Comparison of adiabatic and nonadiabatic inputs in a helical undulator

For matched injection of an electron beam into an undulator, the fringe field must be shaped. In helical undulators adiabatic input has been used from the very beginning, and remains to be the most prevalent means to this day. Such an input consists in a smooth rise of field amplitude over several periods. In recent times, nonadiabatic methods of injection have been proposed and realized. These have a number of advantages. They are shorter, more accurate (minimizing not only angular error but that of displacement as well) and technically simpler, merely a winding of an additional undulator half-period. The article presents an analysis and comparison of both methods of matched injection, their advantages and weaknesses, and applicability in each case of precision correction of distortions arising in shaping the fringe fields.