Stray field of a pulse septum induced by eddy currents

NewSUBARU, a 1.5 GeV storage ring, has a magnetic pulse septum to inject a 1 GeV electron beam from the SPring-8 linac. Its maximum field is 0.36 T with waveform of 1 ms width half-sine. Behavior of a stored electron beam indicated an existence of a stray field whose time structure was cosine like. A beam pipe for an injected beam and a beam duct for a circulating beam made an electric loop path of an eddy current induced by the magnetic field of the septum. Off-beam measurements of magnetic field and the induced current confirmed the existence and paths of the eddy currents. An electric insulation added to a metal support of a beam pipe cut the loop path and reduced the cosine like stray field. There still exists a weak current, which runs long path along a beam transport line.