Robust optimization for magnetic lens design

When the dimensions of magnetic lenses are optimized for the best value of a given target function, the method used should be robust in the presence of numerical noise, should allow constraints to be applied and as far as possible should locate global minima rather than local ones. We describe the use of the complex method to minimize the chromatic aberration of a magnetic lens. Geometric parameters were varied by a shell program, while the magnetic solution and aberration calculation were carried out by the CIELAS2 analysis software. The chromatic aberration coefficient was reduced from 1.61 to 0.68 mm, while satisfying given constraints. The method can be applied to optimise any set of lens properties whose numerical values can be combined into a figure of merit.