Design of high power FELs and the effects of diffraction on de-tuning in an FEL oscillator

Abstract

In experiments going back to the first free electron laser (FEL) oscillator at Stanford, the measured width of the desynchronism curve is often significantly greater than predicted by theory and two-dimensional (2D) simulations in (z; t). The results of new four-dimensional (4D) simulations in (x;y; z; t) show that this difference can be explained by the effects of diffraction. When the light is artificially constrained to remain in the cavity fundamental mode, 2D and 4D simulations give similar results, but when the light is allowed to self-consistently develop higher-order modes, the 4D simulations give different results that agree better with experiments. The results of new 4D simulations also show the effects of emittance versus electron beam energy and mirror shift versus mirror tilt on extraction. Analysis of these results examine the robustness of FEL designs.