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The β-induced Alfvén eigenmode (BAE) excited by energetic electrons has been identified for the first time both in the Ohmic and electron cyclotron resonance heating plasma in HL-2A. The features of the instability, including its frequency, mode number, and propagation direction, can be observed by magnetic pickup probes. The mode frequency is comparable to that of the continuum accumulation point of the lowest frequency gap induced by the shear Alfvén continuous spectrum due to finite β effect, and it is proportional to Alfvén velocity at thermal ion β held constant. The experimental results show that the BAE is related not only with the population of the energetic electrons, but also their energy and pitch angles. The results indicate that the barely circulating and deeply trapped electrons play an important role in the mode excitation.

In our previous letter, the geodesic acoustic mode (GAM) induced by energetic particles (EGAMs) was reported in low density ohmic plasma on HL-2A (Chen et al 2013 Phys. Lett. A 377 387). We extend the experimental results of the EGAM mode in this paper. During strong tearing modes (TMs), the beta-induced Alfvén eigenmodes (BAEs) and EGAM-induced density fluctuations are firstly measured by microwave Doppler reflectometers with different work frequencies. As predicted by theory, the measurements of magnetic probes and Doppler reflectometers suggest the EGAM magnetic oscillations have poloidal/toroidal mode numbers of m/n = 2/0, and are localized in the core with a broad radial structure. The mode frequency is less than that of the conventional GAM (i.e. fEGAM/fGAM < 1), and is constant in the radial direction. Our experimental results suggest that a density limit exists for the excitation of the EGAM in the ohmic plasma, and the density limit is improved with electron cyclotron resonance heating + neutral beam injection heating on HL-2A. The auto and cross squared bicoherences of magnetic and density fluctuations indicate that intense nonlinear interactions exist among EGAM, BAEs and strong TMs. These new observations will help us to understand the underlying physics mechanism for the excitation of fluctuations in the sub-Alfvén frequency range.

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