It is found that nonlinear interaction of plasma wakefields driven by counterpropagating laser or particle beams can efficiently generate high-power electromagnetic radiation at the second harmonic of the plasma frequency. Using a simple analytical theory and particle-in-cell simulations, we show that this phenomenon can be attractive for producing high-field (∼ 10 MV/cm) narrowband terahertz pulses with the gigawatt power level and millijoule energy content. Narrowband terahertz sources are also of great interest with regard to the resonant control and manipulation of matter, but generation of multi-cycle terahertz pulses with even moderate fields (1 MV/cm) and mJ energies still remains a challenging problem. Today, the most intense (∼ 1 MW, tens of J) pulses of this radiation are produced by large-scale accelerator facilities such as free electron lasers [12][13][14]. Table-top generation schemes providing J pulses with narrow linewidths (2 − 3%) are based on optical rectification of a temporally modulated chirped pump laser in an organic crystal [15] or in a periodically poled lithium niobate [16]. Plasma is also considered as a promising nonlinear medium for generating high-power multi-cycle terahertz pulses. It supports long-lived oscillations with extremely large electric fields and allows to tune the radiation frequency by a simple change of plasma density.It has been recently proposed a number of generating schemes utilizing conversion of plasma oscillations to the terahertz electromagnetic (EM) waves. In particular, plasma waves driven by laser or particle beams can produce radiation due to the linear mode conversion in a macroscopically inhomogeneous plasma [17] or via the antenna mechanism in a thin plasma with a small-scale longitudinal density modulation [18][19][20]. Plasma wakefields can also generate EM waves in external magnetic fields imposed along [21] or across [22][23][24] the plasma column. In these schemes, however, plasma inhomogeneties widen the frequency spectrum of emitted radiation, and the magnetic field required for the upper frequency part of the terahertz range becomes too strong to be easily implemented in experiments. In this Letter we propose to generate high-field narrowband terahertz radiation by counterpropagating plasma wakes excited in a uniform plasma by short laser drivers. Such a scheme can generate GW, mJ multi-cycle terahertz pulses with the energy conversion efficiency higher than 10 −4 . Let us first study the mechanism of EM radiation produced by counterpropagating plasma wakes independently on the driver nature. This mechanism is similar to that recently discussed in Ref. [20]. Nonlinear interaction of two potential plasma waves oscillating with the plasma frequency and opposite longitudinal wavenumbers ( ( , 1 ) and ( , − 2 )) can generate the superluminal wave of electric current (2 , 1 − 2 ) which, in the bounded plasma, can pump vacuum EM waves. If these plasma wakefields are excited by relativistic drivers with the velocity ≈ , the longitudinal wavenumber of ...
In this paper we study how efficiently electromagnetic radiation can be generated by a relativistic electron beam with a gigawatt power level during its injection into a thin magnetized plasma. It is shown that, if the transverse beam and plasma size is compared with the radiation wavelength and the plasma density is modulated along the magnetic field, such a beam-plasma system can radiate electromagnetic waves via the antenna mechanism. We propose a theoretical model describing generation of electromagnetic waves by this plasma antenna and calculate its main radiation characteristics. In the two-dimensional case theoretical predictions on the radiation efficiency are shown to be confirmed by the results of particle-in-cell simulations, and the three-dimensional variant of this theory is used to estimate the peak power of sub-terahertz radiation that can be achieved in beam-plasma experiments in mirror traps.
The ways to improve the efficiency of electromagnetic waves generation in laboratory experiments with high-current relativistic electron beams injected into a magnetized plasma are discussed. It is known that such a beam can lose, in a plasma, a significant part of its energy by exciting a high level of turbulence and heating plasma electrons. Beam-excited plasma oscillations may simultaneously participate in nonlinear processes resulting in a fundamental and second harmonic emissions. It is obvious, however, that in the developed plasma turbulence the role of these emissions in the total energy balance is always negligible. In this paper, we investigate whether electromagnetic radiation generated in the beam-plasma system can be sufficiently enhanced by the direct linear conversion of resonant beam-driven modes into electromagnetic ones on preformed regular inhomogeneities of plasma density. Due to the high power of relativistic electron beams, the mechanism discussed may become the basis for the generator of powerful sub-terahertz radiation.
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