Abstract
A significant problem in the design of high-energy free-electron lasers (FELs) centers on the technique for outcoupling the output beam. FELs with currently achievable output power usually include a conventional stable resonator with output through a partially transmitting mirror which will not work for arbitrarily high average power. An alternate scheme must be found for high-energy FELs. A high- efficiency grating outcoupler is an attractive possibility, but it is difficult to manufacture. Other suggestions include unstable resonators with an intracavity focus and unstable resonators with an intracavity focus and beam rotation. The intensity distribution at the intracavity focus of a negative-branch unstable resonator has side-lobes that would be scraped off by the faces of the wiggler magnets or by the beam tube through the wiggler. The resulting power loss would be significant. Therefore, it is desirable to develop another type of resonator for use with FELs. The resonator that we have developed is the compact-beam stable-unstable ring resonator. It is a stable resonator in one transverse dimension and an unstable resonator with an intracavity focus in the orthogonal transverse dimension. A scraper mirror outcouples the output beam from one side of the mode only. The resonator can be configured so that it has a small beam waist at the center of the wiggler in the stable direction and has an intracavity focus in the unstable direction. The half- width of the central lobe of the focus is approximately the size of the stable beam waist. In the stable direction, the Gaussian amplitude distribution results in a small loss on the wiggler magnets, or on a beam tube that will fit within the wiggler, if one is used. The beam tube can have an elliptical shape to permit the passage of several side lobes in the unstable dimension. A mode of the CBSUR is a product of the mode of a strip stable resonator with a strip compact-beam negative-branch unstable resonator.
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