The biconic cusp, also known as the picket fence reactor, was one of the earliest suggestions for plasma confinement in a fusion reactor.[1] It consists of two parallel electromagnets with the current running in opposite directions, creating oppositely directed magnetic fields. The two fields interact to form a "null area" between them where the fusion fuel can be trapped.
The concept arose as a reaction to an issue raised by Edward Teller in 1953. Teller noted that any design that had the plasma held on the inside of concave magnetic fields would be naturally unstable. The cusp concept had fields that were convex, and the plasma was held within an area of little or no field in the inside of the device. The concept was independently presented in 1954 by both Harold Grad at the Courant Institute in New York and James L. Tuck at Los Alamos.
At first there was little interest in the design because Teller's problem was not being seen in other early fusion machines. By the late 1950s it was clear these machines all had serious problems, and Teller's was only one of many. This led to renewed interest in the cusp, and several machines were built to test the concept through the early 1960s. All of these devices leaked their fuel plasma at rates much greater than predicted and most work on the concept ended by the mid-1960s. Mikhail Ioffe later demonstrated why these problems arose.
A later device that shares some design with the cusp is the polywell concept of the 1990s. This can be thought of as multiple cusps arranged in three dimensions.