Measurement and Shaping of the Fringing Fields of the Princeton-Pennsylvania Accelerator Magnets

The fringing fields must be included in the determination of the effective length and field index of the P. P. A. weak focusing magnets. For this purpose, a null method was used to compare the magnetic field inside the gap with the fringing field integrated along a line perpendicular to the magnet end. Bz(r, t), the field component normal to the median plane varied between 0.26 and 13.9 Kgauss, at 20 cps. The measuring technique consisted in placing a short line dipole coil in the known and essentially two dimensional field (e. g.- sufficiently far from the end) and a long dipole coil through the fringing field free region. See Figure 1. The outputs of the two coils were summed into an electronic integrator. The output of the integrator was set to zero at a preselected phase of the magnet´s field. Then, the azimuthal position of the coil pair was adjusted so that the output at this phase remained equal to zero when the coils were simultaneously inverted. This procedure was followed for different radial positions. Then another phase was selected. These measurements gave the effective length of the fringing fields to ±5 × 10-3 in., at B minimum, and ±5 × 10-4 in., at B maximum. The results were used to analyze and guide the design of correcting end pole pieces which produced the desired effective length and average field index. This work was performed on the P.P.A. full scale prototyped magnet.