The Discovery of New Turbulence Transition in Fusion Plasmas
In the Large Helical Device (LHD), turbulence is suppressed at a specific condition, using precision laser diagnostics. Furthermore, a comparison experiment of hydrogen plasma and deuterium plasma, and simulations on the Raijin supercomputer reveal that turbulence suppression occurs when the turbulence mode changes. Our findings are expected to be applied to the establishment of innovative operation scenarios for fusion power plants and their design for turbulence suppression.
Fusion energy is released when two light nuclei combine to form a single heavier one (nuclear fusion reaction). Fusion energy-based power generation (in a fusion power plant) uses the energy generated when deuterium and tritium combine to form helium. A nuclear fusion reaction does not produce carbon dioxide. In addition, since it is possible to extract fuel materials from sea water, fusion energy is regarded as a sustainable energy source, and research into its practical application has been progressing rapidly in recent years. To initiate a fusion reaction, deuterium and tritium must be heated to over 100 million degrees Celsius to form plasma, which is then maintained by a strong magnetic “cage”. However, when turbulence is excited in the plasma, the plasma flows out of the magnetic “cage”. Therefore, turbulence is an important topic in fusion research, and its suppression is essential to the realization of a fusion power plant. For turbulence suppression, an understanding of the physical mechanism of turbulence excitation is essential, and LHD is the perfect device with which to tackle this challenge. For example, generally, turbulence measurement is not easy, but we have successfully measured not only its amplitude but also its spatial profile and propagation direction, using precision laser diagnostics. Moreover, these series of experiments were conducted during a period of deuterium experimentation (2017-2022) in LHD, and the ion-mass dependence of turbulence was investigated.
A research group performed experiments in which the density (amount of electrons and ions) of the hydrogen plasma was changed under identical heating conditions to gain a comprehensive understanding of turbulence in LHD. At the same time, the turbulence was measured in detail. As a result, it was found that turbulence is most suppressed at a certain density (transition density), and below the transition density, it decreases with increasing density (purple region in the figure on the right), but above the transition density, it begins to increase (orange region in the figure on the right). Furthermore, it was observed that the turbulence propagation direction reverses after the transition density. This result implies that the turbulence nature changes around the transition density (turbulence transition).
Then, to corroborate the turbulence transition, simulations were performed on the Raijin supercomputer. As a result, we found that the turbulence observed below the transition density was mainly caused by the ion-temperature gradient, while that above it was mainly caused by the pressure gradient and plasma resistivity. Moreover, we identified that this change in the turbulence was an important physical mechanism of turbulence suppression. Thus, by making full use of experiments and simulations, we revealed that the observed turbulence suppression was due to a change in the turbulence mode, i.e., turbulence transition.
In addition, experiments under the same conditions were conducted in deuterium plasmas and compared with hydrogen plasmas. As a result, we found that turbulence transitions occurred at higher density for the deuterium plasma, i.e., turbulence is suppressed at higher density. Furthermore, surprisingly, the turbulence observed above the transition density was clearly suppressed in deuterium plasmas. The turbulence suppression observed at high density in deuterium plasmas implies that it will be further suppressed in the higher-density and heavier-mass deuterium/tritium mixture plasmas envisioned for fusion power generation. This is a favorable result for early realization of fusion power generation.
This work was carried out by a research group led by Professor Kenji Tanaka (National Institute for Fusion Science, National Institutes on Natural Sciences), Assistant Professor Toshiki Kinoshita (Research Institute for Applied Mechanics, Kyushu University) and Professor Akihiro Ishizawa (Graduate School of Energy Science, Kyoto University) et al..
This research result was published in Physical Review Letters, a journal by the American Physical Society, on June 7, 2024.