Simulations of a beam-driven plasma antenna in the regime of plasma transparency

Abstract

In this paper, the theoretically predicted possibility to increase the efficiency of electromagnetic radiation generated by a thin beam-plasma system in the regime of oblique emission, when a plasma column becomes transparent to radiation near the plasma frequency, is investigated using particle-in-cell simulations. If a finite-size plasma column has a longitudinal density modulation, such a system is able to radiate electromagnetic waves as a dipole antenna. This radiation mechanism is based on the conversion of an electron beam-driven potential plasma wave on the periodic perturbation of plasma density. In this case, the frequency of radiated waves appears to be slightly lower than the plasma frequency. That is why their fields enable the penetration into the plasma only to the skin-depth. This case is realized when the period of density modulation coincides with the wavelength of the most unstable beam-driven mode, and the produced radiation escapes from the plasma in the purely transverse direction. In the recent theoretical paper [I. V. Timofeev et al. Phys. Plasmas 23, 083119 (2016)], however, it has been found that the magnetized plasma can be transparent to this radiation at certain emission angles. It means that the beam-to-radiation power conversion can be highly efficient even in a relatively thick plasma since not only boundary layers but also the whole plasma volume can be involved in the generation of electromagnetic waves. Simulations of steady-state beam injection into a pre-modulated plasma channel confirm the existence of this effect and show limits of validity for the simplified theoretical model.