Protection of VUV FEL mirrors using soft orbit bump at duke FEL / HIγS facility
The Duke FEL and High Intensity Gamma-ray Source (HIγS) facility is operated with an electron beam from 0.24 to 1.2 GeV and a photon beam from 190 to 1060 nm. Presently, the energy range of the gamma-beam is from 1 MeV to about 100 MeV, with the maximum total gamma-flux of more then 1010gammas per second around 10 MeV. Production of high intensity, high energy gamma-beams of 60 to 100 MeV, using UV-VUV mirrors of 240 to 190 nm, requires high energy, high current electron beams of 0.9 to 1.05 GeV. Synchrotron radiation damage to the FEL mirrors becomes crucial for VUV FEL operation at or below 190 nm. The edge radiation (ER) from the End-of-Arc (EOA) bending magnet, instead of the radiation of FEL wigglers, is the dominant cause of rapid degradation of the downstream FEL mirror. We describe here a further development of the "soft" orbit bump concept to significantly reduce the radiation exposure to the mirror from the EOA dipole magnet. The bump uses designated "soft" orbit correctors with magnetic field limited to produce radiation with a critical wavelength close to or longer than the FEL wavelength. A first experience of HIγS user runs with 192 nm mirrors utilizing the soft bump is also presented. Copyright © 2013 by JACoW.
