In a working reactor, of course, that beam power would come from the fusion reaction itself, so that the beam and the FRC could keep going as long as the researchers want. That's a milestone TAE hopes to meet with a pB11-burning prototype well before the end of the 2020s, says Binderbauer. This machine will be roughly the size of four double-decker busses parked end to end, he adds - not small, but still just a fraction of the size of ITER. In Bellevue, Washington, meanwhile, another FRC-based reactor is under development at Helion Energy, which was founded by University of Washington researchers in 2013. Company officials are not discussing their plans publicly at the moment, but they have been relatively open about their approach via their website and publications. Helion's reactor, like TAE's, will be a linear tube that uses twin plasma guns to form a stationary FRC in the middle. But instead of trying to sustain the FRC, the Helion device will crush it with an ultrastrong magnetic field until the plasma becomes dense enough and hot enough to fuse. The resulting burst of thermonuclear energy will then cause the ball of plasma to explode outward again, pushing back against the magnetic field and allowing the system to harvest that energy. This cycle will then repeat once per second, generating a steady average power output in much the same way that gasoline explosions do in an internal combustion engine. The Helion reactor will also differ from TAE's in its choice of fuel. Instead of using pB11, it will burn deuterium and helium-3 -an isotope often called a 'helion.' This reaction requires a temperature of several hundred million degrees, intermediate between deuterium-tritium and pB11. But it, too, is aneutronic: the final products are two charged particles, an alpha and a proton. Or rather, this fuel is almost aneutronic: It's impossible to keep the deuterium nuclei in the fuel from reacting with each other and producing at least some neutrons. But those neutrons are low energy and comparatively easy to shield against. And for Helion, the deuterium-deuterium side-reactions are a plus: the products are an almost equal mix of a neutron plus helium-3, and a proton plus tritium - a radioactive isotope that will decay into helium-3 with a half- life of 12.3 years. So in principle, Helion's reactor can make its own helium-3 fuel, which is otherwise available only in trace amounts extracted from natural gas fields, or produced as a byproduct in Canadian CANDU fission reactors. Space Reactor Cohen, for his part, has been pursuing his Princeton Field Reversed Configuration (PFRC) design since 2002, with a strong emphasis on simplicity and compactness (3). The cylindrical plastic vacuum chamber of his current device, PFRC-2, is only 88 centimeters long, with not a plasma cannon or neutral beam injector in sight. Instead, the FRC is generated via a technique first explored by Austrian and Australian physicists and then refined by Cohen himself. He points to four rectangular copper coils that surround the middle of the tube: one each on its front, back, top, and bottom. Each rectangle, in turn, is divided into two smaller rectangles. The idea, says Cohen, is to drive oscillating currents through these coils in a way that sets up a rotating magnetic field inside the tube: a loop of flux that whirls through the plasma like a flipped coin and drags the plasma particles around and around the waist of the cylinder. In the process, he says, 'the fields create, stabilize, and heat the FRC'- all in a single deft maneuver. Indeed, that's what Cohen routinely demonstrates in his lab: Every two seconds, the hydrogen plasma inside is whipped into an FRC, causing a flash. Each flash lasts for only about eight milliseconds, says Cohen, mainly because longer pulses risk melting th e cables that supply the antennas with power. 'So for the next machine,' he says, 'we've got to make better cables'- which should keep the FRCs going indefinitely. In the meantime, Cohen and his group are working on the main goal of PFRC-2, which is to improve the antennas' ability to heat the plasma. This is crucial, notes Binderbauer: 'I know Sam well, we root for each other.' But compared with the temperatures that TAE is already working with, he says, 'his plasmas are cold.' Also, he says, it remains to be seen how well the rotating magnetic field approach will work in a full-scale reactor. 'I'm not trying to say that it can't be done,' says Binderbauer, 'but I th ink those are some of the things that they're going to have to address.'