split and tightly focused on opposing sides of two closely spaced thin-film targets. The first target emits an ion beam whose energy spectrum, flux, and spatial divergence is modified as it passes through the second laser-target interaction. We use 2D PIC simulations to demonstrate that resulting quasi-monoenergetic ion features have central energies robust to total laser energy and laser pointing fluctuations, and are finely controllable by target separation and relative laser timing. We propose three elements for practical experimental success: (1) high-repetition-rate real-time monitoring of particle beam mode to fine-tune relative laser alignment, (2) transforming shot-to-shot fluctuations, especially focal spot overlap, into an automatic parameter scan monitored in an equivalent plane, and (3) focusing the second laser on the opposing side of the second target, enabling tighter target separation for larger solid angles and stronger experimental signatures. The result will be an experiment with a rich, quickly-scanned parameter space and the potential for machine learning applications.