Studies of Interaction Impedance in a TWT Helix

Two reports present further developments in the subject of "Computation of Characteristics of a Helical TWT Slow-Wave Circuit" (LEW-16571), NASA Tech Briefs, Vol. 22, No. 4 (April 1998), page 54. The subject is computation, by use of the MAFIA (Solution of MAxwell's Equations by the Finite-Integration-Algorithm) computer program, of the cold-test electromagnetic characteristics of the helix and surrounding structures in the slow-wave circuit of a traveling-wave tube (TWT). Special attention is given to the on-axis electron-beam/slow-wave interaction impedance, which cannot be measured directly. Heretofore, this impedance has been determined indirectly from cold-test measurement of the change in the resonance frequency resulting from the introduction of a dielectric rod on the axis.