Distribution of Leakage Flux around a TWT-Focusing Magnet-A Graphic Analysis

Formulas and graphs are presented which describe the distribution of leakage flux around a tubular permanent magnet. The importance of this information derives from the fact that the external leakage flux from the magnets used for straight-field focusing of traveling wave tubes frequently interferes with the operation of other equipment in the vicinity. It is noted that the decrease in field intensity with increased distance from the magnet is approximately exponential over the region of interest. The attenuation constant in the exponent is a function of the length-to-diameter ratio of the magnet. This relation is derived and reduced to a graph. This graph, and easily constructed auxiliary graphs, enable one to insert dimensional information about the magnet and to directly read out the relative field strength to be expected at various distances from the magnet. Illustrations, based upon experience, demonstrate the usefulness of the graphs and their applicability to magnets which deviate from the tubular shape.