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Conceptual Design Report

CDR chapter 7 - Undulators


The FERMI undulators are based on PPM arrays, a choice dictated mostly by the need to provide variable polarization. The present design is based on a remanent field Br=1.2 T, which allows to choose material grades with close to the maximum available coercitivity. High coercitivity makes the material highly resistant to radiation, a feature of paramount importance for its long lifetime. The undulator coefficient K must be greater than 1 in order to provide sufficient FEL gain. The minimum acceptable gap height is 10 mm, dictated by residual gas pressure and energy losses induced by resistive wall wakefields in the undulator vacuum chambers.
The modulators for both FEL-1 and FEL-2 have fixed, linear polarization. They must be tunable in the 240-360 nm range, a requirement that can be satisfied by a wide range of period lengths among which the shortest acceptable, ~ 10 cm, is favoured for FERMI. The first stage radiator and the second stage modulator for FEL-2 are also designed for fixed, linear polarization.
All the other undulators are of the variable polarization type, based on the widely used APPLE-II (Advanced Planar Polarized Light Emitter) configuration, the most efficient one for this application. All polarizations are tunable over the full design tuning ranges of 100 to 40 nm and 40 to 10 nm of FEL-1 and FEL-2 respectively. Because no analytical expression is known for the field amplitude as a function of the geometrical parameters of such devices, a semi-empirical formula is used derived by fitting the results of 3D magnetostatic calculations performed on a number of different special cases.
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Ultima modifica il Giovedì, 17 Ottobre 2013 09:25