Comparison of beam-smoothing methods for direct-drive inertial confinement fusion

Analysis and numerical simulations of the smoothing of the speckled illumination of a direct-drive inertial confinement fusion target are presented. In particular, the spatial spectrum of the integrated target fluence is compared across smoothing methods. Two categories of smoothing methods are considered. In one method spatially incoherent light is amplified and directed onto the target, whereas in the other the light is phase modulated and spectrally dispersed before being amplified and then focused through a random phase plate onto the target. The dependence of the smoothed spatial spectrum on the characteristics of phase modulation and dispersion is examined for both sinusoidal and more general phase modulation. It is shown that smoothing with nonsinusoidal phase modulation can result in spatial spectra that are substantially identical to that obtained with the incoherent light method in which random phase plates are present in both methods and identical beam divergence is assumed. (C) 1997 Optical Society of America.