  Three-wave interactions between Langmuir and electromagnetic waves in plasma
with unstable electron flows are believed to be the main cause for type II and
III solar radio emissions. The narrow band of type II bursts requires to assume
that this radiation is generated in some local regions of shock fronts
traveling in the solar corona, where the specific conditions for the
enhancement of electromagnetic emissions near the plasma frequency harmonics
are created. The reason for such enhancement at the second harmonic may be the
formation of counter-streaming electron beams. There are different opinions in
literature on whether the second harmonic electromagnetic emission in the
presence of an additional beam can be efficient enough to markedly dominate
emissions produced by a single beam. In the present paper, we carry out
particle-in-cell simulations of the collision of two symmetric electron beams
in plasma with open boundary conditions and show that the efficiency of
beam-to-radiation power conversion can be significantly increased compared to
models with periodic boundary conditions and reach the level of a few percent
if three-wave interactions with electromagnetic waves near the second harmonic
of the plasma frequency becomes available for the most unstable oblique beam
driven modes.
