Three-Dimensional Simulation of the University of Hawai`i FEL Oscillator: Superradiant Emission and Cavity Desynchronization

TL;DR

The paper addresses understanding superradiant emission in a FEL oscillator using a high-fidelity 3D model. It develops and applies a 3D time-dependent simulation framework based on GINGER-3D with a Matlab pulse-propagation module to UH Mānoa FEL parameters, exploring nominal operation, cavity desynchronization, and short-bunch regimes. The results show nominal operation yielding superradiant scaling $E \propto N_e^2$, cavity desynchronization boosting peak power by about a factor of 5, and short-bunch operation with desynchronization reaching peak powers on the order of $3.92 \times 10^2$ MW and substantially faster saturation; these effects are accompanied by leading spikes in the radiation profile and soliton-like electron dynamics. The framework provides a flexible platform for design, optimization, and experimental validation of high-peak-power, ultrafast FEL pulses at UH Mānoa and can be extended to interferometric configurations and cavity-length tuning to maximize extraction efficiency and ultrafast pulse generation.

Abstract

In this paper, we investigate superradiant emission in a free-electron laser (FEL) oscillator using a comprehensive three-dimensional time-dependent simulation tool. Using beam parameters from the University of Hawai`i (UH) at Mānoa FEL facility, our study shows that at nominal bunch length, the FEL radiation exhibits superradiant scaling in saturation. We then explore how cavity desynchronization enhances this regime by mitigating the laser lethargy effect in oscillators and improving overlap between the electron bunch and the radiation pulse, with peak power increased by more than a factor of five. Finally, we simulate a short-bunch operational mode with bunch length comparable to the slippage length, which accelerates saturation and further amplifies the FEL power. These findings highlight that the UH Mānoa FEL oscillator has the potential to achieve superradiant emission at its nominal operating mode, and that short-bunch operation offers further enhancement while requiring additional optimization.

Figures (6)

• Figure 1: Schematic of the FEL cavity.
• Figure 2: Peak radiation power as a function of oscillator passes, using the nominal beam and machine parameters. We show the integrated transverse profile of the radiation at pass 50 and pass 300. Note that the colorbars are normalized to have a maximum of 1 for both profiles.
• Figure 3: Radiation energy vs. charge squared for the 2 ps bunch length case and the 0.5 ps bunch length case, both for 400 oscillator passes and at perfect synchronization. Simulation data are presented with solid dots, and polynomial fittings using charge squared are presented with solid and dashed lines for linear and quadratic fittings respectively.
• Figure 4: (a) Peak power (blue) and pulse energy (red) at oscillator pass 400 as a function of varying desynchronization values $d$. (b) Pulse profiles for perfect synchronization $d=0$ (yellow), optimal $d=0.005$ based on peak power (blue), and optimal $d=0.01$ based on pulse energy (red). The FWHM of the electron bunch length is indicated by the dashed vertical lines. (c) Electron phase space after pass 400 for perfect synchronization ($d=0$). (d) Electron phase space after pass 400 for the optimal desynchronization based on peak power ($d=0.005$). Phase space images are normalized by their total sums. Time axes in (c) and (d) have been offset to position the electron beam at the center.
• Figure 5: Peak radiation power as a function of oscillator passes, for 2 ps bunch length at zero (dashed) and optimal (solid) desynchronization $d=0.005$ (blue) and 0.5 ps bunch length at zero (dashed) and optimal (solid) desynchronization $d=0.002$ (red).