Design of minimum weight straight-field permanent magnets for traveling-wave tubes

A set of design formulae has been derived which enables one readily to estimate the minimum amount of magnetic material which will satisfy a given set of field strength requirements on the axis of a tubular permanent magnet. Such a magnet is suitable for focusing the beam of a TWT with coaxial rf fittings if small holes are cast in the side of the magnet to admit the coaxial lines. The calculation is further expedited by the use of design. graphs, based upon the formulae, from which one can read off directly the required cross section area at the middle of the magnet. From this value one may calculate, using an empirical formula, the average cross section, and continue with a straightforward calculation to obtain the required volume and weight of magnetic material. A correction factor has been derived which enables one also to apply the results for the simple tubular magnet directly to the case of magnets specially shaped to fit around waveguide.