According to the info out there, the fogbank material absorbs the X rays and turns into a plasma which helps confine the thermo-nuclear fuel while its being heated by the plutonium 'sparkplug' in the middle of it
thus boosting the pressures so that more of the fuel is turned into energy (and if you wanta really big bang, surround the whole lot in a depleted uranium cylinder that catches the high speed neutrons emitted from the reation, turns into plutonium which then fissions extremely well in the high density neutron flux.
See russian "Tsar Bomb" for more details...
Biting the hand that feeds IT
