Joe 4 (Warhead name: RDS-6s (Reaktivnyi Dvigatel Specialnyi; Special Jet Engine)) was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953. Scholars dispute the authenticity of RDS-6 as a thermonuclear device as it did not manage to produce a yield consistent with a true hydrogen bomb.[1] It utilized a scheme in which fission and fusion fuel (lithium-6 deuteride) were "layered", a design known as the Sloika (Russian: Слойка, 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 energy and neutron-initiated ("boosted") fission. A similar design was earlier theorized by Edward Teller, but never tested, in the USA as the "Alarm Clock".[2]

The Soviet thermonuclear weapons program initially researched two weapon designs. One design was the Sloika (RDS-6s), the other design was the Truba (RDS-6t). The RDS-6t was a two-stage gun-type bomb with a deuterium-tritium secondary and was similar to the U.S. “classical Super” design. When the United States detonated a hydrogen bomb in the Pacific in 1952 (Ivy Mike), higher priority was given to the RDS-6s design, which was considered to be more likely to work.[3]

Joe 4 detonated with a force equivalent to 400 kilotons of TNT. The Soviet physicist Yuli Khariton estimated that Joe 4's yield was 15% to 20% fusion, the rest fission boosted by the fast neutrons released in the fusion. Being a single-stage weapon, though, it was not capable of being scaled up indefinitely like "true" hydrogen bombs (see Teller-Ulam design for more details on the distinctions between fusion weapons).

Despite its inability to be scaled into the megaton range, the detonation was used by Soviet diplomats as leverage. The Soviets claimed that they too had a hydrogen bomb, but unlike the United States' first thermonuclear weapon, theirs was deployable by air. The United States didn't develop a deployable version of the hydrogen bomb until 1954. The Sloika model was never widely deployed.

The first Soviet test of a "true" hydrogen bomb was on November 22, 1955 under the directive of Nikolai Bulganin (influenced by Nikita Khrushchev), code-named RDS-37.[4] All were at Semipalatinsk Test Site, Kazakhstan. Like RDS-6, it was a "dry" weapon, using lithium-6 deuteride instead of liquid hydrogen.

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