Stray Field Reduction of ALS Eddy Current Septum Magnets

Stray field from an eddy current septum magnet adversely affects the circulating beam and can be reduced using several techniques. The stray field time history typically has a fast rise section followed by a long exponential decay section when pulsed with a half sine drive current. Changing the drive current pulse to a full sine has the effect of both reducing peak stray field magnitude by ∼ 3x and producing a quick decay from this peak to a lower field level which then has a similar long decay time constant as that from the half sine drive current pulse. A method for tuning the second half-sine (reverse) current pulse to eliminate the long exponential decay section is given. A method for halving the remaining brief peak is also given.