Short-Pulse High-Power THz Generation Using Optical Klystron FELs: Simulation Results

TL;DR

This work investigates extending the optical klystron concept to THz FEL operation, addressing slippage, diffraction, and collective effects with numerical simulations at 10–100 μm for a DALI-like lattice. It analyzes energy modulation, microbunching, and radiation growth in unseeded OKs, showing that harmonic bunching and careful $R56$ optimization are needed at short wavelengths while long wavelengths benefit from slippage but suffer temporal broadening. The authors propose and numerically demonstrate a chicane-embedded optical delay to restore phase and enable staged amplification, achieving multi-megawatt THz pulses. The study concludes that diffraction is manageable in the presented design and outlines concrete paths toward compact THz FEL sources, while highlighting the need to incorporate collective effects in future work.

Abstract

We investigate the feasibility of extending the optical klystron (OK) concept into the terahertz (THz) regime, where strong radiation slippage and diffraction fundamentally challenge efficient free- electron laser (FEL) operation. Numerical simulations were carried out for resonant wavelengths of 10, 30, and 100 μm using parameters relevant to the planned DALI facility. The results show that while long wavelengths exhibit rapid energy growth, they suffer from significant temporal broadening due to slippage, whereas shorter wavelengths require large dispersive strengths R56 to achieve sufficient bunching. Harmonic bunching is demonstrated as a viable alternative to reduce the required R56 at short wavelengths. Diffraction was analyzed and found not to limit the present design, as the radiation spot size remains well within the beamline aperture. To address the slippage challenge, we propose and numerically demonstrate a novel chicane-embedded optical delay scheme, which restores phase alignment between the radiation and microbunched electrons. Simulations confirm that careful tuning of the dispersive strengths allows staged amplification, preserving beam quality and reaching multi-megawatt output power. These results highlight the potential of THz- tailored optical klystrons to generate compact, short, and high-intensity THz pulses, and lay the groundwork for future experimental studies and facility implementation.

Figures (9)

• Figure 1: Optical klystron schematic with radiation dump between undulators. In U1 the electron beam acquires energy modulation at $\lambda_r$. Radiation from U1 continues straight and is absorbed in a dump placed inside the chicane, while the electron beam is deflected by the dipoles. The chicane ($R_{56}$) converts energy modulation into density modulation (microbunching), and the pre-bunched beam radiates coherently from the entrance of U2.
• Figure 2: Simulated energy modulation of the electron beam after the first undulator (U1) for a radiation wavelength of $100~\mu\text{m}$.
• Figure 3: Bunching-factor maps $|b(z)|$ after the first chicane as functions of $R_{56}$ (horizontal axis) and longitudinal position within the bunch $z$ (vertical axis) for $\lambda_r = 100~\mu$m (left), $30~\mu$m (middle), and $10~\mu$m (right). For each $R_{56}$, $b(z)=\langle e^{i k_r z'}\rangle_{\text{slice}}$ is computed in longitudinal slices; the color scale indicates $|b|$. The ridge of maxima marks the optimal dispersion; its shift to larger $R_{56}$ at $10~\mu$m arises from the weaker energy modulation in U1.
• Figure 4: Numerically evaluated 3rf harmonic bunching-factor maps $|b_{3rd}(z)|$ after the first chicane as functions of $R_{56}$ (horizontal axis) and longitudinal position within the bunch $z$ (vertical axis)
• Figure 5: Left: Simulated FEL output power after the second undulator (U2) for radiation wavelengths of $10~\mu\text{m}$, $30~\mu\text{m}$, and $100~\mu\text{m}$. The blue shaded area represents the electron beam current profile. The effect of slippage is evident in the temporal structure of the FEL pulses, with longer wavelengths exhibiting broader pulse durations due to stronger slippage. Right: Corresponding spectra, showing narrowband emission centered at the resonant frequencies for each wavelength.