Beamline description

Optical Klystron

The two APPLE-II type permanent magnet undulators, together with an electromagnetic chicane, constitute modulator (Elettra ID 1.1), radiator (Elettra ID device 1.2) and dispersive section of the optical klystron. Wavelength and polarization of the undulators are independently tunable.

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 polarization 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.

The SR-FEL beamline shares the insertion device with the Nanospectroscopy beamline. During the Nanospectroscopy operations the dispersive section is used as a phase-shifter, which enables the two undulators to be properly phased, thus effectively doubling the undulator length and the useful flux.