Plasma and Fusion Research

Volume 3, S1010 (2008)

Regular Articles

Bispectral Analysis of Harmonic Oscillations Measured using Beam Emission Spectroscopy and Magnetic Probes in CHS

Tetsutarou OISHI, Shinichiro KADO¹⁾, Mikirou YOSHINUMA, Katsumi IDA, Tsuyoshi AKIYAMA, Takashi MINAMI, Kenichi NAGAOKA, Akihiro SHIMIZU, Shoichi OKAMURA and CHS group

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
1): High Temperature Plasma Center, The University of Tokyo, 5-3-5 Kashiwanoha, Kashiwa 277-8568, Japan

(Received 16 November 2007 / Accepted 19 February 2008 / Published 5 August 2008)

Abstract

The coherent MHD oscillation, which consists of the fundamental frequency of several kilohertz and its higher harmonics, (harmonic oscillation: HO) has been observed in Compact Helical System. HO consists of two pairs of harmonic series. One is located in the core region near the ι = 0.5 rational surface (denoted as “HO (core)”), the other is located in the edge region near the ι = 1.0 rational surface (denoted as “HO (edge)”). In the present study, bispectral analysis is applied to the fluctuation data, for which HO is measured by beam emission spectroscopy (BES) and using magnetic probes. The analysis has revealed that fundamental mode of HO in both the magnetic and core density fluctuations have phase correlation with the harmonics including fundamental oscillation, while HO in edge density fluctuation does not have such phase correlation. Mode numbers of HOs are identical for harmonic components having different frequencies, i.e., m/n = -2/1 for HO (core) and m/n = -1/1 for HO (edge). It suggests that the generation of harmonics cannot be interpreted simply as mode coupling because the summation rule for the wavenumber is not satisfied, even though the bicoherence value is significant. The bicoherence value and relative amplitude of higher harmonics correlate with each other, which suggests that bicoherence indicates the degree of distortion of the signals.

Keywords

bispectral analysis, bicoherence, fluctuation, magnetohydrodynamics, edge harmonic oscillation, Compact Helical System, beam emission spectroscopy, edge transport barrier

DOI: 10.1585/pfr.3.S1010

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This paper may be cited as follows:

Tetsutarou OISHI, Shinichiro KADO, Mikirou YOSHINUMA, Katsumi IDA, Tsuyoshi AKIYAMA, Takashi MINAMI, Kenichi NAGAOKA, Akihiro SHIMIZU, Shoichi OKAMURA and CHS group, Plasma Fusion Res. 3, S1010 (2008).