DEVELOPMENT OF A CRYOGENIC PERMANENT MAGNET IN- VACUUM UNDULATOR AT THE ESRF

Lowering the temperature of NdFeB materials increases their field remanence and intrinsic coercivity. This property is potentially interesting for the construction of a cryogenic permanent in-vacuum undulator (CPMU). Around 150K, the coercivity is so increased that the NdFeB material is comparable to the Sm2Co17 material as far as resistance to radiation damage is concerned. The improvement in field remanence is less remarkable (8% at 150K) and is dominated by a reversible Spin Reorientation Transition (SRT) occurring around 135K. Below this temperature, the remanence decreases. The magnetization curves of NdFeB material measured at different cryogenic temperatures are presented. Non-linear models have been constructed and used in the RADIA code in order to compute the field performance of a hybrid NdFeB invacuum undulator. A prototype CPMU is currently under construction at the ESRF. It has a period of 18mm and a magnetic length of 2m. The field integral and local field measurements of the device at cryogenic temperature require new systems operated in vacuum. A stretched wire bench and a hall probe bench are under construction: their main characteristics will be presented.