Performance of cryogenically cooled monochromators at SPring-8

The performance of the SPring-8 cryogenic cooling system was examined extensively, especially, on the features closely related to the actual experimental situations. The cryogenic cooling system has sufficient cooling capability to, handle the permitted highest heat load of 478 W at BL29XUL which equips the SPring-8 standard in-vacuum undulator. The effect of the heat load on the beam profile and the rocking curve width was reasonably small. The rocking curve width with Si 333 diffraction at 18.7 keV was close to the theoretical width 0.86". Thus the monochromator conserves the good spatial coherence originating from the small source size of the undulator. Mechanical vibrations at frequencies of 30-100 Hz degraded the parallelism between the two monochromator crystals. The angular width of the mechanical vibration was measured to be 0.2" independent of the heat load and the flow rate of the circulating liquid nitrogen. The output intensity was stable and had no drift under any heat load. On the other hand, it took a few hours before the output energy stabilized, when the heat load was increased from 0.135 to 112 W.