  In Ruderman & Sutherland (RS75) model, the normal neutron stars as pulsars
bear a severe problem, namely the binding energy problem that both ions (e.g.,
${}_{26}^{56}$Fe) and electrons on normal neutron star surface can be pulled
out freely by the unipolar generator induced electric field so that sparking on
polar cap can hardly occur. {\bf This problem could be solved within the
Partially Screened Gap (PSG) model in the regime of neutron stars}. However, in
this paper we extensively study this problem in a bare strange quark star (BSS)
model. We find that the huge potential barrier built by the electric field in
the vacuum gap above polar cap could usually prevent electrons from streaming
into the magnetosphere unless the electric potential of a pulsar is
sufficiently lower than that at infinite interstellar medium. Other processes,
such as the diffusion and thermionic emission of electrons have also been
included here. Our conclusions are as follows: both positive and negative
particles on a BSS's surface would be bound strongly enough to form a vacuum
gap above its polar cap as long as the BSS is not charged (or not highly
negative charged), and multi-accelerators could occur in a BSS's magnetosphere.
Our results would be helpful to distinguish normal neutron stars and bare quark
stars through pulsar's magnetospheric activities.
