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A fission-fusion thermonuclear weapon in which the burst of neutrons are generated by a fusion reaction. Gamma rays from the explosion form a fireball, as they superheat nearby air and/or other material. For an explosion in the atmosphere, the fireball quickly expands to maximum size, and then cools as it rises through the surrounding still air. It takes on the flow pattern of a vortex ring with incandescent material in the vortex core, i.e., the Mushroom Cloud.