The DiagOn

DiagOn is one of the first components of soft X-ray beamlines, upstream from the monochromator. It uses the special radiation characteristics of an undulator and the highly chromatic1 behaviour of multilayer mirrors to instantaneously create an image that is simple to interpret.
By imaging the beam directly at the undulator output, a spot without any position sensitivity would be obtained (Figure 1). This beam, known as a 'white beam', comprises all the wavelengths of the undulator's emission spectrum, divided into an infinite number of concentric rings.

Figure 1 : Diagram representing DiagOn operation
White beam before DiagOn	Scan of the TEMPO undulator
Beam after reflection by the DiagOn multilayer of CASSIOPEE beamline

As it travels through the DiagOn, this white beam is partially reflected by a multilayer mirror (Figure 1) which selects a wavelength. By creating an image of this reflected beam via a fluorescent screen (scintillator) and a camera, we observe a ring corresponding to the beam at the wavelength selected by the multilayer mirror. On the same image, we also see the projected shadow of the mobile front end diaphragm (Figure 1). The centre of this ring defines the beam propagation axis, and allows easy evaluation of the possible distance between this centre and the centre of the diaphragm defining the theoretical axis of the beamline.

This imager is a detector designed and created internally at SOLEIL based on an original idea from our Optics group. The Detector group took responsibility for the project in close collaboration with members of the Optics group and the Engineering Design office.
This imager is not as simple as it might seem, and took over a year of design work, from the choice of multilayers for the various beamlines, to the special mechanical design (given the critical position of the DiagOn and the power of the beam at the undulator output).
The close collaboration between different departments made it possible to develop a device that meets the requirements of experimental lines and to supply an operational device for the beamline openings.
The multilayer mirrors were manufactured at the Orsay Optics Institute, which has the necessary sputter deposition tools. The knowhow of the Charles Fabry Laboratory was extremely valuable to the SOLEIL teams on this project.

Figure 2 : The DiagOn installed on CASSIOPEE beamline

Whilst a pre-DiagOn was already operational on the TEMPO beamline (a prototype that has since been replaced by a permanent device), the DiagOn had its first real success on 8^th February 2007 with the opening of the CASSIOPEE beamline with a multilayer (Mo/B4C inclined at 45°), allowing selection of an energy in the region of 180 eV. In the wake of this achievement, the DiagOn of the HU640 on the DESIRS beamline was installed and produced its first images on 7^th March 2007 with a multilayer (Mo/Si inclined at 45°) allowing selection of an energy in the region of 60 eV.
In all three cases, the DiagOn made it possible to define the distance between the beam and the theoretical axis of the line and, in conclusion, to certify correct alignment of the undulator and the diaphragms: on average, to within 200 μm, i.e. an angular deviation of approximately 10 μrad, to a precision of 2 μrad.

With tremendous ease, DiagOn can perform beyond its original purpose. This imager lets you carry out a more complete diagnosis of undulator emissions; in particular by interpreting the recordings of images obtained during scans of the undulator deflection parameter.
The DiagOn thus provides answers about beam divergence by determining the relationship between the K of the undulator and the radii of the rings obtained (Figure 3). It can also give the answers in terms of flux (normalised number of photons emitted by the undulator) as a function of the deflection parameter.
This imager has its limitations, however, and these results are naturally only a first approximation, with all the uncertainties generated by the method the DiagOn uses (bandwidth and selectivity of the multilayer, scintillator efficiency, etc.).
Above all, it is important to take into account the rather wide bandwidth, the multilayer, and the problem of spurious reflections (such as the low energies, which reflect at the surface of the mirror).

Figure 3 : Comparison of ring radius measured from the DiagOn and theoretical ring radius on the HU256 undulator of the CASSIOPEE beamline