In 1951 Ivy Mike, part of Operation Ivy, became the first detonation of a thermonuclear weapon, yielding 10.4 megatons of TNT using liquid deuterium. Cousins and Ware built a toroidal pinch device in England and demonstrated that the plasma in pinch devices is inherently unstable. In 1953 The Soviet Union tested its RDS-6S test, (codenamed " Joe 4 " in the US) demonstrated a fission/fusion/fission ("Layercake") design that yielded 600 kilotons. Igor Kurchatov spoke at Harwell on pinch devices, revealing that the USSR was working on fusion.