7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Wave number analysis of meso-scale structures in linear plasmas
T. Yamada1, M. Sasaki2, H. Arakawa3, K. Kikuta4, T. Nishizawa5, C. Moon5,
Y. Nagashima5, Y. Kosuga5, N. Kasuya5, A. Fujisawa5
1
Faculty of Arts and Science, Kyushu University
2
College of Industrial Technology, Nihon University
3
Faculty of Medical Science, Kyushu University
4
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
5
Research Institute for Applied Mechanics, Kyushu Universitye-mail (speaker): takuma@artsci.kyushu-u.ac.jp

Meso-scale structures such as streamers and zonal
flows are produced by nonlinear interaction between
microscopic drift wave turbulence in fusion plasmas.
Zonal flows suppress the radial transport and are wellstudied these days. In other hand, although streamers
enhance the radial transport, to study the generation andsuppression of the streamer structures is important inplasma physics and fusion reactors.
By using the linear plasma device, LMD-U in Kyushu
University, our group succeeded in finding the streamer
structure and its mediator mode for the first time [1].
Owing to the low temperature property, linear plasmas
are well used for basic study of plasma turbulence. In theprevious studies, our group clarified the cross-sectional
structures of the streamer, its mediator mode, and carrier
drift waves [2]. The mediator mode is the trigger for thestreamer generation, and nonlinearly couples with thecarrier drift waves to create the streamer structure.
Several Langmuir probes arranged in the axial direction
including the 64-channel poloidal probe array, andbi-spectral and bi-phase analyses were used to observe
the three-dimensional phase structures. The results werewell compared with theoretical [3] and numerical [4]works. While the streamer structure and carrier drift
waves were radially elongated, the mediator mode had anode in the radial direction. Additionally, the axial
dimensional research revealed that while the carrier drift
waves had an axial mode number one (propagation
direction from the end to the source), the streamer andmediator were revealed to have an axial mode numberzero [5].
To compare the observed meso-scale structures withtheoretical and numerical studies more precisely in thephase space, local information of the wave number isrequired. In this study, we tried to observe the local wave
number of plasma turbulence for phase space analysis.
By applying a window function to the observed dataarray of 64-channel poloidal probe array before theFourier analysis, nearly local information of the wave
number of which the window function is open can bederived. Figure 1 shows the result. Time evolution of theamplitude of poloidal mode number three when astreamer structure was observed is shown in the figure.
The windows are phase shifted for –π/2, –π, –3π/2,Figure 1. Time evolution of the amplitude of poloidal
mode number 3. Window function is applied in thepoloidal direction. The windows are phase shifted for–π/2, –π, –3π/2, compared to the top figure.
compared to the top figure. The streamer is a bunch ofwaves in the poloidal direction and poloidally rotateswith frequency of 1.2 kHz in our linear plasma. In thefigure, we can clearly see that the amplitude of mode
number three is poloidally localized, and the localized
region is rotating in the poloidal direction with thefrequency of 1.2 kHz.
References[1] T. Yamada et al., Nat. Phys. 4, 721 (2008).
[2] T. Yamada et al., Phys. Rev. Lett. 105, 225002 (2010).
[3] K. Nozaki, T. Taniuti, and K. Watanabe, J. Phys. Soc.
Jpn. 46, 991 (1979).
[4] N. Kasuya, M. Yagi, K. Itoh, and S.-I. Itoh, Phys.
Plasmas 15, 052302 (2008).
[5] T. Yamada et al., J. Phys. Soc. Jpn. 87, 034501 (2018)