Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

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Abstract
Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain