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Encouraged by the successful operation of the JLab Demo in 1998, many high current ERLs are now being designed with not only short pulse synchrotron beamlines but also FELs. Such inclusion has major implications on magnet quality, rf feedback requirements, wiggler design, srf cavity QL, halo, etc. Measurements on the JLab ERL FEL have identified new challenges. The JLab Upgrade was designed with a 160 MeV beam of 10 mA in 75 MHz, 300 fs bunches. FEL designers set transverse emittance and longitudinal bunching, but to accommodate an FEL in our ERL also means setting stringent phase stability requirements of (<6x10-9/fm rms) based on a desired FEL detuning tolerance of 1.2 microns. Recovered beam RF loading on the subsequent accelerated beam complicates satisfying these requirements. Gain in the rf feedback limits the accuracy of energy stability when loaded Qs are ~107 . Energy recovery to <10 MeV sets magnetic field tolerances at 10-4. We present measurements on the JLab ERL showing how to set system requirements to tolerate such FEL lasing.
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