New observation of surface pre-breakdown of polymethyl-methacrylate in vacuum with a scanning electron microscope

Unlike the normally observed surface discharging radial streaks, which signify a fast discharging process, curved discharging streams and a halo are observed on a polymethylmethacrylate (PMMA) sample surface by the realization of controlled discharging with a scanning electron microscope (SEM). These experimentally observed discharging curved streams and halo, and some other features in the SEM image, reveal the nature of slow discharging and shed some light on the understanding of the initiation and propagation of discharging, for which disputed models exist