Description
Controlled excitation of zonal flows (ZFs), their interaction with the mean flow, and modulation of mean-flow-driven fluctuations, have been investigated in the cylindrical magnetized plasma device named IMPED. The experiments are carried out at a fixed magnetic field of 550 G, while systematically varying the magnetic field ratio Rm = Bm/Bs and the neutral gas pressure. At low neutral pressure (2×10−5 mbar), where mean
flow is negligibly small, radial localization of Low-Frequency ZFs (0.2–1 kHz), density and temperature gradients shifts towards the edge when Rm is decreased. Drift-wave (DW) (5–10 kHz) and electron-temperature-gradient (ETG) (400–500 kHz) turbulence are observed, which follows the peak ZF power, peak density and temperature gradients. Power fraction analysis indicates that ZFs are dominantly driven by DW turbulence
through asymmetric Reynold stress and partially by ETG turbulence. At higher neutral pressure (1 × 10−4 mbar), mean flow develops. The mean flow shear peaks around 4
cm and drives a conventional Kelvin–Helmholtz instability (KHI) of 5.6 kHz. This mode coexists with a ZF (∼700 Hz) and envelope analysis shows that KHI is strongly modulated by the ZFs. At peak ZF location (around 2 cm), another KHI mode of 2.4 kHz mode is observed. This is also modulated by the ZF and the ZF phase consistently leads that of
KHI phase indicating energy transfer from the ZF to the KHI. This suggests the observed KHI is of a tertiary-type KHI driven by ZF shear. Thus, there exists conventional type KHI mode driven by mean-flow shear, and tertiary type KHI driven by ZF shear in the system. Time-averaged particle flux measurements reveal nearly zero net transport at the ZF peak, while the region of peak mean-flow shear exhibit inward particle transport. This is supported by near-zero skewness at the ZF peak (no net transport), negative skewness at the peak mean flow shear (hole-dominated inward transport), and positive skewness at the plasma edge (blob-like outward transport) in the measured PDF of density fluctuations. These observations indicate that mean flow plays a more crucial role than ZFs in suppressing outward particle transport