Project Excalibur was a United States government nuclear weapons research program to develop a nuclear pumped x-ray laser as a directed energy weapon for ballistic missile defence. It became part of the Strategic Defense Initiative (SDI). Conceived by nuclear scientist Edward Teller, the concept involved packing large numbers of expendable soft x-ray lasers around a nuclear device. When the device detonated, it would fire soft x-ray laser beams in many directions. The goal was to aim these beams to shoot down enemy nuclear missiles near the end of, and after the missiles boost phase stage of flight. The kill mechanism of the X-ray laser was ablative laser propulsion shock; that is, the x-rays would heat the surface of the missile, causing it to vaporize explosively, destroying it or knocking it off course.
Limited accounts in the unclassified press indicate that the device consisted of a small nuclear device surrounded by multiple rods made of a material that served as an x-ray gain medium, releasing x-rays when "pumped" by incident photons. Each rod would function as a separate x-ray laser. The x-ray laser would be optically pumped by the extremely high density of high energy photons that appear in the first nanoseconds of a nuclear detonation. The pumped medium would emit a pulse of coherent x-rays, in the direction of the long axis of the rod. Unlike optical lasers, in which the light is reflected by mirrors at the ends and makes multiple passes through the gain medium, in the x-ray laser the x-ray pulse is generated in a single pass through the rod. The calculations showed that the extremely high gain and high energy pulse from the lasers would occur before the detonation destroyed the lasers and the rest of the satellite. If large numbers of gain media rods were used, each pre-aligned to point at a missile, then a large number of missiles could be destroyed in one fell swoop.
The original proposal was to place many x-ray laser satellites in orbit. There needed to be at least one between the U.S. and its enemies when a massive launch of intercontinental ballistic missiles (ICBMs) occurred. The Soviet Union was the only foe technologically able to accomplish a massive simultaneous launch. Since satellite basing would have violated the Outer Space Treaty which prohibits nuclear weapons in space, a later proposal would have based them on "pop-up" missiles in Alaska and in an effort to be closer still to the point of launch, on SLBMs in the Sea of Okhotsk & Kara Sea. In the event of a Russian ICBM launch the laser could pop-up into space in the path of the approaching ICBMs in anticipation for interception.
The project was proposed as a solution to the problems of using optical lasers in satellites to shoot down missiles: if a large nearly simultaneous launch of ICBMs occurred, the Space Based Laser could not destroy them all, since it was designed to fire upon one at a time. It was felt that the large optics of the SBL could not be re-positioned to point from one missile to the next quickly enough. A considerable amount of research went into rapidly re-targeting the Space Based Laser so that many missiles could be destroyed in time to deal with the massive attack. However, it remained out of reach, giving rise to the Excalibur approach, which was viewed as something of a desperate approach even by those who worked on the project.
Ten known tests of nuclear-pumped x-ray lasers were conducted between 1978 and 1988. The project was determined to be out of reach of current technology and was formally abandoned in 1992. Research was redirected to laser satellites and kinetic weapons under the Strategic Defense Initiative. Since then, its main influence has been its appearances in science fiction, most notably in David Weber's Honorverse military science fiction book series.
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