Electrons passing through a negatively biased grid due to collective trapping

Grids are widely used in plasma experiments for various purposes. For example, the various plasma waves are either excited or detected by grids. Grids of different mesh size are also used as electromagnetic screen of an electron plasma, wave exciter. In electrostatic energy analyzers and ion sources grids serve also for the separation of one charge carrier species from the other. In both double and triple plasma devices the grids are used in order to cut out the electron coupling between the chambers. Usually it is assumed that a negatively biased grid reflects all electrons if two conditions are fulfilled: (i) the mesh size is smaller than Debye length and (ii) the potential of the grid (VG) has to be enough negative so that: |VG|>ε/e, where ε is the kinetic energy of the most energetic electron in the system. In this paper, experimental results are presented which show that a considerable number of electrons is able to pass through a grid in spite of both of the above conditions being fulfilled. (author) 9 refs., 4 figs