  Electrons colliding with spatially fixed ions in strong laser fields are
investigated by solving the time-dependent Schr\"odinger equation. Considering
first simple one-dimensional model systems, the mechanisms and energy spectra
of fast electrons are analyzed, starting from collisions on a single ion. By
using these electrons as projectiles for a second and third collision, the
maximum possible energy obtained can be significantly increased. We then
generalize the analysis to 2D systems where additional angular degrees of
freedom lead to a drastic loss of efficiency. This problem can be overcome by
introducing external confinements, which allow to focus the electrons and
increase the intensity of high-energy electrons.
