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In nuclear fusion research it is critical to understand how radio frequency electromagnetic waves travel or propagate inside the turbulent interior or a nuclear fusion reactor in order to maintain an efficient and continuously operating fusion reactor. RF waves are transmitted by an antenna in the donut-shaped vacuum chamber that is a common component of a magnetic confinement device known as a tokamak. The waves heat the plasma fuel in the reactor and drive its current around the toroidal interior of the tokamak. The efficiency of this process can be influenced by how the wave’s trajectory is altered or scattered by the conditions inside the reactor.
Researcher have attempted to investigate these RF wave processes utilizing sophisticated computer simulations to match the experimental conditions recorded in the fusion reactors. A good match between these two would tend to validate the computer model. This would raise confidence in using the computer simulations to explore new physics. It would also help design future RF antennas that would perform more efficiently. Current simulations can accurately calculate how much total current is driven by the RF waves. However, they do a poor job in predicting exactly where in the plasma this current is produced.
MIT researchers have just published a new paper in the Journal of Plasma Physics. In the paper, the researchers have suggested that existing computer models of RF simulations have not properly taken into account the manner in which these waves are scattered as they encounter dense turbulent filaments present in the edge of the plasma. This edge is known as the “scrape-off layer (SOL).
Bodhi Biswas is a graduate student at the Plasma Science and Fusion Center (PSFC) under the direction of Senior Research Scientist Paul Bonoli with the School of Engineering, Distinguished Professor of Engineering Anne White and Principal Research Scientist Abhay Ram who is the lead author of the journal article. Ram compares the scattering of the plasma that takes place in this situation to a water wave hitting a lily pad. He said, “The wave crashing with the lily pad will excite a secondary, scattered wave that makes circular ripples traveling outward from the plant. The incoming wave has transferred energy to the scattered wave. Some of this energy is reflected backwards (in relation to the incoming wave), some travels forwards, and some is deflected to the side. The specifics all depend on the particular attributes of the wave, the water, and the lily pad. In our case, the lily pad is the plasma filament.”
Up to the present, fusion researchers have not taken these filaments and their patterns of scattering into consideration when modeling the turbulence inside a tokamak. This has led to a serious underestimation of wave scattering. Biswas used data from PSFC tokamak Alcator C-Mod to show that employing the new method they developed for modeling RF-wave scattering from SOL turbulence provides results that are considerably different from the older models. They provide a much better match with the data derived from the operation of the tokamak. The “lower-hybrid” wave spectrum is crucial to driving plasma current in a steady state tokamak. It appears to scatter asymmetrically which is an important effect that is not accounted for in previous models.
Please read Part 2