Controlling wiggler harmonic radiation to reduce damage to FEL cavity mirrors
In an oscillator FEL, higher-order harmonic radiation from wigglers can cause serious damage to the downstream FEL resonator mirror or limit the maximum electron beam current for FEL operation due to thermal overload. With a helical wiggler, higher-order harmonic radiation is peaked off-axis. By blocking the off-axis wiggler harmonic power, the radiation damage to the FEL resonator mirror can be reduced. In this paper, we report a recently developed scheme to control the off-axis harmonic radiation from helical FEL wigglers using a set of motorized, water-cooled, in-vacuum apertures. These apertures can reduce the harmonic power load on the downstream FEL resonator mirror by one order of magnitude. With these apertures, high power FEL operation with a high electron beam current will become feasible in the UV-VUV wavelength range with Duke storage ring FELs.
