A hole in the ring ... to see photons at atmospheric pressure

Analysis of samples in the VUV is generally done under high vacuum conditions, to avoid absorption of this radiation by O₂ molecules. However, the study of biological and/or liquid samples is not compatible with such low pressures. In addition, no material offers a transparent window to VUV. To balance the needs for air pressure and VUV, a differential pumping system has been developed by the Vacuum, Design Engineering, Optical and Detector groups and the DISCO beamline at SOLEIL, in collaboration with the Mass Spectroscopy Laboratory in Gif-sur-Yvette.

From left to right: Nicolas Bechu, engineer in the Group “Vacuum & Cryogeny”, and Matthieu Réfrégiers, in charge of DISCO beamline, pointing a finger at the extremity of the differential pumping system.

The principle is as follows: photons produced by the light source of the DISCO beamline, a bending magnet, are guided to the experimental station across four chambers in which the prevailing pressure becomes higher and higher, until reaching atmospheric pressure.
To avoid the maximum absorption of VUV, no window is used to separate two successive chambers - most materials usually used for this type of window (LiF, MgF₂) not being transparent to VUV rays. Everything rests on the differential pumping system, which can make the transition from ultra-vacuum in the storage ring (that is to say the vacuum chamber in the bending magnet) to the end of the device receiving the sample at atmospheric pressure in a flux of rare gas (neon or argon, which do not absorb VUV).
The idea is to gradually reduce the conductance of a section to the next, using openings or tubes of the correct size between these sections. Given the fact that a bending magnet is a light source producing a rather divergent beam, thus requiring the openings to be wide enough to conduct the radiation, conductance of the system is inherently high. Designing this system was therefore far from being a trivial matter.

The final device, with a total length of 84 cm, can provide, on one of the three branches of the DISCO beamline, VUV photons up to 60 nm or 20 eV; that is to say, in an energy range that is inaccessible if MgF₂-type windows are used. It causes no disruption to the vacuum level of the storage ring and has sufficiently low impact on the rest of the beamline that this experimental station can be used continuously in the future. This will be of interest to many groups, especially in the biology field.
 

¹- VUV - Vacuum Ultraviolet: high energy ultraviolet photons (30-200 nm, i.e. 5-40 eV), absorbed by the air.

Reference :
Giuliani, A., Yao, I., Lagarde, B., Rey, S., Duval, J. F. L., Rommeluere, P., Jamme, F., Rouam, V., Wien, F., de Oliveira, C., Ros, M., Lestrade, A., Desjardins, K., Giorgetta, J. L., Laprévote, O., Herbeaux, C., & Refregiers, M. "A differential pumping system to deliver windowless VUV photons at atmospheric pressure". Journal of Synchrotron Radiation, 18(4): 546-549. (2011).

Further reading:
“Le Vide ou le Rien” teaching kit