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 and developed by nuclear scientists Edward Teller, George Chapline, Jr. Peter L. Hagelstein and Lowell Wood 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.
The Excalibur project was proposed as a solution to the problems of using more conventional satellite based optical lasers to shoot down missiles. This original proposal was to place many infrared or x-ray laser satellites in orbit - as there needed to be at least one between the U.S. and its enemies when a massive launch of intercontinental ballistic missiles (ICBMs) occurred. With the Soviet Union viewed as the primary foe technologically able to accomplish such a massive simultaneous launch. If a large nearly simultaneous launch of ICBMs occurred, in a saturation attack, the single beam and power limited Space Based Laser(SBL)s would not have enough time to destroy them all, since each satellite would only be capable of firing upon them one at a time. Moreover, it was felt that the large optics of the SBLs could not be re-positioned to point from one enemy missile to the next quickly enough. A considerable amount of research went into rapidly re-targeting the Space Based Laser concept so that many missiles could be destroyed in a short space of time to deal with the massive saturation attack. However, this approach 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.
Mechanism and capabilities
Limited accounts in the unclassified press indicate that the Excalibur device consisted of a small nuclear explosive 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.
Testing & results
The US project was the result of a 1977 development by George Chapline, Jr. of Lawrence Livermore's "O-Group". Livermore had been working on X-ray lasers for some time, but Chapline found a new solution that used the massive release of X-rays from a nuclear warhead as the source of light for a small baseball-bat sized lasing crystal in the form of a metal rod. The concept was first tried out in 1978s underground nuclear test "Diablo Hawk" but had failed. Peter Hagelstein, new to O Group, set about creating computer simulations of the system in order to understand why. At first he demonstrated that Chapline's original calculations were simply wrong and the Diablo Hawk system could not possibly work. But as he continued his efforts, he found that using heavier metals appeared to make a machine that would work. Through 1979 a new test was planned to take advantage of his work. The follow-up test in November 1980s "Dauphin" appeared to be a success, and plans were made for a major series of experiments in the early 1980s under "Excalibur".
Ten known tests of nuclear-pumped x-ray lasers were conducted between 1978 and 1988.
Move to launch on warning
Since satellite basing of defensive nuclear explosive devices may have violated the Outer Space Treaty which prohibits nuclear weapons in space, a later proposal would have been similar to Launch on warning with basing of the Excalibur packages/modules in the payload shroud of "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 this manner, 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.
Following the end of the Cold War, the project was determined to be out of reach of current technology and was formally abandoned in 1992. Research was redirected back to the likes of the space based laser satellite(SBL) concept, using SDI chemical lasers and kinetic energy weapons under the Strategic Defense Initiative such as the Lightweight Exo-Atmospheric Projectile presently(2014) still in use on the Aegis SM-3 missile.
The fundamental kill mechanism of the Excalibur modules is shared with its conceptual predecessor, the enhanced radiation(X-ray) W71 that tipped the Spartan (missile). However as this warhead's kill range was limited due to the inverse-square law, unlike the range of focused beams afforded by Lasers, this in part led the W71 to be fielded only for a very short period of time in the US.
Likewise the same mechanism of destruction is used in the Soviet Union/Russian counterpart to Spartan, the 1980s "ABM-4 Gorgon" missile of the A-135 anti-ballistic missile system, which remains active.
In the 1986 Operation Charioteer series of underground nuclear testing, test shot "Mighty Oak" was designed to expose a US Trident (missile) warhead to X-ray flux to observe its damage response. The choice of shot name, "Mighty Oak", is likely a reference to the popular fable, The Oak and the Reed, in which; 'A reed before the wind lives on, while mighty oaks do fall'/'Better bend than break'. Alluding to the best way to reduce "wind" damage, which in this case is countermeasures against the ablation generated shock wave. By utilizing warhead components that like the reed, in the fable, survive or can attenuate shock waves, such as ultra low density aerogels. With these aerogel fillers being components of the trident missiles W76 warhead, having the code-name "FOGBANK".
Evidence supporting the idea that warheads could be "hardened" against the effects of ablation generated shock waves, exists in the two lethal ranges stated for the Excalibur concepts predecessor, the W71 warhead, which had a lethal exo-atmospheric radius of 12 miles (19 km) against warheads without countermeasures against soft x-ray generated shock waves, but a range as little as 4 miles (6.4 km) against warheads that did.
Non-nuclear tests to ascertain the capabilities of the fogbank material are still ongoing, with Silver acetylide(SA) - a chemical explosive chosen for testing due to its unusual chemical property of not forming any gaseous products upon detonation, but instead releasing heat only. SA is deposited onto the outside of the W76s re-entry vehicle(RV) and detonated all at once, this approximates the shock response that the US warhead will have when it encounters the Russian Gorgon anti-ballistic missile. A "cold x-ray blowoff impulse generated by an exo-atmospheric hostile nuclear encounter".
Related Excalibur program
Unlike Project Excalibur, the "Excalibur Program" is a non-nuclear optical phased array, high energy laser system funded by DARPA. While the nuclear powered Project Excalibur was intended for space interceptions, the Excalibur program is primarily intended for the atmospheric environment and does not emit x-rays. As of 2014 it reportedly achieved a number of design goals, intent on solving the problem of atmospheric attenuation/blooming, by making sub-millisecond adjustments to each laser's power and frequency in the array. This is the laser equivalent of adaptive optics, which likewise makes adjustments to the shape of telescope mirrors to eliminate the interference caused by the atmosphere.
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