Free‑electron lasers (FELs) make the brightest coherent X‑ray pulses available, but at very high peak current a self‑field inside the electron bunch spoils the lasing across most of the bunch. This paper shows in three‑dimensional simulations that using a quasi‑neutral electron–positron pair beam cancels that self‑field. The cancellation preserves the resonance condition across the whole bunch and lets the FEL amplify light to far higher peak power and shorter pulse duration than an electron‑only beam.

The authors ran particle‑in‑cell (PIC) simulations of a single‑pass, untapered magnetic undulator — the standard device that makes FEL light. They compared a conventional electron‑only beam with a beam that carries equal charge in electrons and positrons so the net charge is nearly zero. In a high‑current, ultracompressed “pancake” bunch the steady longitudinal space‑charge (LSC) field gives a slice‑dependent energy shift that detunes slices from the resonant condition and halts gain. A quasi‑neutral pair beam cancels that DC self‑field and so prevents the detuning.

The simulated results are striking. In a soft X‑ray configuration the pair beam reaches about 1.85 terawatts (TW) peak power in a single 345 attosecond pulse, with stronger odd harmonics and better spatial coherence, while the electron‑only beam fails to reach saturation. In a high‑harmonic “pair‑cascade” setup the simulations produce isolated spikes of roughly 10 TW lasting about 3.5 attoseconds, with coherent amplification extending up to about 177 kilo‑electronvolts — in the gamma‑ray band. The authors report these outcomes without external compensation schemes or tapering and with saturation reached in meter‑scale undulators in their models.