[Paper Review] The potential for extending the spectral range accessible to the European XFEL down to 0.05 nm

Gianluca Geloni, Vitali Kocharyan+1 more|arXiv (Cornell University)|Jan 20, 2010

Particle Accelerators and Free-Electron Lasers3 references9 citations

TL;DR

This paper proposes a novel method to extend the European XFEL's spectral range down to 0.05 nm by using a 'fresh bunch' technique with magnetic and optical delays to simultaneously generate high-intensity, femtosecond X-ray pulses at 0.15 nm and 0.05 nm in the same undulator. The scheme enables saturation at both wavelengths using a single electron bunch and minimal hardware modifications, offering a low-cost, non-disruptive upgrade to existing XFEL facilities.

ABSTRACT

Specifications of the European XFEL cover a range of wavelengths down to 0.1 nm. The baseline design of the European XFEL assumes standard (SASE) FEL mode for production of radiation i.e. only one photon beam at one fixed wavelength from each baseline undulator with tunable gap. Recent developments in the field of FEL physics and technology form a reliable basis for an extensions of the mode of operation of XFEL facilities. This paper explores how the wavelength of the output radiation can be decreased well beyond the European XFEL design, down to 0.05 nm. In the proposed scheme, which is based on the use "fresh bunch" technique, simultaneous operation at two different wavelengths possible. It is shown that one can generate simultaneously, in the same baseline undulator with tunable gap, high intensity radiation at 0.05 nm at saturation, and high intensity radiation around 0.15 nm according to design specifications. We present a feasibility study and we make exemplifications with the parameters of SASE2 line of the European XFEL.

Motivation & Objective

• To extend the operational spectral range of the European XFEL beyond its baseline 0.1 nm down to 0.05 nm.
• To enable simultaneous generation of high-intensity, femtosecond coherent X-ray pulses at two distinct wavelengths—0.15 nm and 0.05 nm—using a single electron bunch.
• To develop a technically feasible, low-cost method that maintains baseline operation integrity while adding a new short-wavelength mode.
• To support advanced pump-probe experiments requiring high spatial resolution, reduced absorption, and larger scattering volumes.

Proposed method

• Utilizes a three-stage undulator system: the first stage generates SASE radiation at 0.15 nm in the linear regime.
• Employs a magnetic chicane and mirror-based optical delay to shift the radiation pulse by approximately half the bunch length, creating a 'fresh' electron bunch for seeding.
• The delayed radiation pulse at 0.15 nm (fundamental) and its third harmonic (0.05 nm) is used to seed the third undulator section, which is tuned to 0.05 nm.
• The third undulator section, 11 cells long (67.1 m), is designed to reach saturation at 0.05 nm, while the 0.15 nm radiation is diffracted out and does not interfere.
• The method relies on the 'fresh bunch' technique to initiate SASE amplification at 0.05 nm in the rear portion of the electron bunch after saturation at 0.15 nm.
• Simulations model electron beam energy spread, energy loss, and beam power distribution across stages to validate performance and saturation.

Experimental results

Research questions

• RQ1 Can the European XFEL's spectral range be extended to 0.05 nm without compromising baseline operation at 0.15 nm?
• RQ2 Is it feasible to generate simultaneous, high-intensity, femtosecond X-ray pulses at 0.15 nm and 0.05 nm using a single electron bunch?
• RQ3 Can the 'fresh bunch' technique be adapted to enable SASE lasing at 0.05 nm in a longer undulator section after saturation at 0.15 nm?
• RQ4 What hardware modifications are required to implement dual-wavelength operation, and do they pose risks to existing machine performance?

Key findings

• The method successfully generates a 10 GW-level pulse at 0.15 nm and a 10 GW-level pulse at 0.05 nm simultaneously, both reaching saturation in their respective undulator sections.
• The third undulator section, 67.1 m long (11 cells), is sufficient to achieve saturation at 0.05 nm, with output power reaching the 10 GW level.
• The beam power distribution at the end of the third stage (Fig. 16) confirms strong amplification at 0.05 nm, while the spectrum (Fig. 17) shows a dominant peak at 0.05 nm with minimal contamination from the 0.15 nm fundamental.
• Energy spread and energy deviation (Fig. 18) remain within acceptable limits, indicating stable operation without significant beam degradation.
• The technique requires only a short magnetic chicane and optical delay, making it a low-cost, non-invasive upgrade with no risk to baseline operation.
• The scheme is applicable not only to the European XFEL but also to other XFEL facilities with shorter undulators, such as LCLS and SASE1, especially those with fixed-gap undulators.