Assemblies of multiple microdroplets made of liquid deuterium are illuminated with pulses from an ultrafast (femtosecond) laser, causing the microdroplets to turn into expanding ion clouds in which the ionized nuclei (deuterons) have kinetic energy sufficient to overcome the Coulomb barrier and cause fusion to produce free neutrons, tritium nuclei, and more kinetic energy. The droplets of liquid deuterium are first illuminated with pulses from an infrared laser in order to cause Coulomb explosion of the droplets and resulting formation of the microdroplets. Alternatively, assemblies of microdroplets of a material containing higher Z atoms such as neon or argon are illuminated by an ultrafast laser and the resulting plasma clouds collide and generate recombination x-rays.