The question facing designers was "How do you build a bomb that will maintain the high temperatures required for thermonuclear reactions to occur?" The shock waves produced by the primary (A-bomb) would propagate too slowly to permit assembly of the thermonuclear stage (the secondary) before the bomb blew itself apart. This problem was solved by Edward Teller and Stanislaw Ulam.

Gamma Radiation

To do this, they introduced a high energy gamma ray absorbing material (styrofoam) to capture the energy of the radiation. As high energy gamma radiation from the primary is absorbed, radial compression forces are exerted along the entire cylinder at almost the same instant. This produces the compression of the lithium deuteride. Additional neutrons are also produced by various components and reflected towards the lithium deuteride. With the compressed lithium deuteride core now bombarded with neutrons, tritium is formed and the fusion process begins.