Evaluation of B₄C as an Ablator Material for NIF Capsules

Abstract

Boron carbide (B4C) is examined as a potential fuel container and ablator for implosion capsules on the National Ignition Facility (NIF). A capsule of pure B4C encasing a layer of solid DT implodes stably and ignites with anticipated NIF x-ray drives, producing 18 MJ of energy. Thin films of B4C were found to be resistant to oxidation and modestly transmitting in the infrared (IR), possibly enabling IR fuel characterization and enhancement for thin permeation barriers but not for full-thickness capsules. Polystyrene mandrels 0.5 mm in diameter were successfully coated with 0.15–2.0 µm of B4C. Thicknesses estimated from optical density agreed well with those measured by scanning electron microscopy (SEM). The B4C microstructure was columnar but finer than for Be made at the same conditions. B4C is a very strong material, with a fiber tensile strength capable of holding NIF fill pressures at room temperature, but it is also very brittle, and microscopic flaws or grain structure may limit the noncryogenic fill pressure. Argon (Ar) permeation rates were measured for a few capsules that had been further coated with 5 µm of plasma polymer. The B4C coatings tended to crack under tensile load. Some shells filled more slowly than they leaked, suggesting that the cracks open and close under opposite pressure loading. As observed earlier for Ti coatings, 0.15-µm layers of B4C had better gas retention properties than 2-µm layers, possibly because of fewer cracks. Permeation and fill strength issues for capsules with a full ablator thickness of B4C are unresolved.