Experimental Investigation of Mechanism of Zonal Flow Generation in Drift Wave Turbulence
Feng-Jen Chang1*, Eiichirou Kawamori1
1Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan
* Presenter:Feng-Jen Chang, email:r03222021@ntu.edu.tw
Zonal flows (ZFs), which are laminar shear flows fed by ambient drift wave (DW) turbulence, are ordered structures ubiquitously observed in magnetically confined plasmas. In toroidal plasmas including tokamaks for nuclear fusion researches, ZFs have a fine scale structure in the radial direction, while extended in the toroidal and the poloidal directions. Thus ZFs contribute to suppression of the radial particle transport. Consequently, the investigation of the generation mechanisms of ZFs is of essential importance. To explore the mechanism of zonal flows in our magnetic mirror plasma experiment, we controlled the DW systems by varying the configuration of magnetic fields. A poloidal Langmuir probe array was used to measure the temporal and the angular structure of the DWs. The power spectra of potential fluctuations showed existence of low-frequency (< 1 kHz) modes, along with the broadened spectrum (FWHM ≈ 1-3 kHz) at a higher frequency range (2-10 kHz). The low-frequency oscillations had zero mode number in the poloidal direction, which is an essential feature of ZFs. In addition, we observed variation of central frequencies and linewidths of the high-frequency components for varied mirror ratio and strength of the background magnetic field, indicating that the underlying mechanisms were associated with the flute modes and the universal modes. The investigation of the DWs' structures along the field direction and the radial direction is ongoing to probe the morphology of the fluctuations, which is expected to clarify whether the low-frequency modes have the features of ZFs. Bi-spectral analysis is also applied to the ongoing experiments for investigation of the wave-wave couplings of the DWs, which are considered as key mechanisms of generation of ZFs.

Keywords: Mirror Confinement, Drift Wave Turbulence, Zonal Flow, Flute Mode