Effects of temperature gradient driven turbulence and core MHD instability on particle transport in HL-2A L-mode plasmas

Experiments in L-mode plasmas in the HL-2A tokamak show that the electron and trace Al impurity transport is related to the normalized electron temperature gradient R/L-Te with opposite trends. Increasing R/L-Te in the confinement zone (0.25 <= rho <= 0.5, rho is the normalized minor radius) tends to pump out electrons but accumulate impurity, leading to slightly hollow electron density profiles meanwhile with an overall stronger inward convection of injected impurity. Moreover, after the injection of Al impurity by laser blow-off (LBO), the impurity density profiles remain deeply hollow due to the combined effect of the impurity expulsion by core magneto-hydrodynamic (MHD) activity in the plasma centre and the inward impurity convection driven by turbulence in the outer confinement region. Gyrokinetic simulation results reveal that the local change of R/L-Te affects the relative strength of the trapped electron mode (TEM) and the ion temperature gradient (ITG) mode, eventually determining the electron particle transport in a way consistent with the experimental observations. Comparison of simulations with and without collisionallity shows that the plasma collisionality can significantly reduce the growth rate of the TEM and the associated outward particle flux.