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

The effect of optical prepulses on the performance of imploding inertial co nfinement fusion (ICF) targets has been measured. The neutron yields from n early identical spherical targets imploded by the OMEGA [T. R. Boehly et al ., Opt. Commun. 133, 495 (1997)] laser were measured as a function of prepu lse fluences in the drive laser. These experiments indicate that the cumula tive prepulse fluence on target must be less than similar to0.2 J/cm(2), wh ich corresponds to a prepulse intensity contrast of greater than or equal t o 10(7). This effect is the result of damage to the Al barrier layer used o n OMEGA ICF targets. To verify this, the effect of low-intensity irradiatio n on these Al layers was observed on planar targets using the time-resolved reflectivity and the time-integrated transmission measurements. These expe riments show that damage to the Al layers begins at similar to0.1 J/cm(2), consistent with the implosion performance results. (C) 2001 American Instit ute of Physics. [DOI: 10.1063/1.1324663].