The performance of cyclotron autoresonance maser amplifiers can be significantly improved by using a properly spatially profiled helical wiggler. The proposed configuration is able to alleviate the severe problem of performance degradation by beam velocity spread. Simulation results show that more than 20&percent; output efficiency can still be attained at 94 GHz, even with 5&percent; beam velocity spread. ©1997 American Institute of Physics.