Pair beams unlock beyond-terawatt attosecond free-electron laser pulses

Abstract: Free-electron lasers (FELs) generate the brightest coherent X-ray pulses available, enabling atomic-resolution and femtosecond-timescale studies across physics, chemistry, and biology. Realising their full potential at extreme peak powers and attosecond pulse durations critically depends on sustaining coherent gain across the full bunch length. Yet, the quasi-static longitudinal space-charge field in the ultrahigh-current regime imprints a slice-dependent energy detuning that quenches gain growth, so that current schemes typically sustain efficient lasing only across a limited fraction of the bunch. Here we demonstrate that a quasi-neutral electron-positron pair beam cancels this self-field and enables full-bunch high-gain lasing in ultracompressed beams without external compensation. Three-dimensional particle-in-cell simulations in a single-pass, untapered undulator confirm the mechanism across operating regimes: in the soft X-ray regime, the pair beam reaches $1.85\,\mathrm{TW}$ at $345\,\mathrm{as}$ with enhanced odd-harmonic emission and improved spatial coherence, while the electron-only beam fails to saturate; and a high-harmonic pair-cascade configuration yields ${\sim}10\,\mathrm{TW}$ in isolated ${\sim}3.5\,\mathrm{as}$ spikes with coherent amplification extending to photon energies of ${\sim}177\,\mathrm{keV}$. These results establish a new operating regime for ultrahigh-power attosecond light sources and open a direct route to coherent gamma-ray emission ($\geq!100\,\mathrm{keV}$) currently inaccessible to magnetic-undulator FELs, with broad implications for ultrafast structural, electronic, and nuclear sciences.