  The problem of formation of the Ruderman-Sutherland type inner vacuum gap in
neutron stars with ${\bf\Omega}\cdot{\bf B}<0$ is considered. It is argued by
means of the condition $T_i/T_s>1$ (where $T_i$ is the critical temperature
above which $^{56}_{26}$Fe ions will not be bound at the surface and $T_s$ is
the actual temperature of the polar cap surface heated by the back-flow of
relativistic electrons) that the inner vacuum gap can form, provided that the
actual surface magnetic field is extremaly strong ($B_s\gtrsim 10^{13}$ G) and
curved (${\cal R}<10^6$ cm), irrespective of the value of dipolar component
measured from the pulsar spin down rate. Calculations are carried out for
pulsars with drifting subpulses and/or periodic intensity modulations, in which
the existence of the quasi steady vacuum gap discharging via ${\bf E}\times{\bf
B}$ drifting sparks is almost unavoidable. Using different pair-production
mechanisms and different estimates of the cohesive energies of surface iron
ions, we show that it is easier to form the vacuum gap controlled by the
resonant inverse Compton scaterring seed photons than by the curvature
radiation seed photons.
