Wikipedia calls it an "aerogel", but rumors have it originally being closer to ordinary "closed-cell extruded polystyrene foam", which is often sold under the brand name Styrofoam. It might be something else entirely now. It's role is said to be absorbing the x-rays generated by the fission stage and turning into a plasma to help ignite the fusion stage. Edit: https://www.armscontrolwonk.com/archive/201814/fogbank/