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FEL Vacuum Guidelines for Users
Vacuum Guidelines for Experiments at FLASH
FLASH is a light source directly coupled to a superconducting linear accelerator operated at 2.2 Kelvin. As the whole vacuum system from the FEL to the experiment will be operated without windows there are special requirements on the optics and the vacuum system of the beamline and of the user experiments. Hydrocarbons and dust are expected to degrade the performance of the FEL and the quality of the photon beam and must therefore be kept on a minimum level. HASYLAB will supply an interface between the beamline and the experiment to keep the right vacuum conditions in the beamline. To make sure that the experimental end station fits to the requirements of the interface contact HASYLAB already in an early stage of a planed project. See also the HASYLAB Vacuum Guidelines for Beamlines and Experiments. In the following you will find some general rules for the experiments.
1. Oil free UHV and Vacuum Pumps
In order to protect optical elements in the beamline from degradation the sum of the partial pressures of gases having a mass above 45 amu must be below 10-3 of the total pressure. HASYLAB will provide a residual gas analyser (RGA) in the interface chamber which will allow to check that. Any one or a combination of the following primary pumps may be used: Ion pumps, titan sublimation pumps, and oil-free turbo pumps. Diffusion pumps are not permitted due to their inherent risks of oil contamination. Only oil-free mechanical pumps may be used as roughing pumps. Under extreme circumstances where no alternatives exists, exceptions may be given by the Hasylab vacuum group. Efficient differential pumping stages must be between those pumps and the last valve of the beamline.
2. Dust Particles
Dust particles on optical surfaces are expected to reduce their performance (radiation damage, scattering) an should be kept on the lowest possible level. Before installation of vacuum components dust should be removed by an appropriate cleaning procedure. In the case that this is impossible, pump and purge with dry nitrogen has to be applied until the numbers of particles in the gas flow is sufficient low. HASYLAB provides the tools for this procedure in the experimental hall.
Roughing pumps must be separated from the primary pumps by pneumatic valves which are controlled by the vacuum interlock system of the experiment. In the case of a pressure or power failure the valves will close automatically.
3. Vacuum Interlock Requirements and Venting Procedure
The beamline vacuum is protected from accidental venting by the vacuum interlock of the experiment. The HASYLAB vacuum group will check the function of the interlock before experiments can set into operation. In order to keep dust particles off from sensitive beamline parts a special venting procedure is necessary. Venting must always performed with particle free dry nitrogen. The venting gas flow is directed in such a way that dust particles stream always downstream away from the last beamline-valve towards the experiment. Venting through turbo pumps is not permitted.
