In the review of The Star Builders: Nuclear Fusion and the Race to Power the Planet (Physics Today, October 2021, page 64), reference is made to the old joke that controlled fusion power is 30 years away and always will be. I would like to update that observation, and perhaps make it more rigorous, by noting the history of controlled fusion research dating from the 1950s. In my 60 years of association with plasma physics and both magnetic-and inertial-confinement fusion, I’ve heard countless briefings and promises: When the research programs were 10 years old, the reactor was 10 years away. After 20 years, it was 20 years away; 30 years on, it was 30 years away; and so on. Now a commercially viable reactor could be more than 50 years away. It appears to be a self-similar problem, where the only time scale is the elapsed time.
Such behavior is not restricted to controlled fusion. The same joke, including the 30-year time frame, used to be said of electric rocket propulsion, and there are now hundreds of spacecraft that use electric propulsion. Perhaps the problem is related to the so-called S curve for technology development. Until a technology rises above the early, exploratory stage, predictions about it becoming mature enough for practical use can be driven more by optimism and enthusiasm than by the available hard facts.
The recent surge in fusion startups1 might be encouraging (to some), but it’s reminiscent of the early days of aeronautics, when some folks began attempting heavier-than-air flight—and airplanes with heavy piston engines won out over the early aeronautical success of hot-air balloons. The accumulation of ideas and experience directed toward real systems may eventually make a difference for fusion power on its S curve. Electric rocket propulsion was helped by frequent, short-turnaround iterations, thereby providing a time scale for progress apart from the elapsed time. Unfortunately for fusion, the cost and size of useful technical demonstrations may preclude such iterations. Startup fusion concepts that substantially reduce cost and size offer optimism for faster progress.