We study plasma flows above pulsar polar caps using time-dependent simulations of plasma particles in the self-consistent electric field .
The flow behavior is controlled by the dimensionless parameter \alpha = j / c \rho _ { GJ } where j is the electric current density and \rho _ { GJ } is the Goldreich-Julian charge density .
The region of the polar cap where 0 < \alpha < 1 is a “ dead zone ” — in this zone particle acceleration is inefficient and pair creation is not expected even for young , rapidly rotating pulsars .
Pulsars with polar caps near the rotation axis are predicted to have a hollow-cone structure of radio emission , as the dead zone occupies the central part of the polar cap .
Our results apply to charge-separated flows of electrons ( j < 0 ) or ions ( j > 0 ) .
In the latter case , we consider the possibility of a mixed flow consisting of different ion species , and observe the development of two-stream instability .
The dead zone at the polar cap is essential for the development of an outer gap near the null surface \rho _ { GJ } = 0 .