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.

Second harmonic electromagnetic emission in a beam-driven plasma antenna

“…To obtain correct angles of generated radiation, this wavenumber should lie in the close vicinity of 𝑘 ‖ = 1.25. The appearance of such a shift is also confirmed by the Fourier transform of plasma fields and agrees with our previous simulations [16]. Thus, in order to exclude variations of radiation efficiency caused by variable amplitudes of plasma waves, it is more suitable to analyze agreement between theory and simulations by comparing the quantities 𝒫/𝐼.…”

“…3b reaches the maximum at the greater plasma thickness 2𝑙 = 10 in a good agreement with the theoretical prediction, but the theory fails to predict the absolute value of radiation power in this case. The similar excess of the simulated TE radiation power over the theoretical prediction in a beam-driven plasma antenna has been also observed for the fundamental harmonic emission [16]. It is apparently explained by the greater sensitivity of TE-polarized waves to transverse currents caused by the nonuniform transverse structure of the dominant beam-driven mode.…”

“…Generalization of this theory to arbitrary angles [15] has allowed to find the regime in which efficiency of -antenna radiation is substantially enhanced even in a thick magnetized plasma due to the effect of plasma transparency to radiated EM waves lying slightly below the cut-off frequency . Further, these theoretical predictions have been successfully confirmed by particle-in-cell (PIC) simulations [16]. Numerical experiments [17] on the steady-state injection of an electron beam into a previously created rippled-density plasma channel have shown that the maximal efficiency of -radiation reaches the level of 10%.…”

Efficient generation of -radiation in a beam-driven thick plasma with oblique density modulation

“…The frequency and the wavenumber of the pump wave under the Cherenkov resonance condition are given by the expressions However, as it was shown earlier by Timofeev et al. (2017), in the state of nonlinear saturation, slightly increases in comparison with the prediction of the linear theory due to a lower velocity of the trapped beam. Moreover, in the problem of continuous beam injection, we are dealing with a wave packet localized in space instead of a plane wave.…”

“…2020). The first attempts to obtain high radiation efficiency due to the antenna mechanism in thick plasma were based on the transition to the regime of its complete transparency (Timofeev, Berendeev & Dudnikova 2017) for the generated EM radiation. In this regime, there is a window in the longitudinal modulation wavenumber , inside which EM oscillations emerging into vacuum propagate through the magnetized plasma as ordinary and extraordinary eigenmodes.…”