In 1951 Edward Teller and Stanislaw Ulam at Los Alamos National Laboratory (LANL) developed the Teller-Ulam design for a thermonuclear weapon, allowing for the development of multi-megaton yield fusion bombs. Fusion work in the UK was classified after the Klaus Fuchs affair.
In the mid-1950s the theoretical tools used to calculate the performance of fusion machines were not predicting their actual behavior. Machines invariably leaked plasma at rates far higher than predicted. In 1954, Edward Teller gathered fusion researchers at the Princeton Gun Club. He pointed out the problems and suggested that any system that confined plasma within concave fields was doomed due to what became known as interchange instability. Attendees remember him saying in effect that the fields were like rubber bands, and they would attempt to snap back to a straight configuration whenever the power was increased, ejecting the plasma. He suggested that the only way to predictably confine plasma would be to use convex fields: a "cusp" configuration.
When the meeting concluded, most researchers turned out papers explaining why Tellers concerns did not apply to their devices. Pinch machines did not use magnetic fields in this way, while the mirror and stellarator claques proposed various solutions. This was soon followed by Martin David Kruskal and Martin Schwarzschilds paper discussing pinch machines, however, which demonstrated those devices' instabilities were inherent.