Abstract
Using a two-dimensional electromagnetic and relativistic particle-in-cell (PIC) code, we show that electromagnetic waves are excited with near fundamental plasma frequency and its second harmonics from relativistic electron beam instability, whose beam velocity is 0.8 c . It is also shown that electromagnetic 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 field. By changing the beam velocity from 0.5 c to 0.8 c , we found that the second harmonics can make transition from R-mode like wave to X-mode. The emission mechanism of the second harmonics with X-mode characteristics is a new nonlinear 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.
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