Improved free-electron laser stability and performance using a hollow electron beam

Abstract

We consider the performance of free-electron lasers (FELs) in the oscillator configuration, using a hollow electron beam distribution instead of the usual Gaussian. Using the three-dimensional, time-dependent FEL code genesis, we show that for FEL oscillators lasing can be achieved over a much broader range of cavity configurations with a hollow electron beam. This occurs because with the hollow electron beam higher-order optical modes and mode competition are suppressed. We also find a substantial increase in the saturated out-coupled power, with the optical mode still remaining the fundamental TEM_{00} mode. For a hollow electron beam, even with a finite mirror misalignment the transverse optical mode profile remains close to a TEM_{00} mode but with higher out-coupled power than with perfectly aligned mirrors. Thus, it is preferable to operate FEL oscillators with a hollow electron beam rather than a Gaussian.