Ultra-intense pulsed source of ionizing radiation based on direct laser acceleration of electrons for studying the FLASH effect

Results of the pilot experiment on the ultra-intense irradiation of a water phantom and various biological media with laser-driven beams of relativistic electrons in context of the FLASH effect are presented. Directed high-current beams of MeV electrons were generated in the interaction of a sub-picosecond PHELIX laser pulse at 10¹⁹ W/cm² intensity with a low-density polymer foam, which was converted into a plasma of near-critical density by a preceding nanosecond laser pulse. The combination of 20-50 Gy of ionizing radiation delivered by the relativistic electron beam in a single laser shot and the world's highest dose rate of 70 Gy/ps makes this source unique for studying the FLASH effect and for applications. The picosecond duration of the electron beam allows for separation of the process of ultrafast (instantaneous) oxygen ionization and the subsequent chemical reactions. In each laser shot, a sudden drop in the molecular oxygen concentration as a function of the delivered dose was measured in water and biological media. The dependence obtained for water is consistent with the results of the Monte Carlo simulations.