Phasing of two undulators with different K values at the advanced photon source

Two full-length 2.4-m-long undulators, with period lengths 2.3 cm and 2.7 cm, were installed in tandem in the 5.6-m-long straight section on the storage ring in sector 14. One part of the user research program requires that both undulators be tuned to 12.0 keV and the x-ray intensity maximized. The total intensity is sensitive to the phasing between the undulators, so the distance between the devices must be optimized and the ends tuned appropriately. Because of the different period lengths, the gaps and K values of the undulators will be different: 10.6-mm gap and a K value of 1.17 for the shorter-period device and 15.7-mm gap and a K value of 0.93 for the longer-period device. A special shield was designed and installed between the devices to eliminate interference. Results of magnetic measurements, tuning, and computer simulations of the spectral performance are presented.