MODIFICATION OF THE BESSY II OPTICS FOR THE IMPLEMENTATION OF A SMALL GAP UNDULATOR

BESSY II faces an increasing demand for photons in the range from 60 eV to 8 keVto be available at the same experimental station. A double undulator scheme is planned where a small period cryogenic undulator with a small gap is combined with a standard circularly polarized undulator. Several optics schemes for the 1.7 GeV BESSY II storage ring are discussed. BESSY II has two different types of straight sections, a high beta straight with βx, min=15 m and βy, min =4.5 m and a low beta straight with βx, min=βy, min=1.2 m. We discuss the present plan using a small detuning of an existing low beta straight to shift the small vertical beta waist to the centre of the small gap cryogenic undulator with only minor impact to the machine.Machine optics tests with a shifted vertical focus started in 2011.