Characterization of a pulsed bifilar helical wiggler

An adiabatic entrance, obtained by placing resistive shunts at half-period intervals between the adjacent clockwise and counterclockwise helical curre nt turns of the wiggler winding, is often used to assure good beam injectio n into the bifilar helical wiggler of a free electron laser (FEL). If the w iggler is pulsed, the time dependence of the magnetic field in the entrance may differ from that of the uniform section of the wiggler, and the magnet ic field as a function of position along the axis may not be gradually incr easing, in other words adiabatic. In addition, if a waveguide is used to co ntain the radiation, as in microwave frequency FELs, the tube itself may pa rtially shield and retard the magnetic field in its interior. We investigat e the time dependence of the magnetic field in a pulsed wiggler both experi mentally and theoretically. Good agreement between measurements and our mod el is found. By using shunts of varying resistance, and by choosing careful ly the time at which the beam enters the wiggler, we obtain an entrance and an exit which are approximately adiabatic. The best result is achieved whe n the 60-ns-long electron bunch traverses the wiggler about 100 mu s after the pulsed wiggler current has reached its maximum. (C) 2000 American Insti tute of Physics. [S0021-8979(00)07709-4].