DIII-D tokamak.jpg
I recently blogged about cutting edge research into stabilizing fusion plasmas being carried out the U.S. Department of Energy Princeton Plasma Physics Laboratory. It turns out that another project at the PPPL is making news.
One of the main current approaches to generating fusion power is the tokamak design. This is a donut shaped chamber surround by powerful magnets. A plasma is subjected to enormous heat and pressure in the tokamak to cause fusion in the plasma. One of the big problems with tokamaks is instabilities in the plasma that may cause it to contact the sides of the chamber and quench any fusion reaction.
There are sudden bursts of heat that can occur in tokamaks that can damage the walls of the confinement chamber. These bursts of heat are called “edge localized modes.” Recently researchers at the PPPL have observed a “possible and previously unknown process” that can trigger ELMs.
An experimental physicist named Ahmed Diallo and a theoretical physicist named Julien Dominski who work at the PPPL have teamed up to analyze data from the DIII-D tokamak at the National Fusion Facility run by General Atomics for the DoE in San Diego. They have found a trigger for a particular type of ELM that does not fit into current models of the ways in which ELMs can destabilize tokamak plasmas.
The findings of this research team could illuminate a variety of mechanisms that can cause ELMs. This, in turn, could lead to new tools for suppressing ELMs. Understanding the physics of ELMs is critical to the development of commercial nuclear fusion reactors which could provide abundant, cheap power.
The research team at PPPL made their discoveries when they were studying puzzling data returned by probes that detect the fluctuation of magnetic fields and plasma density during DIII-D tokamak experiments. It turned out that ELMs appeared during periods when the plasma was unusually quiet. Diallo said that “These were special cases that didn’t follow the standard models.” Dominski said, “It was a most interesting collaboration.
During six months of research, the two physicists at the PPPL discovered correlations of fluctuations in the DIII-D that had not been seen before. These correlations showed the formation of two modes or waves at the edge the plasma coupled together to create a third mode. This new mode moved towards the wall of tokamak which resulted in bursts of low-frequency ELMs.
This type of ELM has also been seen in the Joint European Torus (JET) in the United Kingdom. It has also been seen in the ASDEX Upgrade in Germany and other tokomaks following quiet periods. It is possible that these results could also be applied to systems such as solar flares and geomagnetic storms.
The research team at PPPL have discovered and reported on a method for triggering ELMs but they did not completely explain the process they discovered. They need to analyze more data from tokamaks. Diallo said, “If we can fully understand how the triggering works, we can block and reverse it.”