Close control of plasma rotation improves stability in fusion reactors

A real-time map of plasma stability could be key to maximising power output in nuclear fusion

Keeping a nuclear fusion plasma stable is probably the most important aspect of controlling these potential future energy sources. In the type of reactor under the most intense investigation around the world, plasmas are heated to millions of degrees centigrade and induced to circulate at high speeds around doughnut-shaped reactor vessels. But if the plasma becomes unstable, it can collide with the interior walls of the reactor, which reduces the plasma temperature – stopping the fusion reactions – and also damages the reactor walls.

Stability map of fusion plasma in NSTX. Blue is stable and red is unstable. As the plasma decreases collisionality and increases rotation in time it transitions into an unstable region
Stability map of fusion plasma in NSTX. Blue is stable and red is unstable. As the plasma decreases collisionality and increases rotation in time it transitions into an unstable region

Previous plasma physics theory held that making the plasma rotate would keep it stable. However, Jack Berkery and Steve Sabbagh of Columbia University, who work at the Princeton Plasma Physics Laboratory (PPPL) in San Jose, California, have discovered that the situation is more complicated. While some plasmas become unstable if they rotate too fast, others are stable at much lower rotation speeds. The key is to keep the rotation and another property, known as collisionality, within a favourable range, they say. Collisionality, which relates to the frequency with which particles bounce off each other, was thought to reduce stability as it rises, but this is not the case according to Berkery and Sabbagh.

Spherical tokomak: a more efficient and potentially cheaper way to induce fusion
maintaining stability in  fusion plasmas – such as this one inside a spherical reactor in the UK – will be vital to maximising energy output

The researchers have developed a “stability map” that allows a plasma to be monitored in real-time. The map plots collisionality against rotation, to a 1000th of a second resolution, and determines how close it is to instability. One such map, created in experiments at the National Spherical Torus Experiment (NSTX) at PPPL, is shown above. Over time, collisionality decreases and rotation increases, leading the plasmas become less stable. Control engineers on magnetic fusion reactors could use such maps to tune the behaviour of plasma, ensuring that it stays within a stable region where its power output is maximised, the researchers have explained at a meeting of the American Physical Society’s plasma physics division.