But deciding to do it did not make it a reality, and Teller and other U.S. physicists struggled to find a workable design. Stanislaw Ulam, a co-worker of Teller, made the first key conceptual leaps towards a workable fusion design. Ulam's two innovations that rendered the fusion bomb practical were that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion, and the idea of staging or placing a separate thermonuclear component outside a fission primary component, and somehow using the primary to compress the secondary. Teller then realized that the gamma and X-ray radiation produced in the primary could transfer enough energy into the secondary to create a successful implosion and fusion burn, if the whole assembly was wrapped in a "hohlraum" or radiation case. Teller and his various proponents and detractors later disputed the degree to which Ulam had contributed to the theories underlying this mechanism. Indeed, shortly before his death, and in a last-ditch effort to discredit Ulam's contributions, Teller claimed that one of his own "graduate students" had proposed the mechanism.