Vital information related to the way electrically charged gas called “plasma” flows along the edge of doughnut-shaped fusion devices known as “tokamaks” has been unearthed by physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).
The outcomes of the study signify a promising sign for the advancement of machines to generate fusion energy for producing electricity without generating long-term hazardous waste.
The outcome partially confirms past PPPL discoveries that the width of the heat exhaust generated by fusion reactions could be wider by six times, and hence less narrow, damaging, and concentrated than had been considered earlier. “These findings are good news for ITER,” stated C.S. Chang, PPPL physicist and lead author of a description of the study in Physics of Plasmas, talking about the international fusion experiment that is being constructed in France. “The findings show that the heat exhaust in ITER will have a smaller chance of harming the machine,” added Chang.
During fusion—the power driving the stars and the sun, light elements in the form of plasma fuse together to produce energy, where plasma is the hot, charged state of matter formed of free electrons and atomic nuclei. Researchers across the globe are making efforts to reproduce fusion on Earth for a nearly inexhaustible supply of power to produce electricity.
The superhot plasma inside the tokamaks have the ability to reach hundreds of millions of degrees and is confined by magnetic fields that maintain the plasma from the walls of the machines. However, heat and particles can escape from the confined fields at the “magnetic separatrix”—the boundary between the magnetically unconfined and confined plasmas. At this boundary, the field lines cross at what is called the X-point, the site at which the waste particles and heat escape and hit a target known as the “divertor plate.”
The most recent outcomes demonstrate the stunning effect of the X-point on the exhaust by illustrating that a hill-like bump of electric charge appears at the X-point. The plasma is made to circulate around it by the electrical hill, stopping plasma particles from moving between the downstream and upstream areas of the field lines in a straight path. By contrast, similar to cars that move around a construction site, the charged plasma particles take a detour around the hill.
These results were produced by the researchers using XGC, an advanced computer code created with external collaborators at PPPL, which models the plasma as a collection of individual particles and not as a single fluid. The model demonstrates that the connection between the downstream plasma located below the X-point and the upstream plasma above the X-point formed in a way not forecast by simpler codes. The model results in more accurate predictions about the exhaust and renders future large-scale facilities less vulnerable to internal damage.
“This result shows that the previous model of the field lines involving flux tubes is incomplete,” stated Chang—speaking about the tubular areas around the regions of magnetic flux—“and that the current understanding of the interaction between the upstream and downstream plasmas is not correct. Our next step is to figure out a more accurate relationship between the upstream and downstream plasmas using a code like ours. That knowledge will help us develop more accurate equations and improved reduced models, which in fact are already in progress.”
This study was supported by the DOE’s Office of Science. Co-authors of the Physics of Plasmas paper are PPPL physicists Seung-Hoe Ku and Randy Michael Churchill. Computations were carried out on leadership-class supercomputers at the Oak Ridge Leadership Computing Facility, the Argonne Leadership Computing Facility, and the National Energy Research Scientific Computing Center (NERSC), all DOE Office of Science user facilities.