• C. Benabderrahmane, P. Berteaud, N. Béchu, M.-E. Couprie, J.-M. Filhol, C. Herbeaux, C.A. Kitegi, J.L. Marlats, K. Tavakoli, M. Valléau, D. Zerbib
  SOLEIL, Gif-sur-Yvette

The production of hard X rays at SOLEIL, a 2.75 GeV, requires short period and small gap in-vacuum undulators. For shifting further the radiation toward higher energies, the peak magnetic field of the undulators can be further increased by cooling the permanent magnets at cryogenic temperature below 100 K. A R&D programme for the construction of a 2 m long 18 mm period CPMU is launched: the use of PrFeB enables to increase the peak magnetic field at a cryogenic temperature of 77 K. Praseodymium was chosen instead of Neodymium type magnets, because it prevents the appearance of the Spin Reorientation Transition. The magnetic characterisation of different permanent magnet grades at cryogenic temperatures (NdFeB and PrFeB), and the magnetic and thermal measurements on a small 4 period NdFeB cryogenic undulator are presented. The status on the progress of the CPMU conception is given. The magnetic and mechanical design, including the cooling of the girders at 77K, and the thermal budget are described. The designs of the dedicated magnetic measurement benches, which will be required to check the magnetic performance of the undulator at low temperature, are also reported.