Nuclear fusion is a process that combines light elements in a plasma which is a hot, charged state of matter composed of free electrons and atomic nuclei. Physicists are working to create controlled nuclear fusion in a stable process that releases large amounts of energy. This technology would be safer and cheaper than using nuclear fission to generate electricity. An added benefit is that fusion would not create radioactive waste.
One of the most studied technologies for the production of nuclear fusion is called a tokamak. It is a donut shaped machine that can be found in research laboratories all over the world. In a tokamak, a plasma is confined by powerful magnetic fields that must be able to deal with disruptions that can be more powerful than hurricanes.
Recent research at U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) has found that some of the forces released by disruptions cause unexpected effects. The results of the PPPL research will needed to be incorporated in future tokamak development including the huge international ITER reactor being built in France. Some of the forces studied by the PPPL team could result in serious damage to tokamaks unless they are compensated for.
There are two types of processes in a tokamak that can produce "vertical displacement events" (VDEs) which are disruptions that cause the problematic forces. There are “eddy” currents that swirl around the inner walls of the tokamak and “halo” currents that enter and exit the walls of the tokamak. The halo currents can increase in strength without increasing the total forces that are hitting the walls of a tokamak. The simulations at PPPL utilized the PPPL M3D-C1 code. They showed that as halo currents increase they are unexpectedly offset by a decrease in the eddy current. This is similar to the way that credits and debits offset each other in a bank ledger.
Cesar Clauser is a PPPL post-graduate fellow who led the research that was reported in the journal Nuclear Fusion. He said, “What we found was that changing the halo current doesn't affect the total vertical force. This was a surprising and interesting result.”
The researchers at PPPL intend to compare their sophisticated models with the simplified models that the team constructing ITER uses to calculate disruptive forces. PPPL physicist Nate Ferraro is a coauthor of the paper with PPPL physicist Stephen Jardin. Ferraro said, “One implication to draw from our study is that measuring the halo current could be a proxy for the total forces. This could lead to a more complete understanding.”
The advanced PPPL code M3D-C1 uncovered the intimate relationship between the eddy and halo current forces in tokamak plasmas. Their research confirmed that the halo current does not affect total vertical forces. Clauser said, “The simulations covered a wide range of halo current cases since we wanted to look for the worst-case scenario.” The two-dimensional simulations that were used by the ITER team analyzed the total forces that are produced the eddy and halo forces in the walls of the ITER. There will be more three-dimensional studies used to model the distribution of forces to find if there are paths for halo currents that will not be offset by eddy currents.