Gyrokinetic simulations of zonal flows and ion temperature gradient turbulence in HL-2A ITB plasmas

Abstract

The characteristics of zonal flows (ZFs) in ion temperature gradient (ITG) turbulence during the formation of internal transport barrier (ITB) have been investigated by nonlinear gyrokinetic simulations for the HL-2A tokamak experiment. The turbulent ion heat transport and zonal flow dynamics are investigated in the local turbulence limit for a neutral beam heated L-mode plasma. Linear stability analyses have shown that the maximum growth rate, γmax, is decreased across the whole confinement region during the formation of ITB although the critical parameter, ηi, is increased, which is identified to be due to the stabilizing of ITG with an increased ion-to-electron temperature ratio τ. The entropy generated by ion heat flux is significantly decreased together with the enhanced ZF amplitude and reduced ion heat transport when ITB has been fully developed, especially the modes with intermediate radial wavenumbers, implying that the long and medium radial scale turbulences are strongly suppressed by the ZF shear. Meanwhile, the long-range correlation and relative energy of the self-generated ZF are increased while the turbulent energy is decreased when ITB is triggered, indicating that the ZF gains more energy from background turbulence. It is found that the ratio between τ and ηi is a key parameter in determining the ZF shearing rate ωE×BZF and γmax. The value of ωE×BZF>γmax occurs around τ/ηi > 1.4, which is suggested to be responsible for the reduction of ion heat transport and hence the ITB formation.