Microwave axial free-electron laser with enhanced phase stability

Free-electron laser (FEL) amplifiers in the Raman regime have high efficiencies and high output power at microwave wavelengths; however, the phase stability is not sufficient for driving linear accelerators. Fluctuations in the diode voltage are the largest cause of the shifts in the phase of the output power. Present-day pulse-power technology cannot keep the voltage fluctuations less than 14%. In this paper, we study the phase stability by analyzing the dispersion relation for an axial FEL, in which the rf field is transversely wiggled and the electron trajectories are purely longitudinal. It is found that the phase dependence on the beam velocity can be cancelled by the effect of the phase dependence on the beam plasma wave. This cancellation leads to first-order phase stability, which is not possible for standing-wave devices, such as klystrons. Detailed particle-in-cell simulations demonstrate the transverse wiggling of the rf mode and the axial FEL interaction and explicit calculations of the growing root of the dispersion relation are included to verify the phase stability.