Twin Echo-Enabled Harmonic Generation for Enhanced Coherent Bunching at Short Wavelengths

Harmonic up-conversion schemes represent the most successful approach in externally seeded free-electron lasers (FELs) for obtaining radiation with laserlike properties at short wavelengths. They are, however, limited in the shortest achievable wavelengths, as the harmonic conversion efficiency decreases with increasing harmonic number, even for echo-enabled harmonic generation (EEHG). In the following, we introduce a novel externally seeded cascaded FEL scheme based on two EEHG stages, capable of obtaining unprecedented bunching fraction that is independent on the final FEL wavelength. The second stage can be operated both in conventional harmonic up-conversion but also in down-conversion, i.e., going toward (slightly) longer wavelengths. Higher bunching enables emission of laserlike radiation in the soft x-rays with large output power in a compact scheme. Its performance at 4 nm and shorter is investigated, demonstrating the ability to achieve multi-GW level, fully coherent pulses with laserlike properties and paving the way for externally seeded radiation at 1 nm and beyond.