In the world of renewable energy, there is perhaps no more ambitious goal than fusion energy. This involves fusing hydrogen atoms together to create helium – a process that generates an incredible amount of energy. It’s a reaction that happens every moment in the sun, but replicating it on Earth is a much more arduous process. However, if we succeed, we will have a clean source of renewable electricity that meets our ever-growing energy needs.
To that end, researchers are chasing a phenomenon called “ignition,” in which a fusion reactor generates more energy than was needed to create the initial reaction. Several major efforts are underway to achieve this goal, including the International Thermonuclear Experimental Reactor (ITER) in France. That effort uses powerful magnets in a machine called a tokamak to create superheated plasma made with hydrogen fuel.
But therein lies a problem: there’s only so much hydrogen fuel you can put in a tokamak before everything goes horribly wrong.
“One of the limitations of making plasma in a tokamak is the amount of hydrogen fuel you can inject into it,” Paolo Ricci, a researcher at the Swiss Plasma Center, said in a press release. “Since the early days of fusion, we’ve known that if you try to increase the fuel density, at some point there would be a ‘disruption’ — basically you lose the confinement completely and plasma goes everywhere.”
To solve this problem, scientists began exploring several equations to measure the maximum amount of hydrogen you can put in a tokamak before disturbance. A law that eventually stood and became a mainstay in the world of fusion research is known as the “Greenwald Limit,” which says the amount of fuel that can be used in a tokamak is directly correlated to the radius of the machine. The researchers behind ITER have even built their machine based on this law.
But even the Greenwald limit wasn’t perfect.
“The Greenwald limit is what we call an ’empirical’ limit or law, which basically means it’s a rule of thumb based on observations made in previous experiments,” Alex Zylstra, an experimental physicist at Lawrence Livermore National Laboratory in California, told The Daily Beast in an email. “These are very useful, but we always have to be careful when applying them outside conditions where we have data from experiments.”
That’s why Ricci and his team challenged this long-held belief in a new paper published May 6 in the journal. Physical assessment letters. In it, they argue that Greenwald’s limit can actually be increased — nearly doubling the amount of hydrogen fuel that can go into a tokamak to produce plasma. Their findings could lay the groundwork for future fusion reactors like DEMO – a successor to ITER currently under development – to finally ignite.
“This is important because it shows that the density you can achieve in a tokamak increases with the power you need to run it,” Ricci said. “Actually, DEMO will run at a much higher power than current tokamaks and ITER, meaning you can add more fuel density without limiting output, contrary to Greenwald’s law. And that is very good news.”
Zylstra believes the team’s finding is significant because it sheds light on why exactly fusion reactors also have such a limit. It also shows that the designs for tokamaks like ITER or DEMO “may be less constrained than previously thought”. Increasing fuel density by a factor of two could result in a massive boost in their power output from tokamaks — eventually triggering us to ignite.
“Fusion is an extremely challenging problem – both scientifically and technologically, and to realize fusion power, many advances are needed, one step at a time,” Zylstra added. “If this study is further validated, especially on machines like ITER, it will certainly help the magnetic fusion community to credibly design and optimize future designs for experimental and power generating facilities.”
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