Microwave axial free-electron laser with enhanced phase stability

Free-electron lasers (FELs) amplifiers have demonstrated high efficiencies and high output power at microwave wavelengths. However, measurements and simulations have indicated that the present level of phase stability for these devices is not sufficient for driving linear accelerators. Fluctuations in the diode voltage, which is needed to accelerate the electron beam, 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 1/4%. However, we have found a scheme that win make the output phase much less sensitive to these fluctuations by exploiting the traveling-wave nature of the FEL interaction. In this paper we study the phase stability issue by analyzing the dispersion relation for an axial FEL, in which the rf field is transversely wiggled and the electron trajectories are purely longitudinal. The advantage of using the axial FEL interaction instead of the common transverse FEL interaction is that the dispersion relation is not additionally complicated by how the transverse electron motion depends on the diode voltage and such a device is simpler and less expensive to construct than a transverse-coupling FEL because there is no wiggler. By examination of the dispersion relation it is found that the effect of the phase dependency on the beam`s velocity can be cancelled by the effect of the phase dependency on the beam`s plasma wave, for an annular electron beam. This cancellation leads to first-order phase stability, which is not possible for standing-wave devices, such as klystrons. Detailed particle-in-cell simulations are included to demonstrate the transverse wiggling of the rf mode and the axial FEL interaction