Design and construction of an eight millimeter period undulator for use in a high average power far-infrared free electron laser

An eight millimeter period hybrid undulator with 187 periods was developed and tested and tested for use in the CREOL far-infared free electron laser. Typically, undulators have periods between 2–10 centimeters. However, a shorter period undulator allows an FEL to operate at a shorter wavelength for a given electron energy. The CREOL FEL is expected to produce close to a kilowatt of continuous power at a wavelength of 225–800 $m$m with a maximum electron beam energy of 1.7 MeV. This FEL is based on an electrostatic accelerator that will use electron beam recirculation to achieve continuous operation at high beam current levels. FEL operation and the use of electron beam recirculation places very stringent requirements on the undulator's magnetic field quality. In order to build a high quality undulator, the sources of magnetic field errors in hybrid and permanent magnet undulators were studied. The error sources include magnetization strength and direction variations in permanent magnets, size variations in permanent magnets, magnet positioning errors, and steel pole positioning errors. The error sources that were most important in the CREOL hybrid undulator were the magnetization direction variations and the pole positioning errors. A detailed knowledge of the error sources was used to significantly reduce the effects of these field errors during the assembly of the undulator. Initially, the root mean square deviations in the peak magnetic field were as high as 14% but the final undulator had a root mean square deviation of only 0.3%. The electron beam transport through this undulator was tested and the undulator satisfactory met the requirements for electron beam recirculation and FEL operation.