Synchrotron monochromator heating problem, cryogenic cooling solution

We report on a new prototype of a cryogenic cooling system which is running on the beamline BM30-A/FIP at the ESRF Grenoble. This system has been specially designed to cool the first crystal of a double crystal monochromator subject to the white beam delivered by a bending magnet X-ray source on a third generation synchrotron. The proposed cryogenic cooling loop represents an alternative solution between the usual water cooling method and the very efficient but expensive cryogenic system already existing for insertion devices. The new concept of this cooling loop, an open thermodynamic system, is "optimal" from the point of view of the economical versus performance ratio to dissipate a heat load up to 1 kW. For such a heat load range the efficiency of classical water cooling system begins to fail, and the thermal dilatation of silicon affects the monochromating process. Some details of the loop and the running principle are exposed in this paper. A particular attention has been paid to the conception of the crystal and the heat exchangers. The distortion of the crystal has been verified in practice from the measurements of the double crystal rocking curves under different experimental conditions and compared with the expected theory. (C) 2001 Elsevier Science B.V. All rights reserved.