Impact of local bunching factors in single-pass THz free electron lasers

TL;DR

This work shows that in THz free-electron lasers, local bunching factors arising from the current profile within a resonant wavelength can dominate the initial seeding, challenging the conventional shot-noise-dominated view. The authors derive slice-based local bunching factors for Gaussian profiles, providing explicit expressions for the amplitude and phase, and identify that the maximum coherent contribution occurs near $z_s = \pm \sqrt{2}\,\sigma_z$ with potential $n_e|b_s|^2$ exceeding unity. Through benchmark simulations at very low charge and full-particle Genesis1.3 runs, they validate a method that injects the actual current profile (One4One) and show close agreement with full-electron results, unlike quiet-loading seeds. Applying the PITZ THz FEL setup, they demonstrate substantially higher exit pulse energies and dramatically reduced energy jitter when the current profile is included, consistent with measurements and suggesting relaxed design constraints for THz FELs. These findings highlight the practical impact of current-profile effects on seeding, gain dynamics, and stability in long-wavelength FEL operation.

Abstract

In simulations for modern free-electron lasers (FEL), shot noise plays a crucial role. While it is inversely proportional to the number of electrons, shot noise is typically modeled using macroparticles, with their bunching factors corresponding to the bunching factors of the much larger number of electrons. For short-wavelength FELs, the macroparticles are assumed to be uniformly distributed on the scale of the resonant wavelength, since shot noise dominates the initial radiation - for instance, in the self-amplified spontaneous emission (SASE) regime. In this paper, we show that this assumption does not hold at longer wavelengths, particularly in the THz range, where the bunch current profile is not uniform even within the length of the resonant wavelength. Instead, the current profile dominates the initial bunching factors, which can be several orders of magnitude higher than shot noise. The slice-based bunching factors and bunching phases are derived for Gaussian distributions and compared with shot noise under the assumption that the current within each slice remains constant. Using the THz FEL at the photoinjector test facility at DESY in Zeuthen (PITZ) as a case study, the influence of the current profile has been benchmarked through simulations under very low bunch charge, where the full number of electrons can be modeled using the Genesis1.3 code. Additional simulations with the nominal working parameters of PITZ THz FEL have been compared with experimental data, indicating better agreement when the actual current profile is taken into account.

Figures (7)

• Figure 1: (a) Form factors, (b) bunching phases, (c) ratios of coherent radiation over spontaneous radiation, and (d) current profiles along the bunch slices for $k\sigma_z = 50, 500, 5000$.
• Figure 2: (a) Square of the absolute bunching factors and (b) bunching phases along the bunch slices, with solid lines calculated from current profiles generated by random samples and dashed lines directly from random samples.
• Figure 3: The layout of the THz beamline at PITZ. The L-band photocathode RF gun and the booster accelerate the beam momentum to 17 MeV/c, followed by four quadupole triplets to transport and match the space charge dominated electron beam to the undulator.
• Figure 4: (a) Current profiles, (b) Slice bunching factors and (c) gain curves from benchmark simulations.
• Figure 5: (a) Initial bunching amplitudes and (b,c) initial bunching phases in Genesis1.3 simulations with multiple numerical seeds, obtained using the actual current profile (solid curves) or quiet loading (dashed curves) in each slice.