Efficiency enhancement of a pre-bunched free-electron maser oscillator by locking to a single eigen frequency of the resonator

We present simulations results and experimental evidence of radiation energy extraction efficiency enhancement by use of pre-bunching in a free electron maser (FEM). This enhancement is attained by locking the oscillator to operate at a distinct resonator eigen frequency (longitudinal mode) which is different from the maximum-gain frequency of the free running oscillator. The mode selection is accomplished by means of pre-bunching at the desired eigen frequency. It was shown that maximum extraction efficiency is obtained if the dominant resonator mode (obtained by pre-bunching at that frequency) is the one having the lowest eigen frequency under the net gain larger than unity curve (which is below the maximum gain eigen-frequency mode that would normally dominate in a free running oscillator). The numerical simulations predict that it should be possible to obtain a basic extraction efficiency enhancement of the oscillator of 50%. Experimental results so far have shown an improvement of about 30%.