cm thick hollow cylindrical rod of plutonium, nested in the steel canister. This was the spark plug, a tritium-boosted fission device. It was assembled by plutonium rings and had a hollow volume inside that measured about 0.5 cm in diameter. This central volume was lined with copper, which like the liner in the primary's fissile core prevented DT gas diffusion in plutonium. The spark plug's boosting charge contained about 4 grams of tritium and, imploding together with the secondary's compression, was timed to detonate by the first generations of neutrons that arrived from the primary. Timing was defined by the geometric characteristics of the sparkplug (its uncompressed annular radius), which detonated when its criticality, or k, transcended 1. Its purpose was to compress the fusion material around it from its inside, equally applying pressure with the tamper. The compression factor of the fusion fuel and its adiabatic compression energy determined the minimal energy required for the spark plug to counteract the compression of the fusion fuel and the tamper's momentum. The spark plug weighed about 18 kg, and its initial firing yielded . Then it would be completely fissioned by the fusion neutrons, contributing about to the total yield. The energy required by the spark plug to counteract the compression of the fusion fuel was lower than the primary's yield because coupling of the primary's energy in the hohlraum is accompanied by losses due to the difference between the X-ray fireball and the hohlraum temperatures. The neutrons entered the assembly by a small hole through the ≈28 cm thick U blast-heat shield. It was positioned in front of the secondary assembly facing the primary. Similar to the tamper-fusion capsule assembly, the shield was shaped as a circular frustum, with its small diameter facing the primary's side, and with its large diameter locked by a type of mortise and tenon joint to the rest of the secondary assembly. The shield-tamper ensemble can be visualized as a circular bifrustum. All parts of the tamper were similarly locked together to provide structural support and rigidity to the secondary assembly. Surrounding the fusion-fuel–spark-plug assembly was the uranium tamper with a standoff air-gap about 0.9 cm wide that was to increase the tamper's momentum, a levitation technique used as early as Operation Sandstone and described by physicist Ted Taylor as hammer-on-the-nail-impact. Since there were also technical concerns that high-Z tamper material would mix rapidly with