In the world of renewable energy, perhaps there is no goal more ambitious than fusion energy. This involves fusing hydrogen atoms to form helium – a process that generates an invalid amount of energy as a result. It’s a reaction that happens every moment in the sun, but replicating it on Earth is a tedious and rare process. However, if we are successful, we will have access to a clean source of renewable electricity that meets our growing energy needs.
To this end, the researchers are pursuing a phenomenon called “ignition,” which is when a fusion reactor generates more energy than is needed to create the initial reaction. A few major attempts are underway to achieve this goal, including the International Thermonuclear Experimental Reactor (ITER) in France. This effort uses powerful magnets in a machine called a tokamak to create superheated plasma created using hydrogen fuel.
But here’s the catch: There’s only so much hydrogen fuel you can put in a tokamak before everything starts to go wrong.
“One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it,” said Paolo Ricci, a researcher at the Swiss Plasma Center. He said in a press release. “From the early days of fusion, we’ve known that if you try to increase the density of the fuel, at some point there will be what we call a ‘turbulence’ – you basically lose the entrapment completely, and the plasma goes where it is.”
To solve this problem, the scientists began looking for different equations to measure the maximum amount of hydrogen you can put inside the tokamak before the break. One of the laws that has stuck to it and has become a mainstay in the world of fusion research is known as the “Greenwald Limit,” which states that the amount of fuel a tokamak can use is directly related to the radius of the machine. The researchers behind ITER even built their devices based on this law.
But, even Greenwald’s limit wasn’t perfect.
“The Greenwald limit is what we call an ‘experimental’ law or limit, which basically means that it’s like a general rule based on observations made on previous experiments,” Alex Zilstra, an experimental physicist at Lawrence Livermore National Laboratory in California, told The Daily Beast at e-mail. “These are very useful, but we always need to be careful when applying them outside of circumstances where we have data from trials.”
That’s why Ritchie and his team challenged this firm belief in new paper Published on May 6 in the magazine Physical review letters. In it, they hypothesized that the Greenwald limit could in fact be raised by nearly twice as much — nearly twice the amount of hydrogen fuel that would go into a tokamak to produce plasma. Their findings could lay the groundwork for future fusion reactors like DEMO – a successor to the ITER reactor currently under development – to finally reach ignition.
“This is important because it shows that the intensity you can achieve in a tokamak increases with the power you need to run it,” Ritchie said. “In fact, DEMO will operate at much higher power than current tokamaks and ITER, which means you can add more fuel density without reducing the output, unlike Greenwald’s Law. And this is very good news.”
Zylstra thinks the team’s discovery is important because it sheds light on why fusion reactors have limits, too. It also states that tokamak designs such as ITER or DEMO can be “less restrictive than previously thought”. With twice the fuel density, this can dramatically improve the power output of the tokamak – and finally get us to ignite.
“Fusion is a very challenging problem – both scientifically and technologically speaking, and making the power of fusion a reality requires many advances step by step at a time,” Zilstra added. “If this study is further validated, especially on machines like ITER, it will certainly help the magnetic fusion community to design and improve future designs for experimental facilities and power generation.”