Gyrokinetic simulation of electrostatic turbulence in HL-2A ITB plasma

Comprehensive gyrokinetic simulations have been performed utilizing the numerical Lie transform code to investigate the dynamics of ion internal transport barrier (ITB) in the HL-2A tokamak, with a focus on the interaction between drift wave turbulence and zonal flow (ZF). The simulations reveal a self-regulatory cycle responsible for ITB sustainment, in which ion temperature gradient turbulence drives ZF that in turn suppresses the turbulence. Within this cycle, a key finding is that the ZF is primarily driven by the turbulent ion energy flux, rather than the poloidal Reynolds stress (PRS) as held in the classical predator–prey model. This result provides a self-consistent explanation for the experimentally observed weak effect of turbulent PRS on radial electric field in HL-2A and validates the ZF drive theory proposed by Wang et al in 2017. Furthermore, the ITB collapse is attributed to the transition to strong trapped electron mode (TEM) turbulence triggered by electron cyclotron resonance heating, as the short wavelength TEM turbulence is inefficient at driving ZF to suppress transport. Finally, we demonstrate that ITB recovery can be achieved by enhancing core ion heating, which simultaneously reduces the linear drive of instabilities and strengthens the nonlinear ZF shear. These findings offer a comprehensive physical picture of the ITB lifecycle and provide valuable insights for ITB control in fusion plasmas.