  A beam of super-thermal, hot electrons was injected into maxwellian plasma
with a density gradient along a magnetic field line. 1.5D particle-in-cell
simulations were carried out which established that the EM emission is produced
by the perpendicular component of the beam injection momentum. The beam has a
positive slope in the distribution function in perpendicular momentum phase
space, which is the characteristic feature of a cyclotron maser. The cyclotron
maser in the overdense plasma generates emission at the electron cyclotron
frequency. The frequencies of generated waves were too low to propagate away
from the injection region, hence the wavelet transform shows a pulsating wave
generation and decay process. The intensity pulsation frequency is twice the
relativistic cyclotron frequency. Eventually, a stable wave packet formed and
could mode couple on the density gradient to reach frequencies of the order of
the plasma frequency, that allowed for propagation. The emitted wave is likely
to be a z-mode wave. The total electromagnetic energy generated is of the order
of 0.1% of the initial beam kinetic energy. The proposed mechanism is of
relevance to solar type III radio bursts, as well as other situations, when the
injected electron beam has a non-zero perpendicular momentum, e.g. magnetron.
