by Researchers in Japan have demonstrated, for the first time in a fusion reactor, a type of fuel that is abundant and does not produce harmful particles. Although the reactions were nowhere close to achieving clean energy and required even higher temperatures than standard fusion fuel, the result is a proof of principle for private fusion startup TAE Technologies, which says its path to a practical power plant faces fewer mechanical obstacles than conventional approaches.
The results show how the alternative fuel, a mix of protons and the element boron, “has a place in utility-scale fusion power,” TAE CEO Michl Binderbauer said in a statement. Not everyone is convinced. “It’s an interesting experiment,” but it will do little to convince skeptics to switch fuels, says Dennis Whyte, director of the Center for Plasma Fusion and Science at the Massachusetts Institute of Technology.
Fusion is often promoted as a carbon-free energy source that has an abundant and cheap fuel—a mixture of the hydrogen isotopes deuterium and tritium (DT). In fact, tritium is rare and must be “breeded” from lithium in the reactor itself. some scientists worry about future shortages. In addition, when fusing at high temperatures, DT fuel produces abundant high-energy neutrons, which are harmful to humans and reactor structures.
TAE follows a different recipe: fusing hydrogen nuclei – protons – with easily mined boron. The reaction produces no neutrons and produces only harmless helium, but requires temperatures of about 3 billion degrees Celsius—200 times the heat of the Sun’s core and 30 times that needed to fuse DT. Researchers have already shown that they can fuse protons and boron using beams of particles aimed at a solid target or by blasting the plasma with a laser. Now, a team has done it—on a small scale, at least—using a conventional fusion reactor, called the Large Helical Device (LHD), at Japan’s National Institute of Fusion Science. The team presents their work today at Nature communications.
The LHD, which began operations in 1998, is shaped like a twisted donut and has electromagnets that contain the superheated ionized fuel known as plasma. This type of device, known as a stellarator, is not designed to operate at the temperatures required for proton-boron fusion. In the experiments, a boron plasma was heated to about 20 million degrees Celsius and beams of neutral hydrogen atoms were fired into the plasma. Proton-boron fusion produces high-speed helium atoms, and the helium sensors, developed by TAE, registered 150 times more hits with a boron plasma in the machine than when it contained an unreactive gas – a sign that fusion was occurring.
Computer simulations by the team suggested that this translates to about 5 trillion fusion reactions per second. While that may sound like a lot, Whyte says it equates to about 7 watts of power, one-tenth of the power produced by a candle flame. What’s more, Whyte says, most of these reactions were caused by the particle beams. In many fusion reactors, particle beams are used to heat the overall temperature of the plasma enough for it to fuse more widely. But the LHD results suggest that fusion was happening only in the few hot spots where the beams hit the plasma, not elsewhere, Whyte says, because the fusion rate drops off quickly once the beam is turned off.
A power-producing fusion reactor would need a wider fusion burn to provide enough heat to sustain the reactions—plus some extra to be harvested for electricity. LHD is a long way from that, but TAE believes it can get there with a very different plasma device. TAE’s various testbeds have created a rapidly rotating “smoke ring” of plasma that is stabilized and heated by beams of particles. TAE’s largest machine to date, called Norman, achieved a temperature of 60 million degrees Celsius for 30 milliseconds.
In a few years, TAE says it will finish building a successor, called Copernicus, which is intended to reach 100 million degrees Celsius – the temperature required for conventional DT fusion. By the next decade, the company wants to build an even more powerful machine — the Da Vinci — that could bring it close to proton-boron temperatures.
A reactor powered by protons and boron would remove many of the challenges engineers face as they try to move fusion from scientific demonstration to practical electricity generator. The US National Ignition Facility made headlines last year after it showed a “gain”: a fusion reaction triggered by powerful lasers that produced more heat than the lasers pumping inside. However, this explosive form of fusion reactor can be difficult to convert into a power plant. The international ITER reactor under construction in France aims to demonstrate a more steady-state, furnace-like approach. But it won’t show a profit until the end of the next decade – when some scientists worry it will begin to eat up most of the world’s tritium supply.
ITER also has thick concrete shielding to protect operators from neutrons. In a commercial reactor, operating around the clock, these neutrons would also damage the reactor structure and shorten its life. Studies are underway to find hard neutron materials for reactors, but no obvious candidates have yet been identified.
Whyte says neutrons are a huge challenge for conventional fusion, but he believes getting the plasma to temperatures measured in billions could be just as difficult. Even if the TAE gets there, each proton-boron reaction yields only one-tenth the energy of fusing deuterium and tritium. To make it worthwhile, proton-boron fusion “would need strong mechanical advantages,” Whyte says.
