2020s, he says, all of them are eventually going to have to build a real, power-producing test reactor - something that's not likely to happen for a decade or more. Pluto Power That's why Cohen takes the long view. His goal is an ultra -compact reactor that will use a fuel mix containing helium-3, an isotope that yields a particularly clean form of fusion with minimal radiation risk. But the stuff is exceedingly rare, he says: 'So we're not trying to make power for everybody.' Instead, the goal is niche uses such as spacecraft propulsion, in which the reactor would fire a very tenuous plasma from one end so that it functions as a rocket (1). Such a direct fusion drive (DFD) would produce only the most infinitesimal hint of acceleration, says Cohen - about like pushing an 18-wheel truck with your fingertip. But in space, that push would have nothing to resist it. After a year or two, such a rocket could get a 10-ton spacecraft halfway to Pluto, traveling well over 50 kilometers per second. 'Then you'd turn around and decelerate,' says Cohen. 'And when you got to Pluto, you'd go into orbit.' At that point, the reactor would turn off the ion rocket and convert itself into a one-megawatt electrical power source. 'Some of that power you can use to send high- definition video back,' says Cohen. 'And some of it you can beam down to a lander that you've placed on the surface, so it could drive around and drill holes in the ice.' The same type of DFD rockets could also be used to explore the moons of Jupiter and Saturn, says Cohen, or the icy bodies of the Kuiper Belt beyond Pluto, or anywhere else in the outer solar system. Plasma Problem Of course, there's a reality check, says Dean: 'If you want to make a fusion exhaust system, you still have to be able to make the fusion plasma.' It's a trick that neither Cohen nor anyone else has yet managed. Researchers have been trying to harness fusion power since the 1920s and 1930s, when they first realized that stars like the sun get their energy from thermonuclear reactions at their core. And yet, as the many delays and cost overruns on ITER have made clear, success is still years away at best. Still, old hands like Cohen know the pitfalls of fusion research as well as anyone. Until the late 1990s, his professional life revolved around ITER, which is supposed to be the ultimate expression of the oldest and most promising approach to fusion energy: magnetic confinement. In theory, this is just a matter of ionizing an appropriate mix of light isotopes, trapping them in a magnetic field, and heating them to millions of degrees while simultaneously squeezing them to densities approximating the sun's c ore. The isotopes will then start fusing into larger nuclei while releasing vast amounts of energy. In practice, though, hot, ionized plasma doesn't like being confined by a magnetic field; it twists and tries to escape like a living thing. Thus the appeal of the tokamak design, which was a major breakthrough when Soviet physicists introduced it in the 1960s. Thanks to strong magnetic fields that guide the ionized isotopes around and around its donut-shaped vacuum chamber, a tokamak could keep the plasma under control better than almost anything else at the time. And thus the funding agencies' willingness to keep sinking billions of dollars into ITER: a gargantuan tokamak whose 23,000-ton weight will be three times that of the Eiffel Tower, and whose 29 by 29-meter vacuum chamber will be as tall as a seven-story building. This is the scale that a tokamak w ill need to achieve the elusive goal of 'break - even,' in which the plasma produces more fusion energy than the machine requires to operate. Except that to Cohen and an increasing number of other fusion researchers, ITER has laid bare the tokamak's many drawbacks as a practical power source. These start with the facility's size, cost, and complexity, which are so far beyond what power companies are willing to accept that they have all but given up on fusion, says Dean: 'I can't even talk to anyone in the utilities who knows what a tokamak is anymore.' And then there's the neutron problem. The physics of tokamaks limits them to burning a mix of the hydrogen isotopes deuterium and tritium. This fuel is by far the easiest to ignite, requiring comparatively low plasma temperatures of about a hundred million degrees Kelvin. But when the two nuclei fuse to form a helium-4