Investigation of zonal flows by using the collective scattering measurement of density fluctuations

Abstract

The poloidal × E B r T rotation velocities in the core plasma region are studied using the instantaneous frequency method (IFM) with the density fluctuations measured by the CO 2 laser collective scattering diagnostics on the HT-7 tokamak. A coherent mode is observed in the fluctuations of poloidal velocities with the mode frequency from 10 to 20 kHz. It is identified as geodesic acoustic mode (GAM) zonal flow with poloidal symmetry (m = 0) and its mode frequency coinciding with the theoretical expected GAM frequency. The nonlinear interactions are investigated by applying the envelope analysis on the density fluctuations. The results confirm that the envelope modulation in the high frequency density fluctuations only comes from the shearing by GAM. The comparison between IFM and envelope analysis is also discussed.

Study of turbulence modulation and core density peaking with CO2 laser collective scattering diagnostics in the EAST tokamak

“…Research on several tokamaks and stellerators in various plasma conditions has shown that the fluctuation velocity is often dominated by the E × B term [27][28][29][30][31]. But the hypothesis of negligible phase velocity is not completely trivial in some cases, especially in the confinement mode transition [25] and in Ohmic confinement with low injected torque [32,33].…”

“…where S k (t, f) is the spectrogram of turbulence at wave number k, and the integrated power of turbulence at k can be calculated by S k (t) = S k (t, f). Some research on poloidal velocity has already been carried out with the diagnostics during steady-state discharges [31,36].…”

“…The intensites of the turbulence (S k=12 cm −1 , S k=22 cm −1 ) in two radial regions (core ρ = 0 − 0.4, outer ρ = 0.4 − 0.8) are measured simultaneously by the CO 2 laser collective scattering diagnostics in the EAST tokamak. During steady discharge, the frequency of the turbulence ω k under a fixed scale is assumed to be constant, and the effect of turbulent phase velocity on the calculation of core poloidal speed is negligible [31]. The mean poloidal rotation speed in the core is employed to describe the change of V θ .…”

Calibration of phase contrast imaging on HL-2A Tokamak

“…Keen tools are important to diagnose turbulent parameters in wave-number domain. A conventional diagnostic is far-forward laser scattering [5,6]. But this diagnostic is not very suitable for low k circumstances, and the usage of this method is strongly limited by the inevitable refraction of the incident laser beam as the great improvement of plasma density in tokamaks.…”

Optical path design of phase contrast imaging on HL-2A tokamak

“…The turbulent energy spectrum will show an exponential decay as the increasing of k, which means that we should investigate not only the turbulence with high k such as the electron temperature gradient mode [1] but also that with low k such as the ion temperature gradient mode [2] and trapped electron mode [3]. The far-forward laser scattering diagnostic [4][5][6] which can measure density fluctuations with high k, is not suitable for low k circumstances for the limited scattering angle. Phase contrast imaging (PCI) [7], which can diagnose long wavelength density fluctuations, is a keen tool to investigate the wide-spectrum characteristics and is developed in many magnetic confinement devices, such as DIII-D [8], Alcator C-Mod [9], LHD [10], TEXT-U [11].…”