Development of Far-Infrared Free-Electron laser using microwiggler of millimeter period

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A core type electromagnetic helical microwiggler has been developed with the intention of generating radiation with a wavelength of few tens mu m using an electron beam with a moderate energy of 7-9 Mev by RF-linac accelerator. However, it is more diffucult to obtain a higher magnetic field according to shorter period of wiggler, because of increasing the leakage fields. Under the circumstances, we use a "one period three poles" type herical wiggler which has the least leakage field in the core type electromagnetic wiggler. A threefold-enlarged trial wiggler with the same structure as the original design was manufactured. The field measurements are used to adjust the final design of the microwiggler. The main items obtained from the measurements are as follows.1. There are two field structures in the phase of helical field on z azis. One is the pole-field which is obtained at the pole position on z axis, the other is compositin-field which is contributed from neighboring poles. The former gives the focussing actions for electrons, whereas the latter gives the defocussing actions.2. In order to improve the defocussing at the composition field, a canted pole-face wiggler was adopted of which pole-face has a slope of 8ﾟ. The focussing action to electron beam was shown by computer simulation using TM-code.3. The difference of the peak values between pole-field and composition-field is modified to equal value by controling the six coil currents.4. Computer simulations by TG-code showed that the light with wavelength of 10-20mu mgrows to satulation level of 10 kW, using final wiggler.