  The formation and evolution of coherent structures in a low-energy electron
beam produced in a Malmberg-Penning trap is investigated by means of CCD
diagnostics. The electrons are emitted from a thermionic cathode and their
energy is controlled by an acceleration grid. By varying the spatial
distribution of the energy of emitted electrons, different space charge effects
are observed, as, e. g., a sharp or a gradual transition to a space charge
dominated regime. The variation of the coherent structures along the beam is
studied by varying the electron density or/and the value of the confined
magnetic field. The observed processes are interpreted using a tridimensional
particle-in-cell code which solves the Vlasov-Poisson system in zeroth order
drift approximation.
