High repetition-rate neutron generation by several-mJ, 35 fs pulses interacting with free-flowing D_2O

摘要

Using several-mJ energy pulses from a high-repetition rate (1/2 kHz), ultrashort (35 fs) pulsed laser interacting with a ～10 μm diameter stream of free-flowing heavy water (D_2O), we demonstrate a 2.45 MeV neutron flux of 10~5/s. Operating at high intensity (of order 10~(19)W/cm~2), laser pulse energy is efficiently absorbed in the pre-plasma, generating energetic deuterons. These collide with deuterium nuclei in both the bulk target and the large volume of low density D_2O vapor surrounding the target to generate neutrons through d(d, n)~3He reactions. The neutron flux, as measured by a calibrated neutron bubble detector, increases as the laser pulse energy is increased from 6 mJ to 12 mJ. A quantitative comparison between the measured flux and the results derived from 2D-parti-cle-in-cell simulations shows comparable neutron fluxes for laser characteristics similar to the experiment. The simulations reveal that there are two groups of deuterons. Forward moving deuterons generate deuterium-deuterium fusion reactions in the D_2O stream and act as a point source of neutrons, while backward moving deuterons propagate through the low-density D_2O vapor filled chamber and yield a volumetric source of neutrons.