  The charging of insulating samples degrades the quality and complicates the
interpretation of images in scanning electron microscopy and is important in
other applications, such as particle detectors. In this paper we analyze this
nontrivial phenomenon on different time scales employing the
drift-diffusion-reaction approach augmented with the trapping rate equations
and a realistic semi-empirical source function describing the pulsed nature of
the electron beam. We consider both the fast processes following the impact of
a single primary electron, the slower dynamics resulting from the continuous
bombardment of a sample, and the eventual approach to the steady-state regime.
