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@INPROCEEDINGS{Ferrari:622552,
      author       = {Ferrari, Eugenio},
      title        = {{T}win {E}cho-{E}nabled {H}armonic {G}eneration for
                      {E}nhanced {C}oherent {B}unching at {S}hort {W}avelength},
      reportid     = {PUBDB-2025-00389},
      year         = {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)},
      month         = {Jun},
      date          = {2025-06-23},
      organization  = {European Conference on Lasers and
                       Electro-Optics, Munich (Germany), 23
                       Jun 2025 - 27 Jun 2025},
      cin          = {MPY},
      cid          = {I:(DE-H253)MPY-20120731},
      pnm          = {621 - Accelerator Research and Development (POF4-621)},
      pid          = {G:(DE-HGF)POF4-621},
      experiment   = {EXP:(DE-MLZ)NOSPEC-20140101},
      typ          = {PUB:(DE-HGF)1},
      url          = {https://bib-pubdb1.desy.de/record/622552},
}