In an experimental study of the runaway electron generation during major disruptions in the HuanLiuqi-2A (commonly referred to as HL-2A) [L. W. Yan, Nucl. Fusion 51, 094016 (2011)] tokamak, detailed time and space resolved x-ray images of the long-lived runaway electron beam in flight have been observed and these allow a detailed analysis of the generation and evolution of the disruption produced runaway electron beam in a major disruption, where the conversion efficiency of pre-disruption plasma current into runaway current is up to 55% on HL-2A tokamak. Moreover, a delay of about 7 ms between the start of the disruption and the formation of runaway electron beam has been found. With the aid of the Equilibrium FIT (EFIT) code, magnetic configuration reconstruction has made possible a detailed observation of the magnetic flux geometry evolution during major disruptions. The EFIT magnetic configuration reconstructions show that the delay is due to the transient strong deformation of the magnetic configuration in the initial stage of the current quench, which may provide a possibility of suppressing or mitigating the runaway electron beam during this period by massive gas injection or other methods.

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 ⩽ ρ ⩽ 0.5, ρ 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.