The basic concept has been raised on several occasions, with the first known references being part of the Project Orion nuclear-powered spacecraft project in the 1960s. This used beryllium oxide to convert the X-rays released by a small bomb into longer wavelength radiation, which explosively vaporized a tamper material, normally tungsten, causing it to carry away much of the bomb's energy as kinetic energy in the form of tungsten plasma. The same concept was explored as a weapon in the Casaba/Howitzer proposals.