High harmonic excitations in relativistic electron beam propagation

Summary form only given, as follows. Propagation properties of intense relativistic electron beams have been extensively investigated for various applications including high power microwave generations and amplifications, sterilization, electron beam welding, electron beam sputtering, and beam-plasma interactions. Although these intense electron beams have different goals and objectives, they have the common factors to be understood in their propagation. One of the most fascinating common features in the electron beam propagation is excitation of the high harmonics. For example, whenever an electron beam propagates through background plasma, multiple harmonic Langmuir waves are excited. This phenomenon was observed in the laboratory experiments, as well as in space observations. In this context, we reinvestigate properties of the electron beam propagation, particularly concentrating on high harmonic excitations caused by the beam current modulation during propagation. A self-consistent nonlinear theory of the current modulation in an intense electron beam propagating through a reactive device is developed. The beam propagates downstream, executing the space-charge oscillation and experiencing an instability caused by the reactive device through the AC electric field. A theoretical model for the harmonic excitations in the beam current modulation is developed based on the self-consistent nonlinear theory. In the later stage of propagation, the modulation current has a cusped waveform with a narrow peak, which generates high harmonics. The mode strength of high harmonics increases to saturation, as the beam propagates further downstream.