Evaluation of boron-doped beryllium as an ablator for NIF target capsules

We are studying the feasibility of using boron doping to refine the grain s tructure of sputter-deposited Be for NIF ignition capsule ablators. The goa l is to improve the surface finish and homogeneity of these coatings. Films deposited on flat silicon substrates display a pronounced change in struct ure at a concentration of similar to 11 at.% B. At lower levels of B, grain sizes of about 200 mm are observed. AFM images show the roughness of these films to be about 20 nm rms. At higher levels of B, the grains size drops to below 50 nm and the roughness decreases to less than 2.5 nm rms. Films d eposited on capsules do not show the same behavior. In particular, at 15 at .% B, the capsule coatings have nodular structure with an rms roughness of greater than 50 nm. When viewed in cross section, however, no structure is seen with either the flat films or the capsule coatings. We believe that di fferences in substrate temperature may be largely responsible for the obser ved behavior.