Multipactor Analysis in Circular Waveguides Excited by TM₀₁ Mode

A series of detailed numerical simulations are used to investigate the properties of multipactor breakdown in circular waveguides propagating the TM01 mode. A Monte Carlo model is constructed to track the motion of the electrons, study the multipactor scenarios, and predict the multipactor thresholds. The theoretical and numerical analyses indicate that the product of the frequency and the gap ( ${f}\cdot {D}$ ) affects both the intensity of the ponderomotive force and its spatial distribution, which results from the nonuniformity of the radio frequency (RF) field and significantly influences the electrons’ trajectories and multipactor trends. The decrease in ${f}\cdot {D}$ results in a remarkable enhancement in the magnitude of the ponderomotive force, while the maximal intensity gradually moves toward the half radius R/2 area. Low values of ${f}\cdot {D}$ correspond to high ponderomotive potential, which sustains the short-range electrons and triggers the single-sided multipactor. In contrast, high values of ${f}\cdot {D}$ correspond to low ponderomotive potential, contributing to long-range electrons and exciting the double-sided multipactor. Fitting to the susceptibility diagram produces the border line and a modified ${f}\cdot {D}$ threshold of ( ${f}\cdot {D}$ )th $\approx ~338.4$ GHz mm, which separates the susceptibility diagram into single-sided, double-sided, and mixed-sided zones. The initial electron energy influences their trajectories at high ${f}\cdot {D}$ and low RF power. This effect tends to dominate the multipactor behavior in the mixed-sided region.