Experimental study of drift wave–streamer system in linear magnetized plasmas
直線磁化プラズマにおけるドリフト波・ストリーマー系の実験研究
Takuma Yamada
山田 琢磨Graduate School of Frontier Sciences, The University of Tokyo
5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
東京大学新領域創成科学研究科 〒277-8561 千葉県柏市柏の葉5-1-5Itoh Research Center for Plasma Turbulence, Kyushu University
6-1, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
九州大学伊藤極限プラズマ研究連携センター 〒277-8561 福岡県春日市春日公園6-1
By using a poloidal multi-channel probe array and a radial mobile probe, a streamer (state of bunching ofdrift waves) in a linear cylindrical magnetized plasma was identified for the first time. It was revealed thatthe streamer was produced by nonlinear mode coupling process, that is, the nonlinear phase locking
between drift waves and a certain mediator mode. By investigating the radial structures of the streamer andmediator, the streamer had a radially elongated structure, while the mediator had a phase node in the radialdirection. These features agreed well with the pair of fast and slow modes derived by a nonlinear
Schrödinger equation based on the Hasegawa-Mima model.

1. Introduction
For understanding anomalous transports inmagnetized fusion plasmas, study of drift wave
turbulence has been an important subject.
Theories and simulations have predicted that thenonlinear couplings between the drift waves cangenerate meso-scale structures, such as zonal
flows, zonal magnetic fields, and streamers,
which should have a strong influence on theanomalous transports [1]. Recently, experimental
approaches are also in progress, and haveobserved nonlinear couplings between drift wave
turbulence and meso-scale structures.
2. Experimental Results
The first experimental identification of astreamer (state of bunching of drift waves) in a
linear cylindrical magnetized plasma wasachieved [2]. The streamer is a poloidally
localized and radially elongated global structurethat lives longer than the characteristic turbulencecorrelation time. By using a 64-channel poloidal
probe array [3] and a radial mobile probe, it wasrevealed that the streamer was produced by
nonlinear mode coupling process, that is, thenonlinear phase locking between drift waves anda certain mediator mode, which has the samewave number and frequency with the streamer
structure. Figure 1 shows the spatiotemporal of
the streamer structure and mediator component
measured with the poloidal probe array. The
envelopes of the bunching of waves and themediator have the same structures. By applying

two-dimensional (frequency and poloidal wavenumber) bi-spectral analysis, it was confirmedthat the carrier waves of the streamer and themediator were strongly coupled [3].

Fig.1. (a) Spatiotemporal behavior of the fluctuation
measured with the poloidal probe array. (b) Frequency
component under 1.6 kHz extracted from (a).

be longer than the time lag. Therefore, therelation between the time window and time lag
was investigated in detail and it was showed that
this problem would be solved by choosing proper
time window [6].
Acknowledgments
This work was supported by a Grant-in-Aid for
Scientific Research of JSPS (21224014).

Fig.2. (a) Two-dimensional power spectrum of thefluctuation. A and B are the linear instability modes, C is
the mediator for the streamer, and C–O are thequasi-modes.

Furthermore, the radial structures of the streamer
and mediator were investigated by calculating thebi-phase between the mediator observed by thepoloidal probe array and the carrier wave
observed by the radial mobile probe. As a result,the streamer had a radially elongated structure,
while the mediator had a phase node in the radialdirection. These features agreed well with the pairof fast and slow modes derived by a nonlinear
Schrödinger
equation
basedon
theHasegawa-Mima model [4].
The two-dimensional bi-spectral analysis
showed another results. By the analysis, theformation of the plasma turbulence was regarded
as a result of nonlinear interaction of a small
number of parent modes, which were driven bylinear instabilities such as drift mode. As isshown in Fig. 2, the two-dimensional power
spectrum showed three original parent modes,
around ten quasi-modes, and broadband
fluctuations. The two-dimensional bi-spectral
analysis identified the nonlinear couplings
between coherent–coherent, coherent–broadband
and broadband–broadband components [5]. To
identify the parent modes, amplitude correlation
technique was applied. This technique simplydeduces the causal relation by only measuring thetime lag between two modes. This technique hada problem that the evaluation of amplitude of themode requires signal in a time window, which canReferences[1] P. H. Diamond, S.-I. Itoh, and K. Itoh: Modern
Plasma Physics Vol. 1 (Cambridge University Press,2010) Chaps. 5, 7, and 8.
[2] T. Yamada et al.: Nature Phys. 4 (2008) 721.
[3] T. Yamada et al.: Rev. Sci. Instrum. 78 (2007)
123501.
[4] T. Yamada et al.: Phys. Rev. Letter 105 (2010)
225002.
[5] T. Yamada et al.: Phys. Plasmas 17 (2010) 052313.
[6] T. Yamada et al.: J. Phys. Soc. Jpn. 79 (2010)
085001.