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Journal Articles

Bispectral analysis applied to coherent floating potential fluctuations obtained in the edge plasmas on JFT-2M

Nagashima, Yoshihiko*; Ito, Kimitaka*; Ito, Sanae*; Fujisawa, Akihide*; Hoshino, Katsumichi; Takase, Yuichi*; Yagi, Masatoshi*; Ejiri, Akira*; Ida, Katsumi*; Shinohara, Koji; et al.

Plasma Physics and Controlled Fusion, 48(4), p.S1 - S15, 2006/04

 Times Cited Count:34 Percentile:75.65(Physics, Fluids & Plasmas)

This paper presents the results of bispectral analysis of floating potential fluctuations in the edge region of ohmically heated plasmas in the JFT-2M tokamak. Inside of the outermost magnetic surface,coherent modes were observed around the frequency of geodesic acoustic mode which is a kind of the zonal flow. The squared bicoherence shows significant nonlinear couplings between the coherent fluctuations and the background fluctuations (which are likely to contain drift wave turbulent fluctuations). The experimental results that the total bicoherence is proportional to the squared amplitude of the coherent fluctuation, and that the biphase of the coherent modes localizes around a constant value $$pi$$, are consistent with the theoretical prediction on the drift wave - zonal flow systems based on the Hasegawa-Mima model.

Journal Articles

Observation of nonlinear coupling between small-poloidal wave-number potential fluctuations and turbulent potential fluctuations in Ohmically heated plasmas in the JFT-2M tokamak

Nagashima, Yoshihiko*; Hoshino, Katsumichi; Ejiri, Akira*; Shinohara, Koji; Takase, Yuichi*; Tsuzuki, Kazuhiro; Uehara, Kazuya; Kawashima, Hisato; Ogawa, Hiroaki; Ido, Takeshi*; et al.

Physical Review Letters, 95(9), p.095002_1 - 095002_4, 2005/08

 Times Cited Count:99 Percentile:92.97(Physics, Multidisciplinary)

By a reciprocating electrostatic probe, two types of low frequency coherent electrostatic modes (about1kHz and 10-15kHz) are found,for the first time, in the edge region of ohmically heated plasmas in the JFT-2M tokamak. These modes exists in the last closed surface. The higher frequency mode has features of the geodesic acoustic mode (GAM), though the low frequency mode is not identified yet. A bispectral analysis revealed that significant nonlinear couplings (parametric modulation) between these coherent fluctuations and broadband background turbulent potential fluctuations occur. In the other words, the GAM and the back ground turbulence are modulated by the newly found low frequency electrostatic mode (further, the GAM modulates the background turbulence, also) for the first time. The newly found low frequency mode (frequency about 1kHz) is supposed to contribute to the turbulent diffusion nonlinearly.

Journal Articles

Reflectometry study of mode coupling in fusion plasma turbulence

Bruskin, L. G.; Mase, Atsushi*; Oyama, Naoyuki; Shinohara, Koji; Miura, Yukitoshi

Plasma Physics and Controlled Fusion, 45(7), p.1227 - 1245, 2003/07

 Times Cited Count:7 Percentile:23.5(Physics, Fluids & Plasmas)

The capability of microwave reflectometry to estimate the nonlinear coupling in the plasma turbulence is theoretically investigated. The full-wave 2D model of microwave scattering in plasma is applied to simulate the reflectometer response to the turbulent fluctuations, consisting of random and coherent modes, and the simulated data ensemble is further processed to obtain the auto bicoherence function. Sensitivity of the method was analyzed for different ambient plasma profiles, geometry, fluctuation spectra and amplitudes.

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