What makes fusion so difficult? You have to make the fuel very hot and keep it hot. The fuel is too hot to put inside a container, so you have to contain it in a very powerful magnetic field. Even when you have generated this magnetic 'cage' and made it stable, there is something that lets the heat leak out of this cage and prevents you from make the fuel hot enough... turbulence. Turbulence is a chaotic mess of fluctuations and whirling vortices which flings the heat out from the plasma.
My work is focussed on eliminating turbulence. The figure above shows a calculation of the highest temperature you can achieve without generating turbulence.
In our usual picture of turbulence (pictured in the video), small fluctuations within the plasma are driven unstable by the enormous pressure gradients, and grow to become strong waves, eventually interacting with each other, and breaking each other up to form turbulence. However, sometimes one can get turbulence even if the small fluctuations are stable... This is known as subcritical turbulence.
I am investigating subcritical turbulence within a plasma, and trying to determine when it is present.
Tools to Simulate Turbulence
The turbulent vortices that convect heat out of the plasma are elongated, following the magnetic field lines which wrap around the toroidal plasma. We use a coordinate transform (pictured) to compress these structures into a numerical box to make simulations of them possible. Over the past two decades the tools used to study these vortices have improved beyond all recognition. We can now simulate, within a few hours, the effects of fully saturated nonlinear turbulence.
The challenge now is to do that in a matter of minutes, to make possible an investigation of the effects of turbulence in many different possible scenarios.