Electro-optic sampling based characterization of broad-band high efficiency THz-FEL

TL;DR

This paper demonstrates the first electro-optic sampling measurements of broadband, high-efficiency THz FEL radiation, revealing detailed waveforms and dynamics in a waveguide FEL using a tapered undulator.

Contribution

It introduces EOS-based measurement techniques for broadband THz FEL pulses and provides new insights into the FEL dynamics under zero-slippage conditions.

Findings

Successful reconstruction of THz waveforms in spatial and temporal domains.

Observation of complex FEL dynamics through varying electron beam energy.

First application of EOS to broadband THz FEL characterization.

Abstract

Extremely high beam-to-radiation energy conversion efficiencies can be obtained in a THz FEL using a strongly tapered helical undulator at the zero-slippage resonant condition, where a circular waveguide is used to match the radiation group velocity to the electron beam longitudinal velocity. In this paper we report on the first electro-optic sampling (EOS) based measurements of the broadband THz FEL radiation pulses emitted in this regime. The THz field waveforms are reconstructed in the spatial and temporal domains using multi-shot and single-shot EOS schemes respectively. The measurements are performed varying the input electron beam energy in the undulator providing insights on the complex dynamics in a waveguide FEL.