Vanishing of Dimits Shift in Realistic Fusion Plasmas with Negative Magnetic Shear
arxiv(2024)
摘要
This study employs gyrokinetic simulations to investigate ion temperature
gradient (ITG) turbulence in realistic fusion plasmas featuring reverse
magnetic shear. Negative magnetic shear is found to suppress the ITG
instability due to the scarcity of mode rational surfaces, as evidenced by a
comparison of instabilities for different magnetic shears. This suppression
effect remains observable in nonlinear turbulence with zonal flow artificially
eliminated, where the emergence of turbulence solitons aligns with mode
rational surface peaks. However, the suppression effect diminishes in the
presence of self-consistently generated zonal flow, along with the occurance of
turbulence solitons. The zonal flow is found to originated from a force driven
process by the primary instability, instead of the conventional modulational
instability. The study further reveals a remarkable phenomenon that the Dimits
shift no longer exists for negative magnetic shear, which are attributed to the
weakness of zonal flow around marginal stability. However, away from marginal
stability, the turbulent transport is primarily regulated by the zonal flow
regardless of different magnetic shears.
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