Ultrahigh flux of direct laser-accelerated electrons, MeV photons, and neutrons from overdense polymer foams

Ultrabright, well-collimated MeV bremsstrahlung radiation was generated through the interaction of high-current electron beams produced via direct laser acceleration (DLA) with a high- Z converter. The DLA mechanism was initiated by a 200 TW, subpicosecond PHELIX laser pulse at a moderately relativistic intensity of (1–2) × 10 19 W cm − 2 , delivering approximately 60 J of energy into preionized, overcritical-density foam targets. The electron spectrum measured along the laser axis exhibited an effective temperature of approximately 30 MeV and energies exceeding 100 MeV, with a total charge of about 300 nC for electrons with energies > 1.5 MeV (ponderomotive potential), while 100 nC of them is directed along the laser axis within a half-angle cone of 12 ∘ . The directed fraction of DLA electrons with energies exceeding 7.5 MeV carries a charge of 20 to 30 nC, corresponding to a flux up to 2 × 10 24 sr − 1 s − 1 . These high-current relativistic electron beams efficiently generate MeV x-rays, enabling the subsequent production of isotopes, positrons, and neutrons with exceptional yield and application potential. In laser shots employing overcritical-density foam targets placed in front of a high- Z converter, bremsstrahlung photons with energies up to 70 MeV were generated and analyzed via nuclear activation of tantalum and gold. The formation of the isotopes 174 Ta and 190 Au , whose photonuclear cross-section peaks at approximately 65 MeV, confirmed the presence of high-energy photons. In contrast, no activation was observed in control shots where the laser was directed onto the converter without foam, indicating that high-energy photon generation is intrinsically linked to the DLA process in the preionized foam targets. Autoradiographic measurements revealed a divergence of the bremsstrahlung beam of approximately 22 ∘ (half-angle) in the 14–21-MeV range. These diagnostics indicate an unprecedented photon flux of approximately 2 × 10 22 sr − 1 s − 1 , corresponding to about 10 11 photons per shot with energies exceeding 7.5 MeV. The conversion efficiency of focused laser energy to bremsstrahlung photons is greater than 1% (within the FWHM of x-ray beam and laser beam). More than 2 × 10 9 photoneutrons per shot were emitted isotropically, corresponding to a peak flux of 2 × 10 20 cm − 2 s − 1 ( 4 × 10 18 cm − 2