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  Kuhn, C.; Bäcker, H.-J.; Bahrdt, J.; Gaupp, A.; Schulz, B.: Hall-Probe Bench for Cryogenic In-Vacuum Undulator. In: Zhimin Dai ... [Ed.] : Proceedings of IPAC 2013, Shanghai, China, 2013. - ISBN 978-3-95450-122-9, p. 2126-2128
  http://inspirehep.net/record/1338780/files/wepwa003.pdf?version=1

Open Access Version

Abstract:
The Helmholtz-Zentrum Berlin (HZB) builds a 2m long in-vacuum Hall-probe measuring bench for the characterization of several in-vacuum cryogenic undulators currently under development. Aceurate local magnetic measurements need a positioning control of about 5 }Jlll. Fabrication tolerances and potentially strong temperature gradients require an active correction of the Hall probe movement along a straight line. The HZBbench employs a system of Iaser interferometers and positionsensitive detectors, which are used in a feed-back loop for the Hall probe position and orientation.