Description of an Electromagnetic/Permanent Magnet Helical Undulator for Fast Polarization Switching

A new Electromagnetic/Permanent Magnets Helical Undulator (EMPHU), with a 65 mm magnetic period is under construction at SOLEIL for providing a rapid switching at 5 Hz of the photon polarization required to perform dichroism experiments. The vertical field is produced by coils fed by a fast switching power supply, with a maximum current of 350 A and a polarity switching time shorter than 100 ms. The coils consist of copper sheets shaped by water jet cutting. 25 layers of copper are stacked together. The current flows in 16 layers and nine of them are cooled with thermal drain to a water piping. The current-regulated power supply, in-house developed, should be able to operate in the 4 quadrant with a 50 ppm current resolution over the full scale. It is complemented by eight power supplies for the correction coils required to provide dynamics correction of the field integrals. The horizontal field is generated by NdFeB permanent magnets. The designed vertical and horizontal first harmonic field values in the helical configuration reach 0.24 T at the minimum 14.7 mm gap. The magnetic design, the magnetic measurements (DC and AC) and the correction schemes are described.