Undulators

INSERTION DEVICES: APPLE II UNDULATORS

The Apple II type helical undulator is a pure permanent magnet structure, composed of four arrays, as shown in the Figure. The arrangement of blocks is such that there are four blocks per period. By moving two opposing magnet arrays with respect to the other two longitudinally (a phase shift), the strengths of the vertical and horizontal magnetic field components can be varied, and hence the polarisation of the radiation produced.

The advantage of such a device is that the radiation can be polarised vertically, horizontally, and circularly by moving the arrays, which provide a horizontal field, as well as a vertical one, purely from magnet blocks above and below the electron beam.

Since BACH employs two different undulators for different energy ranges, the user must select properly the undulator to work with. Important Note: the two undulators cannot be used simultaneously!!! When one of them is in use (gap below 100 mm) the other must be at the maximum gap (100 mm) and zero phase and zero tapering. If both gaps are below 100 mm a beam dump will occur and all beamlines will loose the beam!