Characterization of thick cryogenic fuel layers using convergent-beam interferometry: A numerical investigation

The inner-surface roughness of thick cryogenic-fuel layers in inertial conf inement fusion (ICF) targets plays a critical role in determining the overa ll success of an ICF capsule implosion. Imperfections at this surface affec t the growth of Raleigh-Taylor hydrodynamic instabilities during both the a cceleration and deceleration phases of the implosion. Characterization of t his surface is performed using a Mach-Zehnder interferometer that illuminat es the target with a wavefront that is convergent to a point near the targe ts' rear focal point, thereby reducing the strong negative-lens effects of the thick cryogenic fuel layer. The construction of this interferometer is described in the text. Phase-shifting interferometry is utilized to acquire the perturbed wavefronts that have passed through the target. These wavefr onts are subsequently sampled around the target perimeter and decomposed in to a one-dimensional Fourier spectrum, which is Abel transformed into a two -dimensional (2D) spectrum. The validity of convergent-beam interferometry is demonstrated by analyzing numerically generated perturbed wavefronts. Th e wavefronts are analyzed, and the (2D) spectrum obtained is compared to th e actual spectrum imposed on the interior of the ice surface of the target model. Agreement between these spectra is > 80% for Legendre modes between 2 and 50. (C) 2000 American Institute of Physics. [S0021- 8979(00)05017-9].