nucleus (two protons plus two neutrons), they eject the leftover neutron at high energy. And because that particle is electrically neutral and can't be controlled with magnetic fields, it ends up smashing into the tokamak's inner walls and wreaking havoc w ith their structural integrity. So the walls will have to be replaced perhaps once per year - a maintenance burden that no power company wants to shoulder. A Different Configuration By the late 1990s such hurdles were spurring Cohen and others to take a fresh look at FRCs, which had been discovered in the 1960s. The key advantage was that an FRC doesn't keep its plasma in line by brute force, the way a tokamak does. Instead, the FRC plasma is self-organizing. That is, the magnetic fields that hold it together are mostly generated by currents flowing through the plasma itself, rather than in external coils. This self-organizing property can be found in other plasma structures, which have names such as 'spheromak' and 'dense plasma focus,' says Cohen. But all else being equal, FRC plasmas are much hotter and denser t han the others. Once it's set up, an FRC actually looks less like a smoke ring than an elongated American football, or maybe a short cigar. The 'field - reversed' name comes from the way magnetic fields curve around the football's outside and then loop backward through its long axis. This structure tends to dissipate in less than a millisecond, unfortunately - one big reason why only a handful of researchers stuck with the FRCs after tokamaks came along. But the appeal remained: Find a way to stabilize the FRC, and the reactor wouldn't have to be much more than a cylindrical vacuum chamber with a comparatively mild magnetic field running down the midline to hold the plasma football in place. Self-organization also should make it comparatively easy for the dense, hot plasma inside the FRC to reach the threshold required for fusion. And not just deuterium-tritium fusion, either: FRCs could potentially reach the much higher temperatures required to burn aneutronic fuels such as deuterium-helium - 3 or proton-boron - 11. These reactions emit most of their fusion energy in the form of charged particles such as protons or helium-4 nuclei, which - unlike neutrons - can be captured and controlled with magnetic fields. This would make it much easier to extract energy from the fusion products before they can damage the reactor walls, and would allow the reactor to get by with minimal shielding. So in principle, says Cohen, FRC- based reactors could solve the tokamak's size, complexity, and neutron problems at a stroke. But to make that work in practice, he says, researchers have had to make a series of critical design choices: how to form, stabilize, and control the FRC, how to heat it, what kind of fusion fuel to use, and so on. 'You multiply all those options,' he says, 'you get roughly 80 different potential FRCs.' TAE has been working on one such option since 1998, when it was founded with the goal of fusing protons with boron-11 nuclei. This pB11 reaction is in some ways the ultimate in neutron-free fusion: Its output is just a triplet of positively charged helium-4 nuclei, which are commonly known as alpha particles (thus the company's original name, TriAlpha.) But the reaction also has some significant downsides. For example, its multibillion-degree threshold for fusion is about 20 or 30 times higher than the temperatures required for the deuterium-tritium reaction that ITER will use. Also, it has about half as much energy yield per fusion event. So to make pB11 work, TriAlpha's design has to be correspondingly ambitious ( 2). The idea is to cap the reactor on each end with two electromagnetic cannons pointed barrel to barrel. To start things off, each cannon fires a ring of plasma into a central chamber, where the rings merge into a single, furiously spinning FRC. From there, a beam of neutral atoms coming in from the side will simultaneously heat the FRC, supply it with fresh pB11 fuel, and stabilize it by keeping the spin rate up. It took TAE until 2012 to demonstrate this whole process in a prototype machine (albeit with a nonfusing hydrogen plasma), says the company's CEO, Michl Binderbauer. 'We showed these beautiful experiments where, if you start with the standard FRC and you d on't do anything, it dies,' he says. 'But if you start injecting particles, you slow down the decay and expand how long it lives.' Since then, says Binderbauer, the company has shown that this process can sustain the FRC indefinitely - or at least, for the five or 10 milliseconds it takes the 25-megawatt beam to exhaust the energy that researchers are able to store for each shot.