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In tokamaks, it is well established that sheared flows mitigate turbulence that drives heat and particle transport, thereby providing a direct pathway to improved confinement. In particular, achieving the ITER steady-state and long-pulse high-performance operation relies on the internal transport barrier (ITB), a narrow region of steep pressure gradients that improves core plasma confinement.
Fishbone instability, one of the particular concerns in ITER hybrid scenarios, has been found to be beneficial for the formation of ITB in ASDEX Upgrade, HL-2A and EAST. One hypothesis is that fishbone-driven E×B sheared flows generated through the expulsion or redistribution of energetic particles may in turn affect turbulence. However, no experimental evidence has firmly substantiated the hypothesis due to the lack of direct flow diagnostics in the core region. Whether fishbone causes the ITB formation or not is still unclear.
In this work, we report the first direct experimental evidence of E×B sheared flows driven by fishbone instability. The flow exhibits a fine and radially reversed structure within the q=1 rational surface, rather than the global pattern reported in previous simulations [1]. Global nonlinear gyrokinetic simulations using the GTC demonstrate that the flow is driven by a zonal radial electric field generated by the instability. The simulated radial profiles agree quantitatively with Doppler reflectometry measurements [2]. Notably, the fine structure is determined by both the thermal ions and electrons rather than fast ions. The contributions from thermal ions and electrons to the radial electric field E_r are comparable in magnitude but opposite in direction. The zonal electron density response is therefore crucial, as it partially cancels the zonal ion density, reducing the net charge separation and thus significantly weakening the total amplitude of E_r.
[1] G. Brochard, C. Liu, X. Wei, et al., Saturation of fishbone instability by self-generated zonal flows in tokamak plasmas. Physical Review Letters 132, 075101(2024).
[2] Gao L, Liu A, Ding W, et al. Measurements of the E×B velocity fluctuation associated with the fishbone instability using the Doppler reflectometry on EAST[J]. Nuclear Fusion 65, 116019(2025).