Joe 4 was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953, that detonated with a force equivalent to 400 kilotons of TNT. The proper Soviet terminology for the warhead was RDS-6s, , . RDS-6 utilized a scheme in which fission and fusion fuel (lithium-6 deuteride) were "layered", a design known as the Sloika (, named after a type of layered puff pastry) model in the Soviet Union. A ten-fold increase in explosive power was achieved by a combination of fusion and fission. A similar design was earlier theorized by Edward Teller, but never tested in the U.S., as the "Alarm Clock". Description The Soviet Union started studies of advanced nuclear bombs and a hydrogen bomb, code named RDS-6, in June 1948. The studies would be done by KB-11 (usually referred to as Arzamas-16, the name of the town) and FIAN. The first hydrogen bomb design was the Truba (, pipe/cylinder) (RDS-6t)). In March 1948 Klaus Fuchs had provided the USSR with documents of the US 'Classical Super'. In these documents the classical super was described as consisting of a gun-type Uranium-235 primary with beryllium oxide tamper and a secondary consisting of a long cylinder with deuterium, doped with tritium near the primary. The design of the RDS-6t was similar to this classical super. The difference was that the light shell of beryllium oxide was replaced by a heavy shell. The assumption was that the deuterium tritium mixture could be easily heated and compressed and the shock would start the thermonuclear reaction prematurely. A heavy shell opaque to radiation would prevent this unwanted preheating more than the light shell. In September / October 1948 Andrei Sakharov, working in FIAN, came up with a competing idea of alternating layers of deuterium and Uranium-238 around a fissile core (Sakharov's 'first idea'). This second design was code named Sloika (RDS-6s) or 'Layer Cake' after the layering. In March 1949 Vitaly Ginzburg proposed to replace the deuterium by Lithium-6 deuteride ('second idea'). The proposal was based on the better efficiency due to the generation of tritium by the neutron capture of lithium and the Uranium-238 fission by the 14 MeV neutrons from D + T fusion. At that time Ginzburg did not know that the cross section for D + T reaction was much larger than that for D + D reaction. In April 1949 the group