Speaker
Description
Free-electron lasers (FELs) deliver the brightest coherent X-ray pulses for atomic-resolution ultrafast science, yet scaling to extreme peak powers and attosecond durations faces a fundamental barrier: in the ultrahigh-current regime, longitudinal space-charge fields impose slice-dependent energy detuning that suppresses gain and confines efficient lasing to a small bunch fraction. We demonstrate that a quasi-neutral electron-positron pair beam eliminates this self-field entirely, enabling full-bunch coherent amplification without external compensation. Three-dimensional particle-in-cell simulations of a single-pass untapered undulator confirm the mechanism in two regimes: in the soft X-ray range, the pair beam reaches ~1.85 TW at ~345 as with enhanced odd harmonics and improved spatial coherence, while the electron-only beam fails to saturate; in a high-harmonic pair-cascade configuration, ~10 TW isolated spikes of ~3.5 as emerge with coherent amplification to ~177 keV. These results establish a new operating regime for ultrahigh-power attosecond sources and a direct route to coherent gamma-ray emission beyond 100 keV, presently inaccessible to magnetic-undulator FELs.
Footnotes
(*) https://arxiv.org/pdf/2603.15407
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