Operation modes of an APPLE II type undulator

Schematic drawing of an APPLE II device at different settings.

Top: linear mode without shift (horizontally polarized light). Middle: circular mode. The shift is set to the position with equal amplitudes of the horizontal and vertical magnetic fields (nearly quarter periode of the magnetic structure). Bottom: linear mode with shift set to half period of the magnetic structure. Moving the two rows parallel The horizontal and vertical magnetic fields are out of phase by 90° (parallel or elliptical mode).

In the so called antiparallel or inclined mode the two rows move in opposite directions. As a result there is no phase difference between the horizontal and the vertical magnetic fields and the emitted light remains linearly polarized but with a variable angle. This additional mode is realized at the devices UE46, UE52 (angular range: 0...+90°) and UE49, UE112 (angular range -90°...+90°).