A DIFFERENTIAL PUMPING SYSTEM FOR ONE OF THE ESRF ULTRA-HIGH-VACUUM BEAMLINES

When low photon energy ranges (3-40 keV) are required in a synchrotron beamline, the use of beryllium windows to isolate the machine vacuum from the beamline is forbidden. In addition, if the operating pressure of the beamline is three orders of magnitude larger that of the machi ne, the only way of realizing vacuum insulation is to install a differ ential pumping system. The ID20 European Synchrotron Radiation Facilit y beamline dedicated to magnetic scattering studies imposes particular ly difficult conditions: no beryllium window is allowed; it may operat e at 10(-7) mbar pressure while the machine vacuum must be kept at 10( -10) mbar. In addition the beam size may be as large as 12x40 mm(2), w hich imposes rather large sections of the connecting pipes, and the ma ximum power is over 3kW. A differential pumping system meeting these r equirements for most common gases has been built. It includes two 400 I/s ion pumps connected in series with tubes of, respectively, 6.3 and 14.1 I/s. Details on the construction and performances are reported i n this paper. (C) 1997 Elsevier Science Ltd.