Speaker
Dr
Igor Timofeev
(Budker INP SB RAS)
Description
It has been found that the laboratory experiments on the injection of a high-current electron beam into a magnetized plasma at the GOL-3 mirror trap is always accompanied by generation of sub-terahertz radiation lying near the plasma frequency and its second harmonic. It has been also observed that this radiation becomes more efficient if the plasma radius is decreased down to the radiation wavelength and if the guiding magnetic field is corrugated along the plasma column. In our view, the observed enhancement in radiation efficiency may be explained, in the first case, by the antenna mechanism [1] in which the vacuum EM waves can be directly generated by a thin plasma and, in the second case, by the linear mode conversion of the beam-driven plasma waves on the density gradients which are usually produced in the corrugated regime.
In this work, using both analytical theory and PIC simulations, we investigate what part of the total beam power injected with the beam into a thin plasma can be converted into the power of electromagnetic emission. It is shown that, if a thin plasma has a longitudinal density modulation with the period comparable to the wavelength of the dominant beam-driven mode, it radiates EM waves near the plasma frequency like a plasma antenna and the efficiency of this radiation reaches 5-10%. Simulations of the steady-state beam injection into the initially uniform plasma demonstrate that such a quasi-1D density modulation can appear self-consistently due to the development of the modulational instability driven by the large-amplitude beam mode. In this case, the antenna mechanism provides the radiation efficiency of 1%, which has been also observed in the laboratory experiments with 100 keV electron beam [2].
Another way to enhance electromagnetic emission in the laboratory experiments with high-current electron beams is to create the specially oriented large-scale density gradients allowing for the complete linear conversion of the most unstable beam-driven waves into electromagnetic O modes. We find the fastest growing modes in the beam-plasma system for typical beam and plasma parameters in our experiments and calculate the corresponding range of possible orientations of the density gradient in a plane plasma slab.
Since the total power of electron beams reaches tens of gigawatts, the proposed mechanisms can be important for designing a terahertz radiation source with the gigawatt power level.
This work is supported by RFBR (grant 15-32-20432) and the Russian Scientific Foundation (grant 14-12-00610).
1. V.V.Annenkov, E.P.Volchok, I.V.Timofeev, Plasma Phys. Control. Fusion **58**, 045009 (2016).
2. A.V.Burdakov et al. , Fusion Sci. Technol. **63** (1T), 286 (2013).
Primary author
Dr
Igor Timofeev
(Budker INP SB RAS)
Co-authors
Ms
Evgeniya Volchok
(Budker INP SB RAS, NSU)
Mr
Vladimir Annenkov
(Budker INP SB RAS)
