  We propose a new mechanism to screen the electric field in the pulsar polar
cap. Previous studies have shown that if an electron beam from the stellar
surface is accelerated to energies high enough to create electron-positron
pairs, the required electric field parallel to the magnetic field lines is too
strong to be screened out by the produced pairs. We argue here that if
non-relativistic protons are supplied from the magnetosphere to flow towards
the stellar surface, they can provide an anode to screen out such a strong
electric field. Injected electron-positron pairs yield an asymmetry of the
electrostatic potential around the screening point. The required pair creation
rate in this model is consistent with the conventional models.
