24 segment high field permanent sextupole magnets

We report on the design, construction, and magnetic field measurements of a system of high field sextupole magnets made from NdFeB compounds. The magn ets are utilized as a focusing system for neutral hydrogen (or deuterium) a toms in a polarized atomic beam source based on Stern-Gerlach spin separati on. Each magnet consists of 24 segments of permanently magnetized material differing in remanence and coercivity to reduce demagnetization. According to quadratic extrapolation to the pole tip the magnetic flux density reache s values of up to B-0=1.69 T. Three-dimensional field calculations using th e MAFIA code were carried out to optimize the magnet performance and to avo id demagnetization by selecting appropriate materials for the individual se gments. Measurements of the radial, azimuthal, and longitudinal magnetic fl ux density distributions were carried out by means of a small Hall probe (1 00x200x15 mu m(3)). The measurements with the small probe permitted to extr act experimentally higher order multipole components very close (similar to 100 mu m) to the inner surface. Experimental values obtained are compared to predictions based on MAFIA calculations and on the Halbach formalism. (C ) 2000 American Institute of Physics. [S0034-6748(00)05309-0].