The effect of optical prepulse on direct-drive inertial confinement fusion target performance

The effect of optical prepulses on the performance of imploding inertial confinement fusion (ICF) targets has been measured. The neutron yields from nearly identical spherical targets imploded by the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser were measured as a function of prepulse fluences in the drive laser. These experiments indicate that the cumulative prepulse fluence on target must be less than ∼0.2 J/cm2, which corresponds to a prepulse intensity contrast of ⩾107. This effect is the result of damage to the Al barrier layer used on OMEGA ICF targets. To verify this, the effect of low-intensity irradiation on these Al layers was observed on planar targets using the time-resolved reflectivity and the time-integrated transmission measurements. These experiments show that damage to the Al layers begins at ∼0.1 J/cm2, consistent with the implosion performance results.