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A microwave field can be used to accelerate electrons as they lose energy to radiation in an FEL, thereby maintaining synchronism and, used in conjunction with an optical klystron, providing high conversion efficiencies. The microwaves can be programmed to increase in time with the optical power level in the FEL so that high gain is maintained over a wide range of power levels. In this paper, parameter constraints for such an FEL are discussed, leading to a structure design that integrates a wiggler with a linac. It is shown that conversion ettlciencies of 50 percent at m with a 2 m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain.