The PTB electromagnetic undulator for BESSY II.

The Physikalisch-Technische Bundesanstalt (PTB) will operate an electromagnetic undulator designed for radiometry at the BESSY II storage ring. The undulator has a period length of 180 mm, 21 full periods and a maximum magnetic induction of 0.46 T, resulting in a tuning range of the first harmonic from 5 to 150 eV at 1.7 GeV electron energy. Moreover, the electromagnetic design allows the undulator to be operated in a special mode with the period length doubled to 360 mm, thus accordingly shifting the tuning range to lower energies. The main design parameters of the undulator for radiometric applications, as well as measured magnetic field data, are presented.