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

Externally seeded free-electron lasers (FELs) based on harmonic up-conversion schemes represent the most successful approach for obtaining radiation with laser-like 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) [1]. Higher bunching is critical to improve the performance of seeded FELs in the soft X-rays, with large output power and more compact footprint. We analyze the performance at 4 nm and shorter via numerical simulations, demonstrating multi-GW level, fully coherent pulses with laser-like properties, paving the way for externally seeded radiation at 1 nm and beyond.