Abstract

PUBDB-2025-00389

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Twin Echo-Enabled Harmonic Generation for Enhanced Coherent Bunching at Short Wavelength

Ferrari, E. (Corresponding author)DESY*

2025

Abstract: We present a novel externally seeded cascaded free-electron laser (FEL) scheme based on two echo-enabled har-monic generation stages to obtaining unprecedented bunching independently on the final FEL wavelength.Externally seeded free-electron lasers (FELs) based on harmonic up-conversion schemes represent the mostsuccessful approach for obtaining radiation with laser-like properties at short wavelengths. They are howeverlimited in the shortest achievable wavelengths, as the harmonic conversion efficiency decreases with increasingharmonic number, even for echo-enabled harmonic generation (EEHG) [1]. Higher bunching is critical to improvethe 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 coherentpulses with laser-like properties, paving the way for externally seeded radiation at 1 nm and beyond.The setup is schematically reported in Fig.1(a). The first dispersive section (DS1) smears the energy modulationgenerated in MOD1 by seed 1 creating stripes in the longitudinal phase space (LPS) of the e-beam. The seconddispersive section (DS2) is tuned to maximize the bunching at the required intermediate wavelength, emitted by thefirst radiator (RAD1) to produce the seed for the subsequent stage (seed 3) towards the tail of the bunch, leavingenough space for the fresh bunch approach [2]. The second stage is used to remodulate the same e-beam withamplitude A3, in a fresh region where the interaction of seed 2 in ECHO-1 did not take place. The energy stripesgenerated by seed 1 in MOD1 and smeared in DS1 are still present thanks to the long pulse duration of seed 1.By proper tuning of the third dispersive section (DS3) an unprecedented amount of bunching is optimized at thefinal wavelength, emitted by the main radiator (RAD2). While conventionally the cascaded approach relies onthe second stage operated in up-conversion (i.e., shorter wavelengths), the proposed scheme can operate both inup- and down-conversion, going towards (slightly) longer wavelengths. This gives best performance in terms ofminimum required seed 3 power (nJ-level) and maximum achievable bunching. An example of LPS for one e-beamslice, before and after interaction in ECHO-2 is reported in Fig.1(b-e). The predicted bunching at 4 nm is 28.7%,an unprecedented level at soft-X-rays for an externally seeded FEL, as one can see from Fig.1(f, g).References[1] G. Stupakov., “Using the Beam-Echo Effect for Generation of Short-Wavelength Radiation”, Phys. Rev. Lett., 102, 074801, (2009).[2] L. H. Yu and I. Ben-Zvi, “High-gain harmonic generation of soft X-rays with the “fresh bunch” technique”, Nuclear Inst. and Methods inPhysics Research, A, 393, (1) 96–99, (1997)

---

Contributing Institute(s):

1. Beschleunigerphysik (MPY)

Research Program(s):

1. 621 - Accelerator Research and Development (POF4-621) (POF4-621)

Experiment(s):

1. No specific instrument

Appears in the scientific report 2025

---