Commissioning of OPHELIE in the DC mode: an electromagnetic planar/helical crossed 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 setting its three main parameters: the vertical and horizontal driving currents and the longitudinal dephasing φ. After its construction in accordance with strict mechanical tolerances, the undulator was magnetically checked and calibrated with both a Hall probe and the pulsed wire technique showing a very satisfactory behavior with, in particular, a high horizontal/vertical symmetry. OPHELIE has been then the object of a full on-beam commissioning in the DC mode. The effects on the positron beam, in terms of closed orbit distortion and betatron tune shifts, have been calibrated and fully analyzed. 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 20 eV, validation of the superimposition principle of the magnetic fields, and stability of the spectra with respect to a polarity/dephasing switch. Besides, an unexpected φ-dependence of the emitted wavelength has been unravelled and will be taken into account in the computer control of the undulator.