The Use of Periodic Permanent Magnet Focusing in Electron Linacs

In electron Linacs, the radial component of the rf accelerating electromagnetic wave and space charge both tend to increase the radius and divergence of the electron beam. These defocusing forces are commonly counteracted by the use of magnetic lens and solenoids. The use of Linacs as portable, high energy radiographic sources, however, necessitates that the weight and power requirements of each component be reduced to a minimum. This has led to the concept of periodic permanent magnet focusing as a replacement for the conventional magnetic lens and solenoids. In this study the conventional copper discs of the accelerating waveguide have been replaced by a magnetic material which is plated with copper to provide the required resistivity. Permanent magnets are placed around the outside of the accelerating structure to provide a periodic magnetic system capable of focusing the injected beam. A computer program has been developed for the iterative solution of the relativistic equations of motion which give the electron trajectories in the periodic field during the acceleration process. The influence of periodicity and axial magnetic field strength on the beam radius for accelerators operating at 6,000 Mc/sec, and 10,000 Mc/sec are discussed. Materials considerations and mechanical design techniques are discussed briefly.