Explosion characteristics of intense femtosecond-laser-driven water droplets

An efficient acceleration of energetic ions is observed when small heavy-water droplets of ~200.3em0exμm diameter are exposed to ultrafast (~400.3em0exfs) Ti:sapphire laser pulses of up to 10¹⁹0.3em0exW∕cm² intensity. Quantitative measurements of deuteron and neutron spectra were done, allowing one to analyze the outward and inward directed deuteron acceleration from the droplet. Neutron spectroscopy based on the D(d,n) fusion reaction was accomplished in four different spatial directions. The energy shifts of those fusion neutrons produced inside the exploding droplet reflect a remaining deuteron acceleration inside the irradiated droplet along the axis of the incident laser beam. The overall neutron yield of the microdroplets is relatively small as a result of the dominant outward directed acceleration of the deuterons with 12000.3em0exneutrons∕shot. Relying on the ``explosionlike'' acceleration of such spherical droplet targets we have developed a spray target consisting of heavy-water microspheres with diameters of 1500.3em0exnm. Both the high deuteron energies of up to 10.3em0exMeV resulting from the irradiation intensity of ~10¹⁹0.3em0exW∕cm² as well as the collisions between the deuterons and the surrounding spray delivered about one order of magnitude more neutrons than the single-droplet system. The ~6×10³ neutrons per laser pulse from the spray can be attributed to an efficient deuteron release from a significantly smaller laser excited volume as from deuterium-cluster targets.