Simulation of electromagnetic emission near the plasma frequency from relativistic electron beams

Using a two-dimensional electromagnetic and relativistic particle-in-cell ( PIC) code, we show that electromagnetic waves are excited with near fundame ntal plasma frequency and its second harmonics from relativistic electron b eam instability, whose beam velocity is 0.8c. It is also shown that electro magnetic waves with second harmonics can be emitted with a short time delay from the beam instability and the second harmonic waves are extraordinary waves (X-mode) propagating nearly perpendicular to an ambient magnetic fiel d. By changing the beam velocity from 0.5c to 0.8c, we found that the secon d harmonics can make transition from R-mode like wave to X-mode. The emissi on mechanism of the second harmonics with X-mode characteristics is a new n onlinear process, in which the proton dynamics does not contribute at all, in contrast to the conventional three waves process. We discuss a possible emission mechanism for the present simulation result.