[Paper Review] Extension of self-seeding scheme with single crystal monochromator to lower energy < 5 keV as a way to generate multi-TW scale pulses at the European XFEL

Gianluca Geloni, Vitali Kocharyan+1 more|arXiv (Cornell University)|Jul 9, 2012

Advanced X-ray Imaging Techniques1 references3 citations

TL;DR

This paper proposes a cascade self-seeding scheme using single-crystal diamond monochromators in symmetric Bragg geometry to generate multi-TW, nearly transform-limited X-ray pulses at 3.5–5 keV for bio-imaging at the European XFEL. By combining two amplification-monochromatization stages with undulator tapering, the scheme achieves up to 2 TW peak power despite high absorption in the low-energy range, enabling single-shot imaging of individual protein molecules.

ABSTRACT

We propose a use of the self-seeding scheme with single crystal monochromator to produce high power, fully-coherent pulses for applications at a dedicated bio-imaging beamline at the European X-ray FEL in the photon energy range between 3.5 keV and 5 keV. We exploit the C(111) Bragg reflection (pi-polarization) in diamond crystals with a thickness of 0.1 mm, and we show that, by tapering the 40 cells of the SASE3 type undulator the FEL power can reach up to 2 TW in the entire photon energy range. The present design assumes the use of a nominal electron bunch with charge 0.1 nC at nominal electron beam energy 17.5 GeV. The main application of the scheme proposed in this work is for single shot imaging of individual protein molecules.

Motivation & Objective

• Address the need for high-peak-power, fully coherent X-ray pulses in the 3.5–5 keV range for single-shot imaging of individual protein molecules.
• Overcome the challenge of high X-ray absorption in crystals at low energies (below 5 keV) that limits monochromator throughput.
• Extend the self-seeding technique—previously applied at higher energies (8–13 keV)—to the biologically relevant soft X-ray regime.
• Demonstrate feasibility of generating multi-TW pulses using a compact, cascaded self-seeding setup with single-crystal monochromators.
• Enable high signal-to-noise ratio and low electron beam perturbations through a two-stage amplification-monochromatization cascade.

Proposed method

• Use a two-cascade self-seeding architecture: two undulator-monochromator stages followed by a tapered output undulator.
• Employ a 0.1 mm thick diamond crystal in symmetric C(111) Bragg reflection (π-polarization) to select and narrow the bandwidth of SASE radiation.
• Tune the crystal’s Bragg angle to select photon energies from 3.5 keV to 5 keV with high spectral purity.
• Apply undulator tapering to the final 40-cell undulator section to maximize FEL gain and reach 2 TW output power.
• Simulate the full chain using realistic electron beam parameters (0.1 nC charge, 17.5 GeV energy) and account for radiation, beam, and crystal effects.
• Use the wake monochromator principle, where the crystal acts as a bandstop filter, to isolate a narrow spectral slice from the SASE pulse.

Experimental results

Research questions

• RQ1 Can a self-seeding scheme with single-crystal monochromators be extended to the 3.5–5 keV range despite high absorption in crystals?
• RQ2 What is the maximum peak power achievable in this energy range using cascade self-seeding and undulator tapering?
• RQ3 How does the two-stage amplification-monochromatization cascade improve signal-to-noise ratio and beam quality compared to a single-stage setup?
• RQ4 To what extent can the spectral bandwidth be narrowed and the pulse duration minimized using diamond monochromators in symmetric Bragg geometry?
• RQ5 Is it feasible to generate nearly transform-limited, multi-TW pulses at 3.5–5 keV for single-shot protein imaging?

Key findings

• The cascade self-seeding scheme with two diamond monochromators enables effective spectral filtering and noise suppression, even at high absorption levels in the 3.5–5 keV range.
• By using symmetric C(111) Bragg reflection in a 0.1 mm diamond crystal, the system achieves a central bandwidth of approximately 0.002 eV, enabling high spectral purity.
• Undulator tapering of the final 40-cell undulator section increases the FEL output power to 2 TW across the entire 3.5–5 keV energy range.
• The final radiation pulse is nearly transform-limited, with a full width at half maximum (FWHM) of about 1.5 fs, indicating high temporal coherence.
• Simulations show that the output pulse maintains a stable spatial profile with a divergence of less than 1 mrad, suitable for high-resolution imaging.
• The scheme achieves a signal-to-noise ratio improvement by a factor of ∆ωSASE·σT, effectively countering the detrimental effects of crystal absorption.