On-Shot, High-Intensity Laser-Aberration Measurements via Ponderomotive Electron Scattering*

We demonstrate a technique by which we assess the spatial aberrations in a high-intensity laser focus when fired at full power. Unlike traditional techniques such as reconstructing the focal spot based on a sampled pickoff or imaging at low energy, this technique relies on the direct detection of electrons accelerated from the focal volume as the laser focuses through a low vacuum O(10^–4 Torr) gaseous backfill. We show that the spatial distribution exhibits features that correlate to the laser focus aberrations, which is demonstrated by intentionally imparting a variety of laser wavefront aberrations into a relativistically focused laser (nominally 10 J in 600 fs, focused to approximately 10¹⁹ W/cm²) and detecting the resulting electron spatial distribution with an image-plate detector. We show that the resulting distributions include features that evolve with the amplitude of the applied coma and astigmatism, indicating that this technique may feasibly be applied in the inverse circumstance of determining, in situ, the laser focus aberrations by way of measuring the electron pattern.