Thermal Design Of Cryogenic Permanent Magnet Undulator For Sls

The cryogenic permanent magnet undulator (CPMU) is an insertion device in which the permanent magnets are cooled down to cryogenic temperatures to improve the magnetic performance in terms of the remanent field and coercivity. As it was found recently, the peak field and coercivity of permanent magnet materials like NdFeB are increasing as the temperature is decreased and reached a maximum at around 130 K. This temperature is not directly attainable by boiling of standard cryogenic fluids like liquid nitrogen (77 K) or liquid helium (4.2 K). We present a practical and reliable cooling method based on the thermal shunt principle with either liquid nitrogen or a cryocooler as a cold source. Design criteria, cryogenic analysis and the layout of a CPMU based on this principle are presented. A new CPMU with a magnetic period of 14 mm and a magnetic length of 1.7 m, has been manufactured and is presently installed and in operation at the Swiss Light Source (SLS) as part of a collaboration between PSI (Paul Scherrer Institute), SPring-8 and Hitachi Metals, Ltd.