Gene Buckle wrote: Yeah, photon gas. Makes one's head hurt. Lithium deutride is a stable solid. The problem is that the reaction rate is slower that with tritium, so to build a device that works it's necessary to compress the fuel dramatically. At any given temperature the reaction rate goes up with the square of the density, but since the work needed to compress a gas is proportional to its temperature, it's necessary to find a way to keep the second-stage (fusion) fuel from getting hot before the needed density is reached. Thus the Teller-Ulam/Sakharov bomb, wherein the energy of an atomic bomb is used to compress the deuterium mass, followed by the heating of the mass to ignition temperature, hence the term "staged radiation implosion". This gig works by exploiting the fact that most of the energy of a fission bomb -- on the order of 80% -- is emitted as soft X-rays. Since the transport of energy by radiation vastly exceeds the core's expansion rate it's possible to use this energy to compress and then ignite the secondary fuel mass before the expanding core disrupts it. When the trigger detonates the soft X-rays are trapped by the radiation liner, forming a photon gas which ionize the polystyrene filler of the device case, transporting energy around the thermal shield that separates the fuel capsule from the trigger, leading eventually to the ablation of the tamper, which in turn leads to a very uniform compression of the fuel capsule (thanks to the thermal equilibrium of the photon gas with a little help from the hydrogen/carbon plasma formed arrives at the shield, timed to further compress the fuel. The thing finally torches off when a "spark plug" -- a hollow plutonium rod -- gets compressed by the implosion shock waves to make it supercritical (although the compression isn't much -- maybe a factor of four). Thermal neutrons left over from the fission flux initiate a chain reaction when the rod goes critical. As the fuel continues to implode accelerating rate of fission of the rod raises the temperature of the fuel around the rod the heating affects of the shock wave combine to raise the temperature to the point of ignition, at which point self-supporting fusion burning spreads outwards. Total time from initiation to the point where expansion lowers the density to the point where fusion is no longer possible is on the order of 20-40ns. -- Chris Kennedy chris(a)mainecoon.com http://www.mainecoon.com PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97