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 ${\mathrm{TEM}}_{00}$ mode. For a hollow electron beam, even with a finite mirror misalignment the transverse optical mode profile remains close to a ${\mathrm{TEM}}_{00}$ 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.
Highlights
Free-electron lasers (FELs) are well known for their transverse coherence and wide range tunability
Various theoretical [19,20,21,22,23,24] and computational [25,26,27] studies have been performed to investigate FEL stability in terms of both the saturated power and optical mode in the resonator, and are verified with experimental results [28]. Most of these earlier studies explain the stability of the FEL oscillator in terms of the resonator stability parameters g1, g2, size of the out-coupling hole, mirror misalignment [26], optical mode competition [24] and so on
We look at the dependence on the cavity configuration, and investigate in detail how mode competition is suppressed with a hollow electron beam
Summary
Free-electron lasers (FELs) are well known for their transverse coherence and wide range tunability. Various theoretical [19,20,21,22,23,24] and computational [25,26,27] studies have been performed to investigate FEL stability in terms of both the saturated power and optical mode in the resonator, and are verified with experimental results [28] Most of these earlier studies explain the stability of the FEL oscillator in terms of the resonator stability parameters g1, g2, size of the out-coupling hole, mirror misalignment [26], optical mode competition [24] and so on. IV we present a discussion on FELs that do not use hole out-coupling (such as the JLab FEL and proposed x-ray FEL oscillators), before concluding
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