[Paper Review] Control of the amplification process in baseline XFEL undulator with mechanical SASE switchers

Gianluca Geloni, Vitali Kocharyan+1 more|arXiv (Cornell University)|Jan 21, 2010

Particle Accelerators and Free-Electron Lasers1 references7 citations

TL;DR

This paper proposes a mechanical SASE switcher system to enable simultaneous multi-color X-ray pulse generation in the baseline European XFEL undulator by controlling the amplification process via shutters and magnetic chicanes. By selectively switching on/off SASE lasing in different undulator segments, the method allows independent saturation at 0.1 nm, 0.15 nm, and 0.2 nm wavelengths using a single electron beam, enabling a multi-user facility with photon beam distribution to separate beamlines.

ABSTRACT

The magnetic gap of the baseline XFEL undulators can be varied mechanically for wavelength tuning. In particular, the wavelength range 0.1 nm - 0.4 nm can be covered by operating the European XFEL with the SASE2 undulator. The length of the SASE2 undulator (256.2 m) is sufficient to independently generate three pulses of different radiation wavelengths at saturation. Normally, if a SASE FEL operates in saturation, the quality of the electron beam is too bad for generation of SASE radiation in the subsequent part of undulator which is resonant at a few times longer wavelength. The new method of SASE undulator-switching based on the rapid switching of the FEL amplification process proposed in this paper is an attempt to get around this obstacle. Using mechanical SASE shutters installed within short magnetic chicanes in the baseline undulator, it is possible to rapidly switch the FEL photon beam from one wavelength to another, providing simultaneous multi-color capability. Combining this method with a photon-beam distribution system can provide an efficient way to generate a multi-user facility.

Motivation & Objective

• To enable simultaneous generation of three distinct X-ray wavelengths (0.1 nm, 0.15 nm, 0.2 nm) in the baseline SASE2 undulator of the European XFEL.
• To overcome the limitation that saturated SASE radiation degrades electron beam quality, making subsequent SASE lasing impossible at shorter wavelengths.
• To develop a method for switching the FEL amplification process on and off within a single undulator using mechanical shutters and magnetic chicanes.
• To support a multi-user XFEL facility by enabling independent, simultaneous delivery of multi-color pulses to different experimental stations.
• To design a photon beam distribution system that separates and directs the three distinct wavelengths to separate beamlines using x-ray mirrors and horizontal offset optics.

Proposed method

• Mechanical SASE switchers are installed at strategic locations in the SASE2 undulator, each consisting of a weak magnetic chicane and a mechanical shutter.
• The magnetic chicane introduces a transverse offset and washes out electron beam modulation from prior SASE amplification, preserving beam quality for subsequent lasing.
• The shutter controls whether SASE radiation from the first undulator segment is transmitted or absorbed, enabling selective activation of the amplification process in downstream segments.
• The system operates in a 1 Hz repetition cycle with switching times under 100 ms, allowing sequential on-off-on switching for three-color operation.
• Photon beam separation is achieved using x-ray mirrors installed in additional magnetic chicanes after each undulator segment, creating horizontal separation of wavelengths.
• The horizontal offset (≈3 mm) is chosen to fit within the 9 mm vacuum chamber aperture while enabling sufficient beam separation at the experimental stations.

Experimental results

Research questions

• RQ1 Can the SASE amplification process be controlled independently in different segments of a long undulator to generate multiple X-ray wavelengths simultaneously?
• RQ2 Can mechanical shutters and magnetic chicanes preserve electron beam quality after initial saturation, enabling subsequent SASE lasing at shorter wavelengths?
• RQ3 Is it feasible to distribute three distinct X-ray pulses to separate beamlines using horizontal separation techniques within the undulator system?
• RQ4 What are the required switching times and operational frequencies to support a practical multi-user XFEL facility?
• RQ5 Can the photon beam distribution system achieve sufficient spectral and spatial separation of the three colors using x-ray mirrors and offset optics?

Key findings

• The method enables simultaneous generation of three distinct X-ray wavelengths—0.1 nm, 0.15 nm, and 0.2 nm—within the same electron bunch and undulator length.
• When the shutter is off, the second undulator segment reaches saturation at 0.2 nm with a peak power of 10 GW, indicating efficient seeded amplification.
• When the shutter is on, the second segment operates in the linear SASE regime, producing only 0.1 GW, but preserves electron beam quality for subsequent lasing at shorter wavelengths.
• The electron beam remains a good 'active medium' for further amplification after the first switch, enabling successful lasing at 0.15 nm and 0.1 nm in downstream segments.
• Photon beam separation is achieved via x-ray mirrors installed after each undulator segment, with horizontal offset of ~3 mm sufficient to create millimeter-scale separation at experimental stations.
• The system supports a 1 Hz repetition rate with switching times under 100 ms, enabling a practical on-off-on cycle for multi-color operation.