  Ions, protons and possibly a small flux of electrons and positrons are
accelerated outward from the polar cap of a normal or millisecond pulsar whose
rotational spin is antiparallel with its magnetic moment. The Langmuir modes of
this relativistic plasma have several properties of significance for the origin
of coherent radio emission. The characteristics of the mode are determined by
the sequence of singularities in the dielectric tensor at real angular
frequencies, which in turn is fixed by the electron-positron momentum
distribution. We find that under a certain condition on its momentum
distribution, an electron-positron flux two orders of magnitude smaller than
the Goldreich-Julian flux stabilizes the plasma and extinguishes the mode. But
more generally, both the growth rate and wavenumber of the multi-component
Langmuir mode can be as much as an order of magnitude larger than those of the
two-component ion-proton mode. It appears to be a further effective source for
the plasma turbulence whose decay is probably responsible for the observed
emission.
