HZB is in the process of developing a concept for a successor to the BESSY II synchrotron facility. The new facility will build on the strengths developed in Berlin over the last twenty years in delivering flexibly polarised soft X-Rays to dozens of beamlines. The successor facility BESSY III is planned to operate at 2.5GeV, in comparison to the 1.7GeV operation of BESSY II. This makes it easier to achieve the goal of delivering 1keV photons to beamlines on the first harmonic of our APPLE II Insertion Devices. It also makes it easier to achieve the aspiration of delivering tender X-Rays up to 10keV more routinely to users utilising in-vacuum APPLE II devices*, Cryogenic Permanent Magnet Undulators (CPMUs) or Cryogenic APPLE devices[2]. However, it also presents challenges in delivering the low energy photons below 10 eV, as period lengths of the relevant undulators must be increased, which in turn increases on-axis power. APPLE-KNOT designs will be pursued to overcome this issue. The undulator group will also be planning Double Period Undulators[3] to offer beamlines broad spectrum coverage from 50eV to 10keV on the 1st and 3rd harmonics. This paper outlines the first choices of undulators to be available to the successor facility BESSY III.

The worldwide first in-vacuum elliptical undulator, IVUE32, is being developed at Helmholtz-Zentrum Berlin. The 2.5 m long device with a period length of 3.2 cm and a minimum gap of about 7 mm is to be installed in the BESSY II storage ring. The device follows the Apple-II design and features four magnet rows. Both the two bottom and two top rows can be shifted longitudinally. This shift needs to be permitted by the shielding foils that cover the permanent magnets. The proposed solution calls for a longitudinal slit in the top and bottom shielding foils, which gets folded into the gap between the top and bottom magnet rows respectively. The manufacturer states that the folding process can introduce a small sinusoidal error to the slit width. We will present wakefield simulation studies that investigate the effect of different possible foil gap variations.