Commissioning of OPHELIE in the DC mode: an electromagnetic planar/helicalcrossed VUV undulator at super-ACO

OPHELIE is a crossed Onuki-type fixed-gap electromagnetic undulator capable of producing any polarization state of the emitted light in the VUV by set ting its three main parameters: the vertical and horizontal driving current s and the longitudinal dephasing phi. After its construction in accordance with strict mechanical tolerances, the undulator was magnetically checked a nd calibrated with both a Hall probe and the pulsed wire technique showing a very satisfactory behavior with, in particular, a high horizontal/vertica l symmetry. OPHELIE has been then the object of a full on-beam commissionin g in the DC mode. The effects on the positron beam, in terms of closed orbi t distortion and betatron tune shifts, have been calibrated and fully analy zed. The promising results gained during the magnetic measurement campaign have been confirmed by the spectral characteristics of the undulator which are very satisfactory: full tunability of the fundamental between 4.7 and 2 0 eV, validation of the superimposition principle of the magnetic fields, a nd stability of the spectra with respect to a polarity/dephasing switch. Be sides, an unexpected phi-dependence of the emitted wavelength has been unra velled and will be taken into account in the computer control of the undula tor. (C) 2000 Elsevier Science B.V. All rights reserved.