A wiggler magnet for a CW-FEM

In CW-FEMs less than 200 kV and 10 mA, operation is necessarily in the microwave frequency range for wiggler periods of the order of cms, so that a waveguide system is mandatory. Also, because of the relatively low velocity of the electron beam, the wiggle amplitude of the electron beam can be much larger than is normal for highly relativistic FELs. Both these factors mean that the electron beam trajectory must be carefully controlled to avoid beam collision with the waveguide walls. A wiggler system with half poles at entrance and exit is not an acceptable solution because of the offset it gives rise to the electron trajectory. Consequently, we have designed and constructed a wiggler magnet with exponential entrance and exit tapers for a minimal deflection and displacement of the electron beam. The experimental measurements showed that an on-axis trajectory is easily obtainable.