Experiments and simulation of helium breakdown at high pressure and temperature in a non-uniform electric field

•This paper first time measures and simulates the breakdown voltages of helium for a millimeter-scale needle-plate gap at high temperature and pressure (25–180 ℃, 0.01–7 MPa).•Two-dimensional drift-diffusion plasma model is used in COMSOL Multiphysics to carry out the simulation and the results are analyzed.•The effects of non-uniform property of the electric field are presented through the hump and corona stability. Comparisons of the breakdown voltages/field strength with the uniform breakdown are presented.•Time evolution of the current and particle number densities are presented corresponding to different pressures, indicating the key role that penning ionization plays in high-pressure helium discharge.