• J. Bahrdt, W. Frentrup, A. Gaupp, M. Scheer, F. Schäfers, G. Wüstefeld
  Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Elektronen-Speicherring BESSY II, Berlin

At BESSY II photon energies above 2keV can be produced only with bending magnets, a permanent magnet wiggler, superconducting (SC) wavelength shifters and a SC-wiggler. The wiggler brilliance suffers from the depth of field effect and the bending magnets and wavelength shifters produce the X-rays only with a single pole. Experiments such as HIgh Kinetic Energy photoelectron spectroscopy (HIKE) or microspectroscopy on nanostructured materials demand a high brilliance and flux as it is provided by a small period cryogenic undulator. This paper discusses the requirements for the operation of small gap cryogenic devices at BESSY II. A scheme with two adjacent, vertical low beta sections inside of one of the long straight sections is suggested. The straight is divided into two parts by a quadrupole triple in the center. An optic with an increased, vertical beta tune by 0.5 is presently studied. The optics outside of the low beta section and the horizontal tune are kept unchanged.

• J. Bahrdt, H.-J. Baecker, M. Dirsat, W. Frentrup, A. Gaupp, D. Pflückhahn, M. Scheer, B. Schulz
  Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Elektronen-Speicherring BESSY II, Berlin
• F.J. Grüner, R. Weingartner
  LMU, Garching
• D. Just
  Technische Universität Berlin, Berlin
• F.H. O'Shea
  UCLA, Los Angeles, California

In collaboration with the Ludwig-Maximilian-University Munich a cryogenic PrFeB- based undulator has been built. The 20-period device has a period length of 9mm and a fixed gap of 2.5mm. The undulator has recently been installed at the laser plasma accelerator at the Max-Planck-Institute for Quantenoptik. The operation of a small gap device at a high emittance electron beam requires stable magnetic material. A high coercivity is achieved with PrFeB- material which is cooled down to 50K. This temperature is 100K lower as compared to the temperature of a NdFeB-based cryogenic undulator. In this paper we present the mechanic and cryogenic design and compare the predictions with measured data. The results are extrapolated to a 2m-long variable gap undulator.