At the risk of ending up at a black site. A literature review suggests that when it is used as an interstage material - FOGBANK acts as a lens i.e. it directs and focuses the neutron flux from the primary device and does something with the X-Rays produced to help trigger the secondary.
Whatever it does, it is an extremely critical component worth spending god knows how much money on.
IIRC, according to Richard Rhodes in 'Dark Sun', the key insight (with Teller and Ulam disputing who had it first) needed to make a true fusion bomb (as opposed to a boosted fission bomb) is that the fusion part has to be far enough separated, from the fission bomb that starts it, that it can do its thing before it is blasted apart by the shock wave from the fission bomb. That raises the problem of how to transfer energy from the fission bomb to the fusion part, and the solution is to surround the latter with something that will absorb some of the X-rays and vaporize, thus applying a very uniform compression of the fusion part. Wikipedia says that polystyrene foam was used in early bombs [1], and apparently polyethylene was used in the original Ivy Mike device. I have no idea whether this is actually the purpose of fogbank.
One problem with re-creating or replacing any special material critical to the weapons' functioning is that test-ban treaties forbid an end-to-end test of bombs using the new stuff.
But my understanding is that there is a U238 shell that focuses the X-Rays like a mirror. If you've got reflection you don't necessarily need refraction. Also my understanding is that the X-Rays from the primary cause adiabatic compression (as much as possible) of the primary.
The fogbank turns to plasma but I think the action is long done by the time the plasma gets to do anything.
If there was an empty space where the fogbank was and the geometry of the weapon changed during storage and delivery, that definitely would prevent the secondary from firing correctly.
Whatever it does, it is an extremely critical component worth spending god knows how much money on.