Experimental and theoretical study of a differentially pumped absorption gas cell used as a low energy-pass filter in the vacuum ultraviolet photon energy range

In order to separate the fundamental synchrotron radiation from the high ha rmonics emitted by an undulator, a low photon energy-pass filter has been d esigned and built, ensuring a high spectral purity on the vacuum ultraviole t (VUV) SU5 beamline at Super-AGO. It consists bf an absorption cell filled with rare gases and separated from the ultrahigh vacuum of the storage rin g and of the beamline by a double differential pumping obtained with thin c apillaries. Its conception has been optimized by numerical computation of p umping speed. Admission pressures in the range of 100 Pa in the central par t of the filter have been used without any degradation of the upstream or d ownstream ultrahigh vacuum. The measured attenuation factors above the ener gy cutoff are above 10(5) and 10(2) (and certainly above 10(3) with ultimat e pressure of Ne) for argon and neon absorbing gases, respectively, with no measurable attenuation of fundamental radiation. A sophisticated numerical simulation of the pressure distribution, taking into account the geometry of the whole absorption cell including the first pair of capillaries, has b een developed. The corresponding calculated attenuation factors are in very good agreement with the measurements, and thus allow reliable predictions of the expected attenuation factors for any given configuration of the filt er. (C) 2000 American Vacuum Society. [S0734-2101(00)07205-5].