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Wigglers emit both fundamental and harmonic radiation. For oscillator FELs, UV-VUV harmonic radiation can cause serious damage to the downstream mirror of the FEL cavity. Compared to a planar wiggler, harmonic radiation from a helical wiggler is peaked off-axis, and using a helical wiggler can significantly improve the FEL mirror lifetime. With a helical wiggler. However, the off-axis harmonic radiation from helical wigglers can still cause serious damage to the downstream mirror or limit the maximum beam current for FEL operation due to UV-VUV power loading on the mirror. At Duke FEL lab, we have developed a mechanism to control the off-axis harmonic radiation from the helical wigglers by using a set of motorized, water-cooled, in-vacuum apertures. These apertures can reduce the harmonic power load on the downstream mirror by one order of magnitude. With these apertures, operation with high electron beam currents becomes feasible for stable, high power UV-VUV lasing. In this work, we report the effectiveness of the apertures in reducing the wiggler harmonic radiation under various operation conditions and for different configurations of the Duke FELs.
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