Thanks to the experience on insertion devices acquired at SOLEIL, the choice of the undulators tends to select solutions not very different from the ones adopted at SOLEIL. For LEL1, 2, and 4, a conventional High Gain Harmonic Generation scheme comporting a modulator, a dispersive section and a radiator has been chosen.
Different configurations are foreseen, from the 1-1 (modulator and radiator tuned on the seed wavelength), 1-2 (radiator tuned to the second harmonic of the modulator resonant wavelength), 1-3, 1-5 and harmonic cascade (resonant wavelength of the radiator equal to the resonant wavelength of the modulator multiplied by a ratio of integer numbers).
An U26 has been chosen as a modulator for LEL1 and LEL2, allowing efficient bunching by the seed for the configuration 1-3 for example. The use of a short period high magnetic field in vacuum undulator allows the total length to be reduced and the system to be more compact. LEL2 aiming at operating even below 1 nm on the non linear harmonics, the magnetic field has been pushed in this case with a radiator of 18 mm period.
A further analysis on the undulator choices will be carried out during the TDR phase.
Recent development in cryogenics undulators permit to enhance the peak magnetic field by a factor 1.3 using a specific magnet variety operated at low temperature (typically 140 K). First proposed at SPring-8, real advances have been achieved so far in particular at SPring-8, ESRF and BNL. R&D is launched on this subject at SOLEIL.
http://www.synchrotron-soleil.fr/portal/page/portal/SourceAccelerateur/InsertionsGeneral
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Fig.1: U20 undulator
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Fig.2: HU80 undulator
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