In an age of shrinking research and development (RandD) budgets, sustaining big science and technology (SandT) projects is inevitably questioned by publics and policy makers. The fusion enterprise is an exemplar. The effort to develop a viable system to produce unlimited and environmentally benign electricity from fusion of hydrogen isotopes has been a goal for six decades and consumed vast financial and intellectual resources in North America, Europe, and Asia. In terms of prolonged duration and sustained resource investment, the endeavor has developed into a huge fusion enterprise. Yet, no practical system for the generation of electricity has yet been demonstrated. This is the paradox at the heart of the fusion enterprise.

Why, despite unfulfilled visions and broken promises, has the grand fusion enterprise endured? How can such a long-term enterprise persist in a funding culture that largely works in short-term cycles?

Adapting Sheila Jasanoff's thesis of "sociotechnical imaginaries", I examine the relationship of shared and contrasting visions, co-produced expressions of nature and society, and distinctpolitical cultures in the quest for viable fusion. A systematic cultural and technological comparison of three fusion ventures, the National Spherical Torus Experiment Upgrade, the International Thermonuclear Experimental Reactor (ITER), and Wendelstein-7X, exposes how these projects and the institutions they inhabit frame the goals, risks, and benefits of the fusion enterprise and sustain a common set of fusion imaginaries. Positioned within the Princeton Plasma Physics Laboratory in the United States, the international ITER Organization sited in France, and the Max Planck Institute for Plasma Physics in Germany, the three projects are prime examples of big science and technology. Rigorous research and analysis of these cases advance the thesis of the unfulfilled utopian vision of fusion energy that has endured for more than sixty years.