  The problem of electromagnetic emission generation in plasma with electron
beams is relevant both for practical applications and for interpretation of
radio emission processes in astrophysical systems. In this work, we consider
the case of counter-propagating electron beams injection into plasma. Such
systems may occur in cosmic plasma in the case of closely spaced particle
acceleration regions, and they can also be implemented in laboratory
facilities. Using particle-in-cell numerical simulations, we have shown that
high beam-to-radiation conversion efficiency can be achieved in the case when
beams excite small scale oblique plasma oscillations. In this case, the
radiation is generated in the vicinity of the second harmonic of the plasma
frequency. For such waves the surrounding plasma is transparent. It has been
found that the efficiency and spectrum of the radiation are not dependent on
the thickness of the beams. It has been also shown that parameters of the
system necessary for efficient radiation generation by the discussed mechanism
can be found using the exact linear theory of beam-plasma instability.
