For 70 years, fusion has been promoted as the ultimate solution to the energy crisis – but achieving it on the scale required has proven elusive. One reason is an anomaly in spherical tokamak test reactors, when additional input power can cause temperatures to fall, or more commonly flatline, instead of rising as intended. New research provides a possible explanation, meaning one fewer obstacle to commercial fusion.
Fusion power involves forcing small atoms (like hydrogen) together so they become larger ones, losing tiny quantities of mass in the process which convert to immense amounts of energy as the formula e=mc2 describes. We know it's possible because a giant fusion reactor crosses our skies every day, and we can see thousands of more distant ones at night. Indeed, making fusion in the lab is so easy a 13-year-old has done it at home.
Sustained fusion that releases more power than must be put in is a different matter. For that, we need something similar to the conditions at the heart of the Sun. Immense temperatures are one component; a paper in Physical Review Letters offers an explanation of one factor impeding their production.
By far, the most popular idea of producing sustained fusion involves using magnetic fields to confine plasma at temperatures no solid container would survive, and equally immense pressures
The most common design for this involves a toroidal tokamak – that is, a donut shape. Frustrated with these devices' slow progress, some fusion scientists turned to spherical tokamaks instead. These still have a cylindrical hole in their center, but surround it with a ball instead. Debate continues as to which design is best. However, enough people see spherical tokamaks as likely to be more cost-effective that several have been built in this format, including the National Spherical Torus Experiment, where a problem was first encountered.
“Normally, the more beam power you put in the higher the temperature gets,” Dr Stephen Jardin of the Princeton Plasma Physics Laboratory said in a statement. That seems like common sense – we might not expect a linear relationship, but surely more beam power has to raise temperatures at least a bit. Yet, for over a decade, physicists have been reporting the opposite, and proposed explanations have proven insufficient to explain the observations.
Jardin and co-authors have performed computer modeling showing that power input above a certain level increases confining magnetic “surfaces” breaking up, making the constrained plasma unstable and leading to increased movement. As the plasma shifts around, it sheds energy, sometimes overcoming the extra power being put in.
"The results indicate that when designing and operating spherical tokamak experiments care must be taken to ensure that the plasma pressure does not exceed certain critical values at certain locations in the [facility]," Jardin said. "And we now have a way of quantifying these values through computer simulations."
Whether the required power can be applied without these problems emerging – and if not, how it can be resolved – remains to be seen, but knowing the cause at least means there is a chance of solving it.
Although the problem has not been seen as clearly in traditionally-shaped tokamaks, the authors note there have been reports of what could be related issues in at least five toroidally-shaped research tokamaks.
This is just one of the many obstacles to fusion reactors that produce more power than they take to build and run. Despite decades of hype, working reactors, let alone cost-effective ones, are a very long way off, but it might mean one fewer hurdle still before us.