Powerful and narrowband THz emission from a plasma with counterpropagating electron beams
Nowadays development of compact and cheap sources of powerful and narrowband THz radiation is highly demanded in science and technology. One of the most perspective nonlinear medium allowing to generate electromagnetic waves in the terahertz frequency range is plasma. It doesn’t have damage thresholds and can sustain electromagnetic fields with extremely high amplitudes. However, potential oscillations induced in a plasma by laser or particle beams cannot be directly converted into vacuum electromagnetic emission due to the cutoff at the plasma frequency. To avoid these challenges, we have proposed a new scheme  in which nonlinear interaction of counterpropagating laser-induced plasma waves leads to EM emission at the doubled plasma frequency. Such a process can proceed in a uniform plasma without external magnetic fields and is not sensitive to the effect of plasma screening. That is why it becomes possible not only to significantly increase the power and energy of THz pulses, but also to provide a narrow line width of the produced radiation (near 1%). Among other advantages of this generating scheme are possibilities of tuning the central radiation frequency by varying the plasma density as well as a smooth transition to broadband radiation regimes with the increase of plasma waves amplitudes.
Physically, generation of EM radiation in the proposed scheme is based on scattering of one Langmuir wave on the periodic perturbation of plasma electron density produced by a counterpropagating Langmuir wave with the same longitudinal wavenumber. The net nonlinear current produced by both waves is not zero only in the case when transverse profiles of their electrostatic potentials differ from each other. In the recent paper , we have proposed to drive these colliding waves by short laser pulses. Despite the high absolute values of THz power and energy, which can be achieved with the use of petawatt-class lasers, efficiency of laser-to-THz energy conversion in such a scheme does not exceed the level 0.1%. The main reason for that is the low efficiency of wakefields excitation and short life-time of excited wakes. In this work, in order to drive colliding plasma waves more efficiently and increase the duration of THz generation, we propose to use kiloampere relativistic electron beams of nanosecond duration instead of short laser drivers. Such beams can reach high power (tens of GW) and are able to continuously pump plasma waves at ionic times via the two-stream instability. We carry out particle-in-cell (PIC) simulations and demonstrate that the beams-to-THz power conversion can be highly efficient (up to 7%) if colliding plasma waves are driven by long-pulse electron beams with different transverse sizes. It is shown that low-emittance 1 MeV electron beams capable of focusing in mm-scale spots can generate sub-GW terahertz radiation.
Simulations were performed using resources of Novosibirsk State University. This work is financially supported by the Russian Foundation for Basic Research (grant 18-32-00107).
 I.V. Timofeev, V.V. Annenkov, E.P. Volchok. Generation of high-field narrowband terahertz radiation by counterpropagating plasma wakefields. // Phys. Plasmas, 2017, v. 24, p. 103106;