Enhanced Harmonic Generation in Terahertz FELs: Influence of Pre-Bunching and Undulator Geometry on Spectral and Angular Emission

TL;DR

This paper combines analytical and numerical methods to study how undulator geometry and pre-bunching influence spectral, angular, and coherence properties of terahertz radiation in free-electron lasers.

Contribution

It provides new analytical insights into the effects of undulator type and beam pre-bunching on THz FEL emission characteristics, supported by GENESIS simulations.

Findings

Prebunched beams show superior harmonic generation but are sensitive to energy spread.

Undulator period increases radiation coherence and angular broadening occurs at high magnetic parameters.

Planar undulators produce prominent side lobes; helical undulators yield narrower emission profiles.

Abstract

Both theoretical and numerical analyses are conducted to investigate terahertz (THz) radiation emission from free-electron lasers. The angular and spectral characteristics of radiation are analytically evaluated leveraging Linenard Wiechert field formalism, The analysis spanned across varying beam profiles and undulator parameters, including harmonic order, magnetic strength, period, and electron energy. Fourier analysis of the electric field reveals strong sensitivity of spectral content to underlying beam dynamics and undulator structure. Planar undulators tend to generate radiation with more prominent side lobes, especially at higher harmonics, while helical undulators produce narrower, more focused emission profiles. Importantly, the polarization and forward directed nature of the radiation are governed primarily by the resonant conditions and beam dynamics, rather than the undulator geometry itself. Building on these analytical insights, GENESIS simulations are employed to quantify the evolution of the bunching factor and radiation intensity under varying energy spread and plasma dispersion. The results show that increased undulator periods enhance radiation coherence, while transitions to wiggler regimes (at high magnetic parameters) lead to angular broadening and lateral lobe formation. Among all beam profiles studied Gaussian, Lorentzian, biGaussian, and prebunched the prebunched configuration demonstrates superior harmonic generation capabilities, albeit with high sensitivity to energy spread and plasma effects.