Dispersive section undulator

Undulator radiation also has specific polarization properties, e.g., the fundamental radiation of a plane undulator as shown in Fig. 1.9 is completely linearly polarized, an undulator with helical magnetic structure produces circularly polarized light, and two crossed plane undulators with a dispersive section between them are capable of producing optional polarization which depends on the phase shift introduced by the dispersive element. 

Fig. 10. Optical Klystron. Schematics of undulator and dispersive sections. The focused pulsed laser case is discussed in section 4.2.

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