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Two elliptical Apple II undulators are available at BACH. Harmonics and light polarization can be controlled by selecting gap (the distance between the top and bottom magnet arrays) and phase of the magnets.
Gap scan operation has been fully implemented for XAS and XMCD measurements. 

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.

8.1 (High Energy) From 175 eV-1600 eV First harmonic:175 eV-730 eV 2.0 GeV
    First harmonic:247 eV-1030 eV 2.4 GeV
8.2 (Low Energy) From 35/42 eV to more than 600 eV First harmonic (highest flux):35 eV-170 eV 2.0 GeV
    First harmonic (highest flux):42 eV-240 eV 2.4 GeV

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!

Last Updated on Monday, 06 July 2015 14:15