Lockheed Martin’s announcement that it had secretly developed a promising design for a compact nuclear fusion reactor has met with excitement but also skepticism over the basic feasibility of its approach. Nuclear fusion could produce far more energy, far more cleanly, than the fission reactions at the heart of today’s nuclear power plants. But there are huge obstacles and no hard evidence that Lockheed has overcome them. The so-far-insurmountable challenge is to confine hydrogen plasma at conditions under which the hydrogen nuclei fuse together at levels that release a useful amount of energy. In decades of research, nobody has yet produced more energy from fusion reaction experiments than was required to conduct the experiments in the first place. Most research efforts use a method that tries to contain hot plasma within magnetic fields in a doughnut-shaped device called a tokamak. Three research-scale tokamaks operate in the United States: one at MIT, another at a lab in Princeton, and a third at a Department of Energy lab in San Diego. The world’s largest tokamak is under construction in France at an international facility known as ITER, at a projected cost of $50 billion. Tom McGuire, project lead of the Lockheed effort, said in an interview that the company has come up with a compact design, called a high beta fusion reactor, based on principles of so-called “magnetic mirror confinement.” This approach tries to contain plasma by reflecting particles from high-density magnetic fields to low-density ones. Lockheed said the test reactor is only two meters long by one meter wide, far smaller than existing research reactors. “In a smaller reactor you can iterate generations quicker, incorporate new knowledge, develop faster, and make riskier design choices. That is a much more powerful development paradigm and much less capital intensive,” McGuire said. If successful, the program could produce a reactor that might fit in a tractor-trailer and produce 100 megawatts of power, he said. “There are no guarantees that we can get there, but that possibility is there.” The small team developing the reactor at the company’s skunkworks in Palmdale, California, has done 200 firings with plasma, McGuire said, but has not shown any data on the results. However, he said of the plasma, “it looks like it’s doing what it’s supposed to do.” He added that with research partners Lockheed could develop a completed prototype within five years and a commercial application within a decade. The company is even talking about how fusion reactors could one day power ships and planes. Many scientists are unconvinced. Ian Hutchinson, a professor of nuclear science and engineering at MIT and one of the principal investigators at the MIT fusion research reactor, says the type of confinement described by Lockheed had long been studied without much success. Hutchinson says he was only able to comment on what Lockheed has released—some pictures, diagrams, and commentary. “Based on that, as far as I can tell, they aren’t paying attention to the basic physics of magnetic-confinement fusion energy. And so I’m highly skeptical that they have anything interesting to offer,” he says. “It seems purely speculative, as if someone has drawn a cartoon and said they are going to fly to Mars with it.” Hutchinson adds: “Of course we’d be delighted if a real breakthrough were possible, but when someone who shows no evidence of understanding the issues makes a bald claim that they will just make a small device and therefore it will be quicker [to develop], we say, ‘Why do they think they can do that?’ And when they have no answers, we are highly skeptical.” Lockheed joins a number of other companies working on smaller and cheaper types of fusion reactors. These include Tri-Alpha, a company based near Irvine, California, that is testing a linear-shaped reactor; Helion Energy of Redmond, Washington, which is developing a system that attempts to use a combination of compression and magnetic confinement of plasma; and Lawrenceville Plasma Physics in Middlesex, New Jersey, which is working on a reactor design that uses what’s known as a “dense plasma focus.” Another startup, General Fusion, based in Vancouver, British Columbia, tries to control plasma using pistons to compress a swirling mass of molten lead and lithium that also acts as a coolant, absorbing heat from fusion reactions and circulating it through conventional steam generators to spin turbines.