The Princeton Plasma Physics Laboratory (PPPL), U.S. Department of Energy (DOE), has refined the use of magnetic field to increase the performance of tokamaks, which are doughnut-shaped fusion plants. This improved technique prevents internal damage from instabilities called "edge localized modes" (ELMs). Tokamaks can operate longer without halting.
Ricardo Shousha is a Princeton University graduate student who works in the plasma control group. He is also affiliated with PPPL. Shousha is the principal author of the paper in Physics of Plasmas.
The power behind the sun and stars is fusion. Fusion is the combination of light elements in the form plasma. This is the hot, charged state composed of free electrons, atomic nuclei, and that generates huge amounts of energy. Scientists are trying to recreate fusion on Earth to provide an inexhaustible supply for electricity generation.
To study the conditions in which plasma's center becomes dense and hot, the researchers used KSTAR (Korea Superconducting Tokamak Advanced Research) facility. H-mode is the desirable state. This happens when the center of plasma becomes hotter than the edge. Scientists want plasma in H-mode to produce more efficient fusion reactions. Because the temperatures and densities of these two regions are so different, ELM instability can form along the boundary. This is similar to thunderstorms that form when cold and hot fronts meet. These instabilities can lead to spurts that look like solar flares.
These events can cause damage to the internal walls and other components of tokamaks. The machine will need to be shut down to repair. ITER, a multinational tokamak in Cadarache France, is at greater risk. This device will produce plasma with much more heat and power that current tokamak plasmas.
The dilemma is for physicists. The plasma should be in H mode, but this can lead to instability that could cause damage to the tokamak. Shousha and other researchers used magnetic fields to reduce instabilities. This method was discovered in 2003. By allowing particles to flow across the boundary, the applied fields reduce instabilities. One side effect of the applied fields is that plasma cools and fusion reactions become less effective.
This problem was solved by the research team using magnets and a feedback system. This combination allows for the determination of the weakest magnetic field to suppress ELMs and minimizes how the fields affect H-mode conditions. Shousha stated, "That's what we did in our research."
These results were possible because of the hard work of graduate students and the international network of institutions and researchers. Egemen Kolemen is an associate professor at Princeton University's Mechanical and Aerospace Engineering department and co-author of this paper. He has a joint appointment in Princeton University and PPPL. They can conduct experiments in any country around the globe, including the United States, China Germany, South Korea, and Germany. And they can also control powerful machines. They do it very well. We can help them as long as they have the will to do so.
Scientists plan to improve their system to detect ELM signals and prevent them from occurring. Shousha stated that if the precursor signals can be detected quickly, it will be possible to take action and possibly prevent the ELM from appearing.