PSICHÉ, high-energy photons for tomography and studies in extreme conditions

 
The PSICHÉ team,
from left to right: Pierrick Zerbino, Jean-Paul Itié and Nicolas Guignot.

PSICHÉ  was born of a dual requirement: on the one hand, to offer a high-performance, flexible beamline to satisfy all the requirements of the large French scientific community specialising in x-ray diffraction under extreme conditions (pressure, temperature), and on the other hand, a high-energy absorption contrast tomography beamline.
To satisfy these requirements, SOLEIL decided to produce high-energy photons using a wiggler as the light source. The characteristics of this insertion element have been studied on many occasions by the Insertion and Machine Physics groups at SOLEIL. This led to the creation of an in-vacuum wiggler of 38 periods of 50 mm and a 2.1-T field between the magnets.
Several experimental installations are placed on this beamline, to benefit from this light source.

Like bending magnets, wigglers produce a beam with a continuous spectrum; unlike undulators, which emit only certain wavelengths that can be modulated by adjusting the distance between the magnets in the undulator. PSICHÉ exploits this special feature of wigglers. Its first experimental hutch is also the optical hutch of the beamline: the light emission from this insertion element will be used there for ‘white beam’ experiments. These are real-time 2D imaging experiments or diffraction experiments that can be carried out on confined sample environments, with very little angular opening, like multi-anvil presses, for high-pressure studies.
It will be easy to switch from one type of measurement to another by adjusting the opening of a system of slits, which are reduced from 10 mm to approximately 5 µm in width in the case of diffraction.

Another mode of operation of the beamline: in monochromatic light for applications and experiments in the second hutch.
In unfocused mode, using the natural divergence of the source, the beam will be used for full-field tomography experiments at high flux (1013 photons/sec). The important features of PSICHÉ are high flux and the energy spectrum of the beamline, for better detection of heavy elements.

A first horizontal and vertical focusing will provide a 100 x 100 µm beam for high-pressure diffraction experiments. These may be conducted on small samples up to 100 µm in diameter, placed in diamond anvils in which the pressure reaches 100 GPa, i.e. one million times atmospheric pressure. Another option: the use of Paris-Edinburgh cells for slightly lower pressure measurements (approximately 10 GPa), but on large sample volumes, which allows an oven to be inserted into the compressed space and reach uniform temperatures above 1000°C on the sample.

A system of slits arranged at the first focal point will play the role of secondary source, leading to a 10 x 10 µm beam that will be used for diffraction measurements on smaller samples. These experiments will be conducted under the most extreme conditions of pressure (hundreds of GPa) and temperature (thousands of degrees), thanks to the connection of a laser heating system.
A ‘High Pressure’ laboratory is also currently being set up at SOLEIL and, following on from the Chemistry, Biology and Surface laboratories, will soon supplement the resources made available to users, most notably to prepare their samples.

Thanks to the measurements performed under extreme conditions, geophysicists will be able to reproduce the pressure and temperature conditions that exist nearly 3000 km below the Earth’s surface, at the border between the liquid core and the internal mantle, a contact zone where many phenomena, affecting volcanic activity for example, take place.
The high-flux tomography possibilities interest scientists in the field of metallurgy, where the sensitivity of the beamline to heavy elements is a major advantage.
For solid-state chemists, a beamline like PSICHÉ will make it possible to synthesize new materials by varying the temperature and pressure conditions. In situ measurements are necessary to optimise the thermodynamic path leading to the creation of the material (determination of intermediate phases, obtaining the lowest pressure and temperature paths, obtaining pure phases, etc.).

Physicists will study the properties of matter under pressure, at conditions where the interatomic distances are reduced, which greatly modifies the electronic interactions. Insulator-metal transitions can be induced, superconductivity can be made to appear, or ferroelectricity or ferromagnetism can be eliminated from a material for a better understanding of the origin of these properties, thanks to a comparison between the experiments and the ab initio calculations. They will thus be able to study the behaviour of matter at medium pressure and very low temperature; a possibility also provided by PSICHÉ thanks to the presence of a cryostat which is compatible with high-pressure cells.

Biology has not been forgotten: proteins can be studied under moderate pressure (1 GPa!) in order to monitor their conformational changes. These changes, induced by a State function (pressure) will allow certain aspects of the protein folding process to be documented.

Finally, one of the ultimate goals is to develop pressure tomography, which will help to answer such questions as: how are low-density elements, such as metallic foams, compressed when subjected to high hydrostatic pressures? In other words: measuring the compression of holes. A far more complex problem that it might seem!

To summarise: PSICHÉ is suited to a wide range of research topics and offers new experimental possibilities to complement the diffraction beamlines that are already operational at SOLEIL, as well as future tomography beamlines (Nanotomography and PUMA).