Eddy current focusing solenoid

A novel eddy current based focusing solenoid concept was developed, in which the eddy currents are used to generate the main focusing field rather than controlling the field distribution. In general, an air-core solenoid coil with a small number of turns gives a solenoid field that is contaminated with unwanted dipole and higher order transverse fields. In concept this problem can be avoided by using filamentary ring with induced current. The concept was proved using numerical simulation and physical models. A real eddy current focusing solenoid was designed and constructed with a six-turn solenoid as excitation coil and a ∼15 mm diameter filamentary metal ring as secondary. For practical application, the secondary ring had to be implemented as a short hollow cylinder. Slicing this cylinder into segments suppresses transverse fields in the same way as for a series of filamentary rings. The developed solenoid field can reach over 300 mT. The mechanical and electrical design, numerical field simulations and measurements with an electron beam are presented.