Gene Buckle wrote:
> Photon gas. Say WHAT? :)
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
from the filler). Kinetic energy from the trigger then
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_at_mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97
Received on Wed Oct 17 2001 - 17:22:58 BST