W71 (nuclear weapon)
Involving significant advances in nuclear weapons technology, the W71, designed by Lawrence Livermore National Laboratory, was the warhead for the U.S. Army LIM-49A Spartan, an exoatmospheric (i.e., in-space) interceptor of a U.S. ballistic missile defense program of the early 1970s. It had a large 5 Mt yield, unusual in that it was engineered to maximize its production of thermal X-rays, which, rather than blast, were its actual kill mechanism. It was a predecessor of later theoretical systems that used X-ray lasers, driven by a nuclear explosion, and of the class of directed energy weapons.
Theory of operation
Its thermonuclear warhead used a coating of gold to maximize X-ray output. "The choice of gold may have been to tailor the opacity so that the hot X-rays present at the end of the fusion burn could escape without being absorbed. Gold is a good tamper material" and, in another way the W71 anticipated future development, has been used in laser fusion inertial confinement target designs due to its high opacity.  The primary of the W71, which may have been a lighter-weight derivative of the W50, used plastic bonded explosives, variously reported as LX-07 or LX-11.
It was the largest-yield weapon ever tested underground, too large for the Nevada Test Range.  Activists who were to found Greenpeace sailed to Amchitka Island in Alaska, the test site, in protest. The test involved immense technical challenges.
Conducted on Amchitka Island in Alaska, the CANNIKIN underground nuclear test of 6 November 1971 was "the largest underground nuclear test conducted in the United States... The shock registered 7.0 on the Richter scale... Within two days after the explosion, a crater more than one mile wide and 40 feet deep formed."
The anti-ballistic missile system of which the Spartan was part was both made technologically obsolete by evolving multiple warhead technologies, and it was cancelled as a result of a 1972 treaty between the U.S. and Soviet Union.