Study of FEL mirror degradation at the DUKE FEL and higs facility
The Duke FEL and High Intensity Gamma-ray Source (HIγS) are operated with a wide range of electron beam energies (0.24-1.2 GeV) and photon beam wavelengths (190-1060 nm). Currently, the HIγS user operation is carried out in the gamma-beam energy range from 1 to about 65 MeV, with a near-future extension to about 100 MeV using 190 nm FEL mirrors. The maximum total gamma-flux produced at the HIγS facility is up to 1010 gammas per second around 10 MeV. Production of this high level gamma-ray flux requires a very high average FEL photon beam power inside the FEL resonator at one kilowatt or more. The high power FEL operation can cause significant degradation of the FEL mirrors due to higher-order wiggler harmonic radiation, especially when operating the FEL in the UV region at a high electron beam energy. This has limited the high-energy, high-flux HIγS gamma-beam operation mostly to circular polarization when UV mirrors are used, as higher-order harmonic radiation of helical wigglers is peaked off-axis. To ensure the predictability and stability of the HIγS operation for user research program, we have developed a comprehensive program to continuously monitor the performance of the FEL mirrors. This program has enabled us to use a particular set of FEL mirrors for a few hundreds hours of high gamma-flux operation with predictable performance. In this work, we discuss sources and consequences of the mirror degradation for a variety of wavelengths.
