Straight-Field Permanent Magnets of Minimum Weight for TWT Focusing-Design and Graphic Aids in Design

A convenient design formula has been derived which enables one to estimate readily 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 traveling-wave tube (twt) with coaxial rf fittings if small holes are provided 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 formula, from which one can read off directly the required cross section area at the middle of the magnet. From this value one may calculate the average cross section, volume, and weight of magnetic material. A correction factor has been derived which also enables one to apply the results for the simple tubular magnet directly to the case of magnets specially shaped to fit around waveguide. The validity of the formula and of the associated design graphs has been confirmed by tests upon a series of magnets. These magnets covered a considerable range of field requirements and of size, varying in weight from three to twenty pounds.