  Pair cascades in polar cap regions of neutron stars are considered to be an
essential process in various models of coherent radio emissions of pulsars. The
cascades produce pair plasma bunch discharges in quasi-periodic spark events.
The cascade properties, and therefore also the coherent radiation, depend
strongly on the magnetospheric plasma properties and vary significantly across
and along the polar cap. It is furthermore still uncertain from where the radio
emission emanates in polar cap region.
  We investigate the generation of electromagnetic waves by pair cascades and
their propagation in the polar cap for three representative inclination angles
of a magnetic dipole, $0^\circ$, $45^\circ$, and $90^\circ$.
  2D particle-in-cell simulations that include quantum-electrodynamic pair
cascades are used in a charge limited flow from the star surface.
  We found that the discharge properties are strongly dependent on the
magnetospheric current profile in the polar cap and that transport channels for
high intensity Poynting flux are formed along magnetic field lines where the
magnetospheric currents approach zero and where the plasma cannot carry the
magnetospheric currents. There, the parallel Poynting flux component is
efficiently transported away from the star and may eventually escape the
magnetosphere as coherent radio waves. The Poynting flux decreases faster with
the distance from the star in regions of high magnetospheric currents.
  Our model shows that no process of energy conversion from particles to waves
is necessary for the coherent radio wave emission. Moreover, the pulsar radio
beam does not have a cone structure, but rather the radiation generated by the
oscillating electric gap fields directly escapes along open magnetic field
lines in which no pair creation occurs.
