Plasma emission via a beam instability with density modulation

In this article, a new emission process for radiation near the plasma frequency in an unmagnetized plasma (i.e., the plasma emission) is discussed. The process involves a tenuous beam of energetic electrons and low‐frequency ion waves. The key point is that the ion waves can modulate the thermal electron density so that the condition for beam–plasma interaction (ω=k⋅V, where V is the average beam velocity) is Doppler shifted in wave vector space in accordance with the wavelength that characterizes the ion density fluctuations [ω+ω’=(k+k’)⋅V, where ω’ and k’ are the characteristic frequency and wave vector associated with the ion wave; in the present discussion, let set ω’=0]. As a result, the unstable electrostatic beam–plasma mode becomes coupled with the fast electromagnetic mode. Consequently in an inhomogeneous plasma the amplified waves can naturally change to electromagnetic mode and escape from the source region.