Word: atomically
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This was an exciting and ominous prospect, but the trouble with fusion reactions is that they are not self-starting; uranium fission is. When a sufficient amount (critical mass) of U-235 is assembled, a single, slow-moving neutron can start an atom-splitting chain reaction in it and make the whole chunck explode. Light elements are not so accommodating. Their atoms must be slammed together violently to make them group into larger atoms and yield energy...
Ordinary-high temperatures, attainable by chemical means, are not nearly high enough, but the center of an exploding uranium-fission bomb (more than 50,000,000° C.) is as hot or hotter than the interior of the sun. Before the first atom bomb exploded, physicists were speculating as to whether atom bombs might serve as "detonators" to start fusion explosions...
...temperature of an atom bomb at the instant of explosion is fabulously high, but as the fireball expands, it cools off rapidly. If it cools too fast, any fusion reaction that it has started will die out. But if the high temperature lasts long enough, it will "ignite" the light elements. Then the fusion reaction will continue, generating energy to keep the materials hot until a large part of the light-element charge has been fired...
...early postwar period, the prospects for fusion did not look very good. The available light elements-lithium, ordinary hydrogen and deuterium (heavy hydrogen)-seemed to require more heat than could be provided by the first atom bombs. The third hydrogen isotope, tritium, looked more promising. A mixture of tritium (H³) and deuterium (H²) will ignite at a comparatively low temperature, turning into helium (He4) and a free neutron, and giving a big yield of energy...
...made at fantastic cost in nuclear reactors. Optimistic physicists hoped that a small priming of tritium would ignite large amounts of light elements that are not so hard to come by. Pessimists feared that too much tritium would be required. They pointed out that each atom of tritium manufactured in a nuclear reactor costs about one atom of U-235 or plutonium, which could be used to better advantage, they thought, in old-style fission bombs...