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Takeshi Ido, Dynamics of energetic particle-driven oscillatory zonal flow in toroidal plasmas, 第38回 プラズマ・核融合学会, 2021.11. |
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Takeshi Ido, Akihide Fujisawa, Keiji Takemura, Naohiro Kasuya, Shigeru Inagaki,
Atsushi Fukuyama, Yoshihiko Nagashima, Chanho Moon, Kotaro Yamasaki,
Takuma Yamada, Yusuke Kosuga, Makoto Sasaki
, Design of Heavy Ion Beam Probes on the PLATO tokamak, High-Temperature Plasma Diagnostics Conference, 2020.12, Plasma turbulence plays a decisive role in determining dynamics of plasmas not only in laboratories but also in the space. Especially, recent studies have indicated the importance of not only local but also global properties of the turbulence such as symmetry breaking and multi-scale interaction. In order to explore dynamics of plasmas through measuring plasma turbulence precisely and globally, a new tokamak named Plasma Turbulence Observatory (PLATO) is being constructed in Kyushu University.
One of the key diagnostic systems for the PLATO project is Heavy Ion Beam Probe (HIBP), by which the electric potential, its fluctuations, density fluctuations, and magnetic fluctuations are measured directly and simultaneously without any perturbation to the plasmas. The HIBPs are installed in two toroidal sections to observe long range correlation in the toroidal direction. In addition, two energy analyzers in a poloidal cross section to observe the poloidal asymmetry and the correlation in the poloidal direction. The measurable area covers almost the whole plasma with a circular poloidal cross section and approximately 80 % of a poloidal cross section of the plasma with \kappa of 1.5 and \delta of 0.5, where \kappa is the ellipticity and \delta is the triangularity. The beam energy is 10 – 50 keV for Rubidium (Rb) beam or 50 – 150 keV for Sodium (Na) beam. According to a numerical calculation under the condition in which the central electron density is 1x10^19 (m^-3) and the density profile is parabolic, the detected beam intensity will be sufficient for micro-turbulence measurement in the region from the normalized minor radius(rho) of 0.2 to the plasma edge by Rb beam and in whole measurable region by Na beam. Therefore, we will be able to investigate nature of turbulence through multi-scale and simultaneous measurement.
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Takeshi Ido, Akihiro Shimizu, Masaki Osakabe, Kimitaka Itoh, Maxime Lesur, Kunihiro Ogawa, Hao Wang, Makoto Sasaki, Yusuke Kosuga, Shigeru Inagaki, Sanae.–I. Itoh, and the LHD Experiment Group, Nonlinear wave-particle interaction in magnetized high temperature plasmas confined in Large Helical Device, 3rd Asia-Pacific Conference on Plasma Physics, 2019.11, Wave-particle interactions are ubiquitous in plasmas, and they are observed as interesting phenomena. In this presentation, observed subcritical instability driven by nonlinear wave-particle interaction in magnetized plasmas will be shown.
In magnetized high temperature plasmas confined in the Large Helical Device (LHD), it is observed that an instability named geodesic acoustic mode(GAM) is excited abruptly by energetic particles existing through the inverse Landau damping. The frequency of this instability usually increases with the time scale of a few milliseconds, and the temporal evolution of the frequency reflects the evolution of the velocity distribution which is a result of wave-particle interaction.
When the frequency of the GAM reaches twice the ordinary GAM frequency and the amplitude of the GAM exceeds a threshold, another GAM is abruptly excited with the shorter time scale of 1 ms or less. The phase relation between the originally-existing GAM, hereinafter referred to as the primary mode, and the abruptly excited GAM, hereinafter referred to as the secondary mode, is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. These behaviors can be interpreted as the excitation of the subcritical instability of the secondary mode through nonlinear wave-particle interaction triggered by the primary mode. Abrupt excitation phenomena have been wildly observed in laboratory plasmas (e.g. sawtooth oscillation and disruption) and astro-plasmas(e.g. solar flare), and subcritical instabilities are one of the working hypotheses of the onset of abrupt phenomena. The finding of the abrupt excitation of the GAM and the understanding of the phenomena as the subcritical instability demonstrate an experimental path to the understanding of the physical mechanism of the onset of the abrupt phenomena.. |
