Apple II undulator

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.

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