Microwave Multipactor Breakdown Between Two Cylinders

An analysis has been made of the microwave breakdown threshold for multipactor in an open structure comprising two parallel cylinders, approximating, e.g., parts of a helix antenna. The electron motion in the corresponding electromagnetic field is analyzed by separating the motion into a slowly varying drift velocity (driven by the ponderomotive force due to the electric field inhomogeneity) and a rapidly oscillating part (driven by the oscillating electric field). Furthermore, the curvature of the cylindrical surfaces of emission is shown to give rise to a new effect that implies a loss of electrons. This leads to a more stringent multipactor breakdown condition for the two-wire structure than for the classical situation corresponding to the case of two plane parallel infinite plates. The importance of this effect is determined by the ratio of the cylinder radii and the distance between the cylinders, and it is shown that when this ratio is small, multipactor can only occur for surfaces having very large secondary emission coefficients. A detailed analysis is also made to determine the lowest voltage between the cylinders at which multipactor becomes possible.