Research in stability of spherical tokamaks focuses on diagnosing the stable states of equilibrium of fusion plasmas in these devices and understanding the limits of the stability of those states.
Spherical tokamaks contain high-pressure, high-temperature plasmas using magnetic fields. Because of the extreme state of the plasma no material probes can be inserted to diagnose it’s conditions. Therefore the first step to understanding and analyzing these fusion plasmas is to recreate their equilibrium states using external diagnostics, such as magnetic field probes and laser temperature measurements, with complex computer codes.
Secondly, not all states of the plasma are stable; imbalances between the confining magnetic field and the plasma pressure can lead to explosive instabilities. Fusion scientists previously thought that making the plasma rotate would stabilize the plasma, but our research group discovered that there is a more complicated connection between rotation and stability. Some plasmas can become unstable when they rotate too fast, while others can maintain stability at lower rotation rates. When plasma rotation is kept in a favorable range, the charged plasma particles bouncing back and forth in the magnetic field can actually steal some of the energy from the rotational motion, which helps stabilize the plasma. A similar stability condition applies to the frequency with which particles collide and bounce off one another, a property termed their collisionality. Our research group has found that reduced collisionality, as will be found in future fusion plasmas, does not necessarily lead to reduced stability, overturning long-held beliefs on the effect of collisions on stability.
