Electromagnetic sub-THz emission from a beam-plasma system with oblique density modulations

Recent experiments on the injection of kiloampere electron beams into a magnetized plasma at the GOL-PET facility have shown that the power of sub-terahertz radiation escaping from the plasma along the magnetic field increases by more than an order of magnitude if strong radial density gradients are preliminarily created in the plasma. In this work, we study effects of the different periodic density modulations on electromagnetic emission from the plasma. First, PIC simulations were performed with the strong transverse density gradient measured in the experiment. They did not show considerable increase in electromagnetic emission relative to the case of the uniform plasma. However, a small oblique modulation of the plasma density $n = {n_0} + \Delta n \cdot \cos (\vec q \cdot \vec r)$ with $\Delta n \sim 0.05{n_0}$ makes it possible to obtain the power of the output radiation at the level of 1-3% of the beam power due to the antenna mechanism. This mechanism has been previously found to be effective only in a thin plasma with the radial size comparable with the radiation wavelength. In this work, based on analytical and simulation studies, we discuss how efficiently this mechanism can work in a thick plasma typical to GOL-PET experiments.