Plastic deformation of solid hydrogen in fusion targets

Current baseline designs of ignitable inertial confinement fusion targets require smooth layers of solid hydrogen held at a few degrees below the melting temperature on the inner surface of thin-walled spherical capsules. The initially smooth solid/vapor interface of a presumably single crystalline (hexagonal closed packed) hydrogen layer grown from melt develops undesirable roughness on cooling. We attribute such roughness to plastic deformation relieving thermal-contraction-induced elastic stresses. In particular, we identify two major contributors to roughness: surface bands of the basal slip systems and thermal grooves formed on deformation-produced low-angle grain boundaries. These findings have important implications for designing strategies aimed at controlling uniformity of the hydrogen fuel layer in fusion targets.