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IN MAGNETIC focusing of electron beams, the field must be rectilinear and substantially uniform. Rectilinearity is an especially important requirement for traveling-wave tube focusing because small deviations between the magnetic-field and electron-beam axes result in appreciable electron-current interception within the tube. Since the magnetic field cannot be made strictly uniform, it is required to have an axis of symmetry coincident with the mechanical axis of the magnetic structure. The mechanical axis is a convenient reference axis for locating the electron-beam axis. The percentage transmission of electron-beam current through the traveling-wave tube is a measure of the degree of focusing.