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Simulations have been performed to determine the multipactor breakdown threshold in a microwave structure comprised of two parallel cylinders, chosen to be an approximate model of an open helix microwave antenna system. The electromagnetic field between the cylinders is available in closed analytical form and a Monte Carlo software has been developed to calculate the 2D electron trajectories and to simulate the multipactor avalanche in this inhomogeneous electric field for different ratios of cylinder radius and distance of separation between the cylinders. The results are compared with those of a recently published analytical theory and show a qualitatively good agreement. In particular, it is confirmed that for given distance between cylinders, there exists a smallest cylinder radius below which no two-sided multipactor breakdown can occur. The basic physical explanation is a loss mechanism for secondary emitted electrons that is caused by the curvature of the cylinder surfaces together with the strong electric field at the surfaces. The results imply that the breakdown threshold in realistic open helix antennas is significantly higher than those predicted using extrapolations based on resonance theory and the classical two parallel plate model.

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Single surface multipactor on metal surfaces is studied in the case when the microwave electric field is superimposed on a dc electric and a permanent magnetic field. Based on a simple analysis of the electron motion it is predicted that considerable reduction in the multipactor threshold is possible when the electron cyclotron frequency equals the microwave field frequency and the permanent magnetic field has the proper orientation with respect to the metal surface. The prediction is confirmed by numerical simulations, and some experimental indications of the reduced multipactor threshold are also presented.