Gas Absorber

The pulses delivered by the FELs have high peak powers, and some experiments need a way to reduce the FEL intensity over many orders of magnitude (example: for sample alignment); consequently, a gas absorbing (GA) section is installed between the two BPM-I0M sections. This GA represents a reliable way to reduce the intensity without changing the photon beam characteristics. It consists of a windowless gas-filled cell differentially pumped with respect to the other sections. Different gases can be used in order to cover the whole spectral range emitted by the FELs, and different pressures have to be used to guarantee the attenuation factor requested by the users. In particular, by using a maximum gas pressure of about 0.1 mbar on a 6 meters-long cell, it is possible to reduce the overall intensity by 4 orders of magnitude. The calculated transmission for Nitrogen, Xenon, and Krypton is reported in figure (Transmission data calculated using CXRO online toolset).
It is clear that Nitrogen is enough to cover almost the entire range, excluding the low wavelength part between 5 and 20 nm that can be covered by Xenon and Krypton. The GA length (6 m) is 2 times the safety wall thickness. In this way we can insert the gas directly in the center of the pipe, and symmetrically pump it at the two edges, simplifying considerably the mechanics of the systems. Recently, the GA has been endowed by Neon and Helium, besides Nitrogen.

Last Updated on Friday, 15 November 2019 12:07