[Paper Review] Self-seeding scheme for the soft X-ray line at the European XFEL

Gianluca Geloni, V. Kocharyan+1 more|arXiv (Cornell University)|Feb 23, 2012

Particle Accelerators and Free-Electron Lasers1 references3 citations

TL;DR

This paper proposes a compact, cost-effective self-seeding scheme using a grating monochromator and magnetic chicane to enhance longitudinal coherence in the soft X-ray regime at the European XFEL. By seeding the second undulator with transform-limited, narrowband radiation, the scheme enables post-saturation tapering, achieving up to 800 GW of peak power—tenfold higher than SASE saturation—while maintaining sub-10⁻³ relative spectral bandwidth.

ABSTRACT

This paper discusses the potential for enhancing the capabilities of the European FEL in the soft X-ray regime. A high longitudinal coherence will be the key to such performance upgrade. In order to reach this goal we study a very compact soft X-ray self-seeding scheme originally designed at SLAC [1,2]. The scheme is based on a grating monochromator, and can be straightforwardly installed in the SASE3 undulator beamline at the European XFEL. For the European XFEL fully-coherent soft X-ray pulses are particularly valuable since they naturally support the extraction of more FEL power than at saturation by exploiting tapering in the tunable-gap SASE3 undulator. Tapering consists of a stepwise change of the undulator gap from segment to segment. Based on start-to-end simulations we show that soft X-ray FEL power reaches about 800 GW, that is about an order of magnitude higher than the SASE level at saturation (100 GW). The self-seeding setup studied in this work is extremely compact (about 5 m long), and cost-effective. This last characteristic may justify to consider it as a possible addition to the European XFEL capabilities from the very beginning of the operation phase. [1] Y. Feng, J. Hastings, P. Heimann, M. Rowen, J. Krzywinski, and J. Wu, "X-ray Optics for soft X-ray self-seeding the LCLS-II", proceedings of 2010 FEL conference, Malmo, Sweden, (2010). [2] Y. Feng, P. Heimann, J. Wu, J. Krzywinski, M. Rowen, and J. Hastings, "Compact Grating Monochromator Design for LCLS-I Soft X-ray Self-Seeding", https://slacportal.slac.stanford.edu/sites/lcls_public/lcls_ii/Lists/LCLS_II_Calendar/Physics_Meetings.aspx, May 2011 and https://sites.google.com/a/lbl.gov/realizing-the-potential-of-seeded-fels-in-the-soft-x-ray-regime-workshop/talks, October 2011

Motivation & Objective

• To enhance longitudinal coherence in the soft X-ray FEL output at the European XFEL, which is critical for advanced experiments requiring fully coherent, Fourier-limited pulses.
• To address the limitation of SASE FELs, which produce radiation with limited longitudinal coherence despite high transverse coherence.
• To enable a significant power boost beyond SASE saturation through the use of a monochromatic seed and undulator tapering.
• To develop a practical, compact, and cost-effective upgrade for the SASE3 undulator beamline that can be integrated from the start of operations.

Proposed method

• A self-seeding scheme is implemented using a compact grating monochromator (based on SLAC's FENG design) with three mirrors and a VLS grating, providing ~2.5 ps optical delay.
• A 5 m-long magnetic chicane with ~2 mm dispersion is used to delay the electron bunch, compensating for the optical path delay and removing SASE microbunching.
• The electron beam is recombined with the monochromatized radiation at the entrance of the second undulator, where coherent amplification occurs.
• Undulator tapering is applied by stepwise reducing the undulator gap, preserving resonance while extracting more energy from the electron bunch.
• Start-to-end simulations are performed using a 0.1 nC electron bunch, modeling the full FEL process from electron beam to final X-ray pulse.
• The system is designed to be non-invasive, preserving baseline operation and allowing safe return to SASE mode.

Experimental results

Research questions

• RQ1 Can a compact self-seeding scheme based on a grating monochromator significantly improve the longitudinal coherence of soft X-ray FEL pulses at the European XFEL?
• RQ2 What is the maximum FEL power achievable when combining self-seeding with undulator tapering in the SASE3 beamline?
• RQ3 How does the monochromatic seed improve the efficiency of energy extraction compared to SASE from noise?
• RQ4 What is the required seed power level to dominate over shot noise and enable effective seeding in the soft X-ray range?
• RQ5 Can the proposed scheme be implemented cost-effectively and with minimal disruption to existing beamline operations?

Key findings

• The self-seeding scheme enables the generation of transform-limited, fully coherent soft X-ray pulses with a spectral bandwidth (FWHM) narrower than 10⁻³.
• Peak FEL power reaches approximately 800 GW in the tapered case, representing a tenfold increase over SASE saturation power (~100 GW).
• At 1.5 nm wavelength, the source delivers 0.6×10¹⁴ photons per pulse with a 10 fs FWHM pulse duration.
• The scheme maintains a high safety margin: even with a 10-fold degradation in seed power (down to 20 kW), the signal remains well above the 1 kW shot noise level.
• The monochromator is estimated to achieve ~10% efficiency, though detailed calculations are needed to confirm this.
• The transverse beam divergence decreases with tapering, though the beam spot size increases at the undulator exit, indicating improved directivity.