Efficiency enhancement of RF-linac based free electron laser oscillator with electron beam energy ramping

Free electron laser (FEL) oscillator is a very effective method to produce the tunable and high-intense laser in the infrared and terahertz regions. However, in an FEL oscillator, normally only a small portion of the electron beam's kinetic energy is converted into the optical field. Inspired by the earlier work on the Israeli EA FEL, in this paper we investigate increasing the efficiency of RF-linac based FEL oscillators through ramping up the electron beam energy moderately in the post-saturation regime. Based on theoretical analysis, the optimal beam energy ramping amplitude and the possible maximum efficiency enhancement are presented. Three-dimension simulations with the FELiChEM parameters show that, through ramping the electron energy from the saturation point by 1.9% and 2.68% for the FEL wavelengths of $10\mu m$ and $30\mu m$, respectively, about 3.6 and 9.1 times enhancements of the FEL efficiency at the new saturation are achieved. The simulation results are consistent with the analytical prediction. This scheme may also benefit the x-ray FEL oscillators.