A very intense and highly focused high energy X-ray beam is crucial for high pressure diffraction experiments due to the very small sample dimensions, for e.g. at pressures above 100 GPa the typical sample dimensions are of the order of 20 µm or smaller. A high photon flux at high X-ray energies is required because of the limited X-ray aperture of high pressure cells and the highly absorbing pressure windows at energies below 15 keV. So the choice of an optimized X-ray source is of primary importance.

The high-flux X-ray source of ID27 is composed of 2 small gap in-vacuum undulators of 23 mm period (U23) that can be used simultaneously at a minimum magnetic gap of 6 mm. For typical experiments a monochromatic beam of 33 keV ($$\lambda$$ = 0.3738 Å) is selected.

## Undulators

Beamline ID27 is equipped with 2 in-vacuum small gap undulators on a low-ß section (small source but higher divergence in the horizontal direction), shown below. It is composed of two 2-meter long U23 undulators which offer outstanding brilliance in the energy domain of interest for high pressure experiments. The energy dependence of the photon intensity for the U23 in-vacuum undulators installed at beamline ID27 is presented. The calculation was conducted for an incident beamsize of 1x1 mm2 at 30 meters from the source. The two U23 undulators available at ID27 can reach minimum magnetic gaps of 6 mm and generate extremely bright beams in the X-ray energy region of interest.

The low-ß source size and divergence of ID27 are presented. They are ideally suited for micro-focussing in the vertical direction but not optimum in the horizontal direction where only a fraction (20%) of the incident beam can be exploited.  The reduction of the source size and horizontal divergence of a factor 2 and 15, respectively, expected from the new Extremely Brilliant Source (EBS) will induce an important photon flux increase.

Direction Size (FWHM in µm) Divergence (FWHM in µrad)
X (horiz) 134 208
Z (vert) 24 12

## Optics Hutch Layout

### Attenuators

A series of attenuators limit the heat load on the monochromator by absorbing part of the low energy X-rays.

### Monochromators

The monochromator high vacuum (10-9 mbar) vessel shown can accommodate 2 silicon double-crystals (i.e. Si(111) and Si(311)). It is mounted on a high stability granite table with 2 degrees of freedom (YZ). This allows us to interchange the 2 crystals rapidly by translating the monochromator along the horizontal direction. The two crystal types can be chosen depending on the requirements of the experiment in terms of energy range, energy resolution and flux. These crystals are mounted on a high stability mechanics, and inserted in a liquid nitrogen cooling unit to prevent any thermal drift at high heat load.

The dimensions of the Si(111) monochromator, which is the most commonly used at ID27, are chosen such as to cover a broad energy range from 20 to 90 keV (0.14 Å).

## EH1 focusing

Two mirrors in Kirkpatrick-Baez geometry are used to focus the X-ray beam. ID27/EH1 is equipped with a pair of 170 mm long multilayer mirrors developed by the ESRF optics group. These mirrors offer a very broad energy band pass from 20 to 80 keV with a maximum of 80% reflectivity at 30 keV, the optimum energy for DAC experiments.

With this focusing system, a 1.7x2.7 µm2 beam spot diameter at full width half maximum (FWHM) is routinely obtained.

## EH2 focusing

EH2 is entirely dedicated to laser heating experiments, and so the quality and intensity of the focused beam is even more critical than in EH1 because of the large temperature gradients always present during laser heating. The ID27/EH1 170 mm long multilayer mirrors have been upgraded to a pair of 300 mm long mirrors at ID27/EH2. These multilayer mirrors are also made of iridium-alumina multilayers deposited on silicon wafers.

Large focal distances of 800 mm and 1200 mm are used for the horizontal and the vertical mirrors in order to avoid serious loss of spatial resolution and leave enough room for the laser heating setup.  The optical acceptance of these new mirrors is 70% larger than the 170 mm mirrors which gives a substantial gain in photon flux. Moreover, their alignment is simple and fast (less than 1 hour) and they are mechanically very stable. The focal spot measured by translating a tungsten wire at the focal point is similar to the one obtained in ID27/EH1 (typically 1.7x2.7 µm2 FWHM).