Bispectral Analysis of Density and Potential Fluctuations in a High Neutral Density Cylindrical Plasma

Abstract

The structure of fluctuations in the Large Mirror Device-Upgrade (LMD-U) depends on the neutral density. In this study, we focus on the high neutral density plasma in which the potential and density fluctuation waveforms are nontrigonometric functions similar to sawtooth waves, and peaks in the power spectrum lie along a straight line when plotted as frequency versus wavenumber. Two-dimentional auto and cross bispectral analyses ofĨ is and/orṼ f , which consider both the matching conditions for both the azimuthal wavenumber and the frequency, have been performed; significant couplings are observed at particular peak frequencies. Similarities and differences between the auto and cross bicoherence ofĨ is and/orṼ f are discussed.

Dynamic interaction between a solitary drift wave structure and zonal flows in a linear cylindrical device

“…In the LMD, the coexistence, nonlinear couplings and energy transfer between drift waves and zonal flows have been shown [19,20]. In the LMD-U, which has an axial length twice of the LMD, the solitary drift waves [18,[21][22][23] and those spatio-temporal structure [24] have been observed at a fixed radial position.…”

Observation of Nonlinear Coupling between Low Frequency Coherent Modes and Background Turbulence in LMD-U

“…This dynamic behavior of fluctuations is linked to the triangle wave like shape of the raw signal. The existence of mutual nonlinear couplings among low frequency coherent modes ( f < 10 kHz) had been reported earlier [23]. In this article, the dynamic behavior of nonlinear couplings is analyzed by use of wavelet bispectral analysis.…”

“…A double loop antenna producing the RF helicon Argon plasma discharge (3 kW, 7 MHz) is installed on a quartz tube with a length of 400 mm and a diameter of 100 mm at one side of the vessel (a schematic is shown in ref [23]). The typical value of electron temperature and electron density in LMD-U is 2 − 3 eV and 1 × 10 −19 m −3 respectively.…”

“…A significantly strong coherent mode at f = 1.2 kHz, m = 1 (fundamental mode) is observed, besides higher harmonics up to m = 10. The results of spectral analyses (power spectrum, cross-coherence and cross-phase) of the m = 1 ion saturation current fluctuations and the m = 1 floating potential fluctuations are shown in [23]. The m = 1 mode has an amplitude ratio of ∼ 0.6, a cross-coherence of ∼ 1 and a cross-phase of ∼ π/6 between ion saturation current and floating potential, and is presumed to be a linearly unstable resistive drift wave.…”

Transition between continuous and discrete spectra of high-wavenumber turbulence in neutral gas pressure scan experiments on a linear magnetized plasma

“…The fact that the frequency of the nth peak is not an integer (n times) multiple of the first peak suggests that the discrete peaks do not appear due to self-coupling of the fundamental modes, as in the case of soliton and sawtooth waves [53]. Rather, the frequency span of the discrete peaks seems to be about ion cyclotron frequency f ci = 8.6 kHz.…”