|Multistage thermonuclear weapons - the main component of today's strategic nuclear forces - are more complex. These employ a 'primary' fission weapon to serve merely as a trigger. As mentioned above, the fission weapon is characterized by a tremendous energy release in a small space over a short period of time. As a result, a very large fraction of the initial energy release is in the form of thermal X-rays.|
These X-rays are channeled to a 'secondary' fusion package. The X-rays travel into a cavity within a cylindrical radiation container. The radiation pressure from these X-rays either directly, or through an intermediate material often cited as a polystyrene foam, ablates a cylindrical enclosure containing thermonuclear fuel; this can be Li2H (lithium deuteride). Running along the central axis of this fuel is a rod of fissile material, termed a 'sparkplug'.
The contracting fuel package becomes denser, the sparkplug begins to fission, neutrons from this transmute the Li2H into 3H that can readily fuse with 2H (the fusion reaction 3H + 2H has a very high cross-section, or probability, in typical secondary designs), heat increases greatly, and fusion continues through the fuel mass.
It always amazes me to read about this stuff, and realize that it was all put together in the 1950's. (Or even earlier, in the case of the Hiroshima and Nagasaki bombs.)