High-Efficiency Plasma-Based Compressor for Ultrafast Soft X-ray Free-Electron Lasers

The generation of intense, femtosecond-scale X-ray pulses is crucial for probing matter under extreme temporal and field conditions. Current chirped-pulse amplification (CPA) techniques in free-electron lasers (FELs), however, face efficiency limitations in the soft X-ray regime due to the inherent constraints of conventional optical compressors. To address this challenge, we propose a high-efficiency plasma-based compressor utilizing highly ionized noble gas plasma. Exploiting strong refractive index dispersion near ionic resonances, this scheme achieves over 70% transmission efficiency around 5.2 nm, and is extendable to other highly charged ions for operation across the soft X-ray to vacuum ultraviolet range. Simulations demonstrate that a 25 fs FEL pulse can be compressed to 1.4 fs with peak power boosted to over 100 GW, while maintaining high energy throughput. This approach overcomes the long-standing efficiency bottleneck of soft X-ray CPA and opens a scalable path toward compact, high-brightness attosecond FEL sources.