M. Fujimoto, R. Kato, M. Kuwahara, T. Igo, T. Okita, T. Konishi, R. A. V. Kumar, S. Mitani, S. Suemine and G. Isoyama
Institute of Scientific and Industrial Research, Osaka University
It is well known that a planar wiggler exerts focusing force on the electron beam in the direction perpendicular to a plane of wiggling, but it has not been explicitly demonstrated, to our knowledge, how strong it is in the wiggler used for an infrared FEL, where the electron energy is typically as low as a few tens of MeV. We have, therefore, studied the focusing property of the planar wiggler used for the far-infrared FEL and SASE at the Institute of Scientific and Industrial Research, Osaka University. The electron beam size was measured at three positions along the beam axis in the wiggler as a function of the K-value for the electron energy of 12.5 MeV. The wiggler has 32 periods with the period length of 6 cm and the K-value can be varied up to 1.472. Measured beam sizes were analyzed with calculation using transfer matrices and it was found that the measured data were explained well by the theory with the fringing field. It has been demonstrated that the focusing force is so strong that the electron beam transversely oscillates two times in the 1.92 m long wiggler. A new operation mode is proposed to keep the electron beam size low and constant through the wiggler by making use of the focusing force.
