Speaker
Description
Free-electron lasers (FELs) produce intense, coherent light pulses spanning the infrared to hard X-rays, enabling ultrafast studies of matter on atomic scales. Extending FEL operation toward gamma-ray photon energies requires extremely high peak currents, yet in conventional electron beams such compression generates a steady space-charge field that distorts the beam energy and quenches coherent amplification. Here we show, using three-dimensional particle-in-cell simulations and generalised FEL gain theory, that quasi-neutral electron--positron pair beams cancel this field at its source, preserving beam resonance without chirp compensation or slice selection. Simulations demonstrate that while an equivalent electron-only beam fails entirely, the pair beam produces ${\sim}1.85$-terawatt, $345$-attosecond pulses at $5.4\,\mathrm{nm}$ in a single ${\lesssim}\,6$-metre undulator, and ${\sim}10$-terawatt, $3.5$-attosecond spikes with coherent emission to $177\,\mathrm{keV}$ in a projected $18$-metre next-generation stage. These results establish a wavelength-universal operating mode with enhanced energy extraction providing a route to coherent gamma-ray emission for photonuclear spectroscopy and attosecond strong-field science.
[We respectfully request a consideration for the oral presentation in the Forum. This work (arXiv preprint is attached) resolves a fundamental and long-standing gain-suppression mechanism in ultrahigh-current FELs through a conceptually novel solution requiring no external hardware such as tapering: a quasi-neutral pair beam cancels the self-field detuning at its source. The result is a qualitative leap in accessible peak power and pulse duration via full-bunch lasing, with direct implications for the design of next-generation FEL facilities. The numbers obtained from the 3D-PIC simulations speak clearly: terawatt-level soft X-ray pulses and isolated 3.5 as spikes at 10 TW with coherent emission to 177 keV. Poster presentation of this work at the DESY Users' Meeting drew strong engagement from both FEL scientists and accelerator experimentalists. A given oral slot would allow the community to engage directly with the underlying physics and begin shaping the experimental roadmap this result demands. That is the reason then why we are much motivated for such an opportunity of oral contribution in the Forum.]