Design of periodic electromagnets

The use of periodic permanent-magnet focusing structures in TWT research is complicated by the difficulty of changing the level of flux density on the axis. It is necessary to completely disassemble the focusing structure and remagnetize each individual magnet. To overcome this difficulty, periodic electromagnets have come into use. Using electromagnets in place of permanent magnets, the flux density on the axis can be changed simply by varying the magnet current. The construction techniques for such magnets are discussed and formulas are given for finding the necessary ampere turns in the windings.