  In this paper, electromagnetic emissions produced by a beam-plasma system are
investigated using particle-in-cell simulations for the particular case when
the typical transverse size of both 100 keV electron beam and produced plasma
channel is comparable to the radiation wavelength. The interest in this regime
of beam-plasma interaction is associated with highly efficient generation of
electromagnetic waves near the plasma frequency harmonics that has been
recently observed in laboratory experiments on the GOL-3 mirror trap. It has
been found that the radiation power only from the vicinity of the doubled
plasma frequency in these experiments can reach 1% of the total beam power.
Subsequent theoretical and simulation studies have shown that the most likely
candidate for explaining such efficient generation of electromagnetic radiation
is the mechanism of a beam-driven plasma antenna based on the conversion of the
most unstable plasma oscillations on a longitudinal density modulation of
plasma ions. In this paper, we investigate how effectively this mechanism can
work in a real experiment at the GOL-3 facility, when a thin sub-relativistic
electron beam gets a large angular spread due to compression by a magnetic
field, and the gas into which it is injected has macroscopic density gradients.
