  We measured, using Petawatt-level pulses, the average ion energy and neutron
yield in high-intensity laser interactions with molecular clusters as a
function of laser intensity. The interaction volume over which fusion occurred
(1-10 mm^3) was larger than previous investigations, owing to the high laser
power. Possible effects of prepulses were examined by implementing a pair of
plasma mirrors. Our results show an optimum laser intensity for the production
of energetic deuterium ions both with and without the use of the plasma
mirrors. We measured deuterium plasmas with 14 keV average ion energies, which
produced 7.2x10^6 and 1.6x10^7 neutrons in a single shot with and without
plasma mirrors, respectively. The measured neutron yields qualitatively matched
the expected yields calculated using a cylindrical plasma model.
