Short Rayleigh length free electron lasers
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Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of highpower infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.
The article of record as published may be located at http://dx.doi.org/10.1103/PhysRevSTAB.9.030703
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Colson, W.B.; Blau, J.; Armstead, R.L. (2003);High-power, short-wavelength free electron lasers (FELs) can make use of a short-Rayleigh-length (SRL) optical mode in order to reduce the intensity on resonator mirrors. The conventional FEL interaction attempts to optimize ...
Crooker, P.P.; Campbell, T.; Ossenfort, W.; Miller, S.; Blau, J.; Colson, W. (2003);In order to avoid mirror damage on a high-power free electron laser (FEL), the design can utilize a short Rayleigh length optical cavity in combination with a short magnetic undulator. The short Rayleigh length increases ...
Blau, J.; Cambell, T.; Colson, W.B.; Ng, I.; Ossenfort, W.; Benson, S.V.; Neil, G.R.; Shinn, M.D. (2002);The TJNAF FEL can be upgraded to operate at 100kW average power and then explore the use of a short Rayleigh length in order to reduce the power density on the resonator mirrors. The short Rayleigh length can only work ...