Non-uniformities in a P. P.M. focusing circuit

The theory and design of a periodic permanent magnet (P.P.M.) focusing circuit are based on the assumption that both the period and peak magnetic field are constant. This ideal, however, is never attained in a practical circuit. Non-uniformi ti es in the period or magnetic field may be due to various tolerances or mechanical design of the circuit. On the whole, undesirable variations tend to increase the beam ripple which, in the case of the travel-ling-wave tube, may lead to loss of gain, increased frequency sensitivity and high intercepted current.