Plasma emission by a nonlinear beam instability

A new theory for electromagnetic radiation emission with wave frequency at the plasma frequency and/or its harmonic (known as the plasma emission) is presented. According to the new theory, the radiation emission takes place as a result of combined effects of the excitation of electrostatic waves by a nonlinear beam instability, and the concurrent conversion into electromagnetic waves by a nonlinear mode coupling process. The underlying physical mechanism for the nonlinear beam instability is an interaction of energetic electron beam with enhanced ion-acoustic or Langmuir turbulence. The turbulence, which is treated as intrinsic in the present analysis, also gives rise to the nonlinear mode coupling process between the electrostatic and electromagnetic modes. That is, the two processes (nonlinear instability and mode coupling) occur concomitantly. An important aspect of the present theory is that the effective growth rate associated with the radiation is comparable to the usual beam--plasma (or bump-in-tail) instability growth rate, which makes the radiation emission process very efficient, a feature very different from any standard theory. copyright 1995 American Institute of Physics