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Description
Laser wakefield accelerators (LWFAs) have emerged as promising compact drivers for free-electron lasers (FELs), owing to their ability to produce femtosecond electron beams with high peak current over centimeter-scale acceleration distances. However, their relatively large energy spread remains a key challenge for achieving high-gain FEL operation. While magnetic compression can effectively reduce the slice energy spread to a level suitable for FEL amplification, it simultaneously introduces a significant longitudinal energy chirp. This chirp leads to a continuous detuning of the FEL resonance along the planar undulator, resulting in phase slippage between the electron beam and the radiation field, reduced bunching efficiency, and degraded output power and spectral quality.
We investigate the use of a longitudinally tapered undulator to compensate for the chirp-induced resonance mismatch in a self-amplified spontaneous emission (SASE) FEL driven by an energy-compressed LWFA beam. Through three-dimensional, unaveraged simulations, we demonstrate that an optimized taper profile can effectively restore electron–radiation phase synchronization, leading to substantial improvements in both saturation power and spectral characteristics compared to the untapered case. Our results indicate that undulator tapering provides a viable and effective approach to mitigating chirp-induced performance degradation, thereby enhancing the feasibility of compact plasma-based FEL systems.
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