Turbulent fluctuations within a quasi-coherent frequency range (peak frequency of ∼130 kHz, Δf ≈ tens of kHz) are modulated by the rotation of low frequency tearing modes in the core of HL-2A ohmic plasmas. The quasi-coherent modes (QCMs) emerge outside the island boundary as the island O-point passes by in the large island cases (W > 4.5 cm), where the local electron temperature profile is steepened. Statistical analysis shows that for the QCM excitation, a threshold value of temperature gradient is identified and the QCM is solely observed in low density discharges, consistent with the linear ohmic confinement regime. These experimental evidence suggests that the observed QCMs are driven by the trapped electron mode, in agreement with linear stability calculations. Cross-correlation analysis reveals that the QCMs have long-range poloidal correlations and radially propagate outwards. Besides, the bispectral analysis indicates: (i) there exists non-linear coupling between the tearing mode and micro-turbulence, most significantly in the QCM frequency range; (ii) the QCMs nonlinearly couple by themselves to excite the second harmonic, whereas no non-linear interaction is observed between the QCMs and ambient turbulence.
Observation and characterisation of trapped electron modes in Wendelstein 7-X