In the hunt for fusion vitality, understanding how radio-frequency (RF) waves journey (or “propagate”) within the turbulent inside of a fusion furnace is essential to sustaining an environment friendly, constantly working energy plant. Transmitted by an antenna within the doughnut-shaped vacuum chamber widespread to magnetic confinement fusion gadgets known as tokamaks, RF waves warmth the plasma gas and drive its present across the toroidal inside. The effectivity of this course of may be affected by how the wave’s trajectory is altered (or “scattered”) by circumstances inside the chamber.
Researchers have tried to review these RF processes utilizing computer simulations to match the experimental circumstances. A superb match would validate the computer model, and lift confidence in utilizing it to discover new physics and design future RF antennas that carry out effectively. While the simulations can precisely calculate how a lot total present is pushed by RF waves, they do a poor job at predicting the place precisely within the plasma this present is produced.
Now, in a paper printed within the Journal of Plasma Physics, MIT researchers recommend that the fashions for RF wave propagation used for these simulations haven’t correctly taken into consideration the best way these waves are scattered as they encounter dense, turbulent filaments current within the fringe of the plasma often called the “scrape-off layer” (SOL).
Bodhi Biswas, a graduate pupil on the Plasma Science and Fusion Center (PSFC) underneath the route of Senior Research Scientist Paul Bonoli, School of Engineering Distinguished Professor of Engineering Anne White, and Principal Research Scientist Abhay Ram, who’s the paper’s lead writer. Ram compares the scattering that happens on this state of affairs to a wave of water hitting a lily pad: “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.”
Until now, researchers haven’t correctly taken these filaments and the scattering they provoke into consideration when modeling the turbulence inside a tokamak, resulting in an underestimation of wave scattering. Using knowledge from PSFC tokamak Alcator C-Mod, Biswas exhibits that utilizing the brand new methodology of modeling RF-wave scattering from SOL turbulence offers outcomes significantly totally different from older fashions, and a a lot better match to experiments. Notably, the “lower-hybrid” wave spectrum, essential to driving plasma present in a steady-state tokamak, seems to scatter asymmetrically, an necessary impact not accounted for in earlier fashions.
Biswas’s advisor Paul Bonoli is effectively acquainted with conventional “ray-tracing” fashions, which consider a wave trajectory by dividing it right into a sequence of rays. He has used this mannequin, with its limitations, for many years in his personal analysis to grasp plasma habits. Bonoli says he’s happy that “the research results in Bodhi’s doctoral thesis have refocused attention on the profound effect that edge turbulence can have on the propagation and absorption of radio-frequency power.”
Although ray-tracing remedies of scattering don’t totally seize all of the wave physics, a “full-wave” mannequin that does could be prohibitively costly. To clear up the issue economically, Biswas splits his evaluation into two elements: (1) utilizing ray tracing to mannequin the trajectory of the wave within the tokamak assuming no turbulence, whereas (2) modifying this ray-trajectory with the brand new scattering mannequin that accounts for the turbulent plasma filaments.
“This scattering model is a full-wave model, but computed over a small region and in a simplified geometry so that it is very quick to do,” says Biswas. “The result is a ray-tracing model that, for the first time, accounts for full-wave scattering physics.”
Biswas notes that this mannequin bridges the hole between easy scattering fashions that fail to match experiment and full-wave fashions which can be prohibitively costly, offering cheap accuracy at low price.
“Our results suggest scattering is an important effect, and that it must be taken into account when designing future RF antennas. The low cost of our scattering model makes this very doable.”
“This is exciting progress,” says Syun’ichi Shiraiwa, workers analysis physicist on the Princeton Plasma Physics Laboratory. “I believe that Bodhi’s work provides a clear path to the end of a long tunnel we have been in. His work not only demonstrates that the wave scattering, once accurately accounted for, can explain the experimental results, but also answers a puzzling question: why previous scattering models were incomplete, and their results unsatisfying.”
Work is now underway to use this mannequin to extra plasmas from Alcator C-Mod and different tokamaks. Biswas believes that this new mannequin will probably be significantly relevant to high-density tokamak plasmas, for which the usual ray-tracing mannequin has been noticeably inaccurate. He can be excited that the mannequin could possibly be validated by DIII-D National Fusion Facility, a fusion experiment on which the PSFC collaborates.
“The DIII-D tokamak will soon be capable of launching lower hybrid waves and measuring its electric field in the scrape-off layer. These measurements could provide direct evidence of the asymmetric scattering effect predicted by our model.”
Bodhi Biswas et al, A hybrid full-wave Markov chain strategy to calculating radio-frequency wave scattering from scrape-off layer filaments, Journal of Plasma Physics (2021). DOI: 10.1017/S0022377821001033
Massachusetts Institute of Technology
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Radio-frequency wave scattering improves fusion simulations (2021, November 11)
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