Valléau, M. Berteaud, P. Briquez, F. Brunelle, P. Béchu, N. Couprie, M.-E. Da Silva Castro, J. Dubuisson, J.M. Ghaith, A. Herbeaux, C. Idam, J. Kitegi, C.A. Lepage, F. Lestrade, A. Louvet, M. Marcouillé, O. Marteau, F. Mary, A. Nadji, A. Nadolski, L.S. Rommeluère, P. Sebdaoui, M. Somogyi, A. Tavakoli, K.T. Tilmont, M. Weitkamp, T. JACoW Publishing Geneva, Switzerland 978-3-95450-206-6 10.18429/JACoW-FLS2018-THP2WD01 English 193-198 THP2WD01 ion undulator cryogenics vacuum electron Contribution to a conference proceedings 2018 2018-06 https://doi.org/10.18429/JACoW-FLS2018-THP2WD01 http://jacow.org/fls2018/papers/thp2wd01.pdf With permanent magnets undulator operation at cryogenic temperature, the magnetic field and the coercivity can be enhanced, enabling shorter periods with high magnetic fields. The first full scale (2 m long, 18 mm period) hybrid cryogenic undulator [1] using PrFeB [2] magnets operating at 77 K was installed at SOLEIL in 2011. Photon spectra measurements, in good agreement with the ex-pectations from magnetic measurements, were used for precise alignment and taper optimization. The second and third 18 mm PrFeB cryogenic undulators, modified to a half-pole/magnet/half-pole structure, were optimized without any magnet or pole shimming after assembly but mechanical sortings and some geometrical corrections had been done before assembly. A systematic error on individual magnets on the third U18 was also compensated. In-situ measurement benches, including a Hall probe and a stretched wire to optimize the undulator field at room and cryogenic temperature are presented. An upgrade of these in-situ benches will be detailed with the fabrication of a 15 mm 3 m long PrFeB cryogenic undulator at SOLEIL.