Handbook (last updated 1996)

Scientific applications

Polarisation dependent spectroscopies (e.g circular magnetic X-ray dichroism)

Source characteristics

Optics

optical elements	double-crystal monochromator with fixed exit
focusing type	horizontal and vertical focusing mirrors
beam size at sample	~ 0.7 x 0.07 mm2 (HxV) (after commissioning of the VF-2M device)
resolution in E/E	~ 10-4
flux at sample	~ 1011 ph s-1, 0.1A

Detectors

photodiodes for fluorescence, total yield and transmission experiments. XEOL detection using a dissector tube.

Specialised equipment :

7T/1.5 K UHV magnet

Helical undulator sources and beamline concept

The straight section ID 12 of the machine is now equipped with two helical undulators units: Helios-I and Helios II. Their emission spectra cover a wide range of energies from soft to hard X-rays:

Helios I:

K_v = 2.06; K_h = 2.78; K <= 3.46

_l (Minimum)=0.57 keV

Helios II:

K_v = 0.80; K_h = 1.29; K <= 1.52

₁ (Minimum) = 3.06 keV

Fig. ID12A-1:
Layout of Beamlines 6 and 26.

By longitudinal phasing of the magnetic arrays, the polarisation of the undulator emission can be changed from linear to elliptical and circular. Asymmetric variations of the undulator half gaps allow the user to select the ellipticity and the azimuthal angle for elliptically polarised radiation. Helios-I has itself been segmented into two identical modules, separated by a magnetic chicane which causes a misalignment of the injection axes ( <= 364 µrad). Thus, Helios-I generates two beams which are perfectly symmetrical with respect to the central beam of Helios II. A special pinhole device located at 26.6 m from the sources ensures perfect separation between the three beams. The phase of the two modules of Helios-I can be adjusted in such a way that the emitted beams can both be circularly polarised, but with opposed signs: weak circular dichroism signals can therefore be detected with a higher sensitivity using alternately chopped beams.

Two different optics systems have been designed:

1. "low energy" experiment (E <= 1.8 keV) will be carried out on a side branch (BL 26) equipped with grating optics;
2. Bragg optics is used on the straight branch (BL 6) dedicated to experiments at higher excitation energies.

During the commissioning phase, the two branches have been operated in a mutually exclusive time-sharing mode, but the first tests carried out in November 1994 confirmed that Beamlines 6 and 26 can perfectly well be operated in parallel, provided that Beamline 6 uses only the emission of Helios-II. The layout of the two beamlines is shown in Fig. 1.

Main beamline components

Common section and UHV requirements

In order to preserve good performances in the soft or medium-soft X-ray ranges, Beamlines 6 and 26 were designed to operate windowless, and therefore must be strictly compatible with the clean UHV requirements specified by the Machine Division. No new component can be inserted without appropriate Mass Spectrometer control. In order to avoid any further contamination of the mirrors of the side branch with carbon, it is possible to operate the common section at very low pressures (10^-10 mbar). The common section includes a prepumping vessel plus a non-standard multipinhole device. The latter device replaces the primary slits and allows the user to select 1, 2 or 3 beams, given the constraint that their separation varies with the Helios-I half gaps. A procedure has been elaborated so that the pinhole device can be properly set up with respect to the emission axes of the three undulator segments.

Four Mirror device (4M-D)

The primary function of this instrument, inserted in the first optics hutch, is to filter out the harmonics and keep the heat load on the monochromator <= 8 W. Moreover:

1. it may be used to refocus each beam individually in the horizontal plane;
2. it may be used to keep the separation between the two beams independent of the undulator gap, and to recondense the two beams to a single spot at the sample location.

The CVD-SiC mirrors are within ESRF specifications regarding both the surface micro-roughness (3-4 Å rms) and the figure slope error. The mechanics was designed in-house. The commissioning of the 4 mirror device is still under way, but a reflectivity in excess of 0.91 has been measured at 6 keV, whereas the rejection of the harmonics proved to be very efficient.

Monochromator and components in the second hutch

The key component of the beamline is a UHV compatible 2-crystal monochromator which was manufactured under ESRF supervision by KoHzu Seiki Co. The reception tests confirmed the exceptional quality of the mechanics, in particular the high precision translation stages used to obtain a fixed exit (within ±10 µm). The design of a cryogenic cooling loop is based on the circulation of cooled He gas at a few bars. Note that this monochromator can be used as a linear polarimeter for measuring the polarisation properties of the source.

Additional components in the second hutch upstream of the monochromator include: standard ESRF secondary slits and variable attenuators; a non standard (UHV compatible) beam chopper, which transmits either the first or the second beam of Helios-I at modulation frequencies <= 1 kHz; a 2-beam position monitor (BPM), made in-house, which can permanently image the 2 beams for diagnostics purposes, and can monitor their position with a typical resolution of 2 µm.

Vertically focusing Optics

A 2 mirror (VF-2M) device is under construction. It will be installed downstream of the monochromator and should also contribute to more efficient rejection of the harmonics for delicate experiments at low energy. Moreover, this instrument will enable the beam to be refocused vertically down to a spot size of 0.07 mm (fwstd). This should be useful for a variety of experiments including reflectivity measurements in the soft X-ray range.

Endstations

Three endstations are available:

1. A white-beam experimental station with very high circular polarisation rates is located in front of the monochromator. This station has been used during the commissioning phase to install a 1/4 wave plate device and to characterise the polarisation state of the Helios-I radiation. Preliminary time-resolved XEOL experiments have also been carried out here.
2. A second experimental station using unfocused monochromatic X-ray beam is available immediately after the monochromator; it is currently used for the commissioning of the beamline. Another beam position monitor (BPM), which can be operated with monochromatic beams, is very useful for alignment diagnostics.
3. A third experimental station, designed to exploit a vertically focused monochromatic beam, will be located in the third hutch. A UHV X-ray fluorescence chamber, to be operated with a multi-element Si-drift diode detector, is under construction and will be installed in this hutch.

A high magnetic field, low temperature, end station is desirable for Circular Magnetic X-ray Dichroism measurements (CMXD) on paramagnetic systems, and a sophisticated cryomagnet, to be operated either on BL 6 or BL 26, is foreseen. Further commissioning will also be needed to investigate whether or not the detectors can be operated under such high magnetic fields.

Preliminary commissioning tests

During the commissioning phase, several tests were carried out to evaluate the present performances of the beamline:

• Circular polarisation rates were measured at 2.8 keV using a UHV compatible 1/4 wave plate device; a preliminary experiment confirmed the expected circular polarisation rates, of the order of 97% upstream of the monochromator.
• CMXD spectra recorded with the two beams of Helios-I suggest that asymmetry factors as low as 2.10^-4 may be detectable under favourable conditions. The quality of the data is unfortunately not as high in an experimental configuration restricted to the single beam of Helios-II.

Work is continuing to extend the limits of sensitivity of the beamline instrumentation.