What is Operation Greenhouse 1951?

Operation Greenhouse was a series of nuclear tests conducted at Eniwetok Atoll in early 1951 to test various design principles that would later become pivotal in the development of thermonuclear weapons. The purpose of these tests was to reduce the size, weight, and amount of fissile material necessary for nuclear weapons, while simultaneously increasing their destructive power.*

Source:

*http://www.atomicheritage.org/history/hydrogen-bomb-1950