  Foam has been suggested to reduce laser imprint because of its low density.
In this paper, the two-dimensional radiation hydrodynamic code FLASH is applied
to investigate and characterize the strength of laser imprint through analyzing
areal density perturbation. There are two important factors for the mitigation
of laser imprint besides the thermal smoothing of the conduction region
(between the ablation front and the critical density surface) and the mass
ablation of the ablation front. First, radiation ablation dynamically modulates
density distribution not only to increase the frequency of the perturbed
ablation front oscillation but also to decrease the amplitude of oscillation.
Second, a larger length of the shocked compression region reduces the amplitude
of the perturbed shock front oscillation. The smaller the perturbation of both
ablation front and shock front, the smaller the areal density perturbation.
Based on the above physical mechanisms, the optimal way of mitigating laser
imprint with foam is that the dynamically modulated density distribution
further reduces the amplitude of perturbation reaching the solid CH when the
areal density perturbation of foam oscillates to the first minimum value. The
optimal ranges of foam parameters to mitigate laser imprint are proposed with
the aid of dimensional analysis: the foam thickness is about 2~3 times the
perturbation wavelength, and the foam density is about 1/2~3/2 times the mass
density corresponding to the critical density.
