A reactor experiment shows an alternative fusion scheme | Science

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 not close to achieving net energy and required even higher temperatures than conventional fusion fuel, the result is a proof of principle for private fusion startup TAE Technologies, which insists its engineering hurdles are lower. path to a more practical than traditional power plant. to him.

The results show how the alternative fuel, a combination 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 won’t do much to convince skeptics to switch fuels, says Dennis Whyte, director of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology.

Fusion is often promoted as a carbon-free energy source with an abundant and cheap fuel – a mixture of the hydrogen isotopes deuterium and tritium (DT). In reality, tritium is rare and must be “breeded” from lithium in the reactor itself; some scientists are concerned about future shortages. In addition, when fused at high temperatures, DT fuel produces massive high-energy neutrons, which are harmful to both people and reactor structures.

TAE follows a different recipe: combining hydrogen nuclei – protons – with boron that is easily mined. The reaction does not generate neutrons and produces only harmless helium, but it requires temperatures of about 3 billion degrees Celsius – 200 times hotter than the core of the Sun and 30 times hotter than needed to fuse DT. Researchers have already demonstrated that they can fuse protons and boron by using particle beams aimed at a solid target or by blasting plasma with a laser. Now, a team has done it – at least on a small scale – using a conventional fusion reactor, called the Large Helical Device (LHD), at Japan’s National Institute of Fusion Science. The group reports its work today i Nature Communication.

The LHD, which began operating in 1998, is shaped like a twisted donut and contains electromagnets containing the super-ionized fuel, called plasma. This type of device, called a stellarator, is not designed to operate at the temperatures required for proton-boron fusion. In the experiments, boron plasma was heated to 20 million degrees Celsius or so and beams of neutral hydrogen atoms were sent into the plasma. Proton-boron fusion produces high-speed helium atoms, and helium sensors, developed by TAE, registered 150 times more hits with boron plasma in the machine than when there was a non-reactive gas – a sign that fusion was taking place.

Computer simulations by the team suggested this amounted to about 5 trillion fusion reactions per second. While this may sound like a lot, Whyte says it equates to about 7 watts of power, a tenth of what a candle flame produces. Beyond that, says Whyte, most of those reactions were caused by the particle beams. In many fusion reactors, particle beams are used to get the overall plasma temperature hot enough for more extensive fusion. But the LHD results suggest that fusion was only happening at the few hot spots where the beams hit the plasma, not anywhere else, says Whyte, because the rate of fusion slows down rapidly 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—and some extra to harvest for electricity. The LHD is a long way off, but TAE believes it can get there with a very different plasma device. Various TAE test beds have created a “smoke ring” of plasma that is stabilized and heated by particle beams. TAE’s largest machine to date, called Norman, reached a temperature of 60 million degrees Celsius for 30 milliseconds.

The twisted, spiral shape on the inside of the Large Helical Device
Researchers say trillions of proton-boron fusion reactions took place every second within the complex bowels of Japan’s Great Helical Gear.National Institute of Fusion Science, Japan/Science Source

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 traditional DT fusion. Within the next ten years, the company wants to build a more powerful machine – Da Vinci – that could approach the temperature of proton-boron.

A reactor on protons and boron would eliminate many of the challenges engineers face in trying to move fusion from a scientific demonstration to a practical electricity generator. The US National Ignition Facility made headlines last year after demonstrating a “breakthrough”: a fusion reaction induced by powerful lasers that produces more heat than the lasers pumped in. However, turning that explosive type of fusion reactor into a power plant can be difficult. The international ITER reactor under construction in France aims to demonstrate a more stable and furnace-like approach. But it won’t show a gain until the end of the next decade—when some scientists worry it will begin to deplete most of the world’s tritium supply.

ITER also has a thick concrete shield to protect operators from neutrons. In a commercial reactor, running around the clock, those neutrons would also damage the structure of the reactor and shorten its working life. Studies are underway to find hard neutron materials for reactors, but no obvious candidates have yet been identified.

Whyte says that neutrons are a huge challenge for traditional fusion, but he thinks that getting plasma to temperatures measured in the billions may be just as difficult. Even if TAE does exist, each proton-boron reaction yields only a tenth of the energy from fusing deuterium and tritium. To make it worthwhile, proton-boron fusion would require “strong engineering advantages,” Whyte says.

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