The Effect of Pole Piece Saturation in Magnetic Electron Lenses

The imaging properties of magnetic lenses in the absence of iron saturation are now well known, but there has been some doubt up to what flux densities the calculated lens performance would be correct. It is shown here that the effect of iron saturation in the pole piece tips is equivalent to an apparent increase in lens dimensions, apart from a loss of m.m.f. in the iron, which depends on the design of the magnetic circuit as a whole. Published data for unsaturated magnetic lenses can still be used beyond the onset of pole piece saturation if these effects are taken into consideration. A practical limit of lens operation is reached for a flux density in the iron near the air gap which would make μ approximately equal to 50.