Laser wakefield acceleration of high-quality electron beams to 300 MeV and efficient initiation of photonuclear reactions

By focusing 40-TW, 30-fs laser pulses to the intensity of 10¹⁹ W/cm² onto a supersonic He gas jet, we generated quasi-monoenergetic electron beams for plasma density in the narrow range of 1.5≤ n_e ≤3×10¹⁹ cm³. We show that the energy, charge, divergence and pointing stability of the beam vary sensitively with n _e within this range. The observed variations can be explained physically by the interplay among pump depletion and dephasing between accelerated electrons and plasma wave. An optimized quasi-monoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of n_e∼1.5×10¹⁹ cm³. The quasi-monoenergeic electron beams with energy of 100-150 MeV and charge of 0.5 nC have been used to perform γ-nuclear photofission of natural uranium targets. Through bremsstrahlung emission, such electron beams are well suited for the efficient generation of high energy (tens of MeV) γ-rays necessary to induce photofission reactions. Quantitative γ-ray spectroscopy of the irradiated U sample shows that more than ∼10⁵ photonuclear reactions have been produced per Joule of laser energy, which is 10 to 50 times higher than in a previously reported experiments.