BE Coatings on Spherical Surface for NIF Target Development

Beryllium is one of the preferred ablators for achieving ignition in inertial confinement fusion (ICF). Thin and thick coatings of Be on CH shells have been deposited using a sputter coater established at UCSDʹs PISCES facility and examined using a variety of characterization techniques. Due to the spherical nature of these substrates, shadowing effects are expected to play significant roles in film growth as well as the expected surface diffusion length of deposited atoms. Be coatings on flat surfaces and spherical surfaces have been deposited and compared to understand the material growth behaviors on different surfaces and as a function of processing parameters. On flat surfaces, Be film developed polycrystalline morphology with columnar growth. On spherical surfaces, Be film also showed columnar growth at lower powers, which then transitioned into a twisted grain structure at higher powers. Cycling of parameters has been used to investigate possible grain growth interruption during growth and improving morphology. Initial results also suggest that copper doping during deposition does not change the columnar growth morphology. Measurements of the surface roughness of beryllium-coated shells indicate roughness growth proportional to the thickness with an exponent of 0.8 to 1.2, which is consistent with shadowing dominated roughening. As ion-beam-assisted growth may improve the surface finish and micro-structure of deposited films, we have also studied the effect of process parameters on the flux and energy of the ions reaching the substrates using an offline energy dispersive mass spectrometer system.