Porous structure can strongly change the electric behavior of material submitted to bombardment by energetic electrons: strong electric fields (10 V/m) can grow within the pores, because of the small sizes of these pores. We give a model of porous material that enables us to emphasize and illustrate the enhancement of the yield of secondary-electron emission (and the broadening in frequency of its profile) in a porous medium, compared to that in bulk material. Here we have considered materials of astrophysical interest [graphite, iron, aluminum (A1203), and silica]. The enhancement is characterized by two emission peaks for small energies of the primary electrons and by an increase of the yield roughly by a factor of 4 for large energies ( & 1 keV). PACS number{s): 79.20.Hx, 96.50.Dj, 95.30.Qd

The structure of the sheath surrounding a cylindrical or spherical body immersed in an isotropic collisionless plasma is investigated for a wide range of parameters corresponding to regions in space where the electron photoemission by the surface can produce small, strong, or even dominant effects (in this last case, the photoemission is also assumed to have cylindrical or spherical symmetry). The collected currents are also considered.

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