EVALUATION OF B4C AS AN ABLATOR MATERIAL FOR NIF CAPSULES

Boron carbide (B4C) is examined as a potential fuel container and abla tor for implosion capsules on the National Ignition Facility (NIF). A capsule of pure B4C encasing a layer of solid DT implodes stably and i gnites with anticipated NIF x-ray drives, producing 18 MJ of energy. T hin films of B4C were found to be resistant to oxidation and modestly transmitting in the infrared (IR), possibly enabling IR fuel character ization and enhancement for thin permeation barriers but not for full- thickness capsules. Polystyrene mandrels 0.5 mm in diameter were succe ssfully coated with 0.15-2.0 mu m of B4C. Thicknesses estimated from o ptical density agreed well with those measured by scanning electron mi croscopy (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 MF fill pressures at room t emperature, but it is also very brittle, and microscopic flaws or grai n structure may limit the noncryogenic fill pressure. Argon (Ar) perme ation rates were measured for a few capsules that had been further coa ted with 5 mu m of plasma polymer. The B4C coatings tended to crack un der tensile load. Some shells filled more slowly than they leaked, sug gesting that the cracks open and close under opposite pressure loading . As observed earlier for Ti coatings, 0.15-mu m layers of B4C had bet ter gas retention properties than 2-mu m layers, possibly because of f ewer cracks. Permeation and fill strength issues for capsules with a f ull ablator thickness of B4C are unresolved.