Theoretical considerations of the electron multipactor have mostly been restricted to the simplified case of plane-parallel geometry with a uniform field. However, a nonuniform field may not only affect the results quantitatively, but may also lead to qualitatively new effects. In the present work, the effects of a nonuniform radiofrequency field on the properties of multipactor initiation are analyzed in the case of a cylindrical coaxial transmission line. A useful approximate solution of the nonlinear differential equation of motion for the electrons is derived and simple analytical estimates are developed to show that in this system the multipactor mechanism becomes qualitatively different for sufficiently small inner coaxial radii. When the inner radius is of the order of the outer one, the multipactor properties are very similar to those of its counterpart in plane-parallel geometry. However, when the inner radius is less than a certain threshold value, single-surface multipactor becomes possible.

A particular software ‘MuSLi’ has been developed and applied for simulations of the multipactor effect in shielded microstrip lines with cross-sections that are partially filled with dielectric material. The software combines an electromagnetic field solver, determining the electric field structure in the microstrip line, and a Monte Carlo algorithm, calculating the corresponding electron trajectories taking into account a spread of the electron initial velocity and different secondary emission properties of the metal and dielectric surfaces. The simulations were carried out for a number of structures of interest for communication systems, currently being discussed for application in coming space missions. The microstrip line system is shown to be strongly resistant to multipactor growth and the main reason for this is identified as the ponderomotive (or Miller) force, which governs the average electron motion in non-uniform rf fields and which tends to push the electrons out of regions with strong rf fields.

Multipactor breakdown in a single waveguide iris is analyzed using the quasistatic approximation for the spatial distribution of the rf field in the iris. Based on the conformal mapping approach, an analytical description is given of the rf field structure in the iris. It is shown that in the central part of any iris with a length to height ratio greater than approximately 0.5, the rf field structure is close to that between two parallel plates. The multipactor threshold for the iris is determined mainly by electron losses from the central part of the iris where the losses are due to the tangential component of the emission velocity of secondary electrons. The effective length of the iris central part is determined and an estimate of the multipactor threshold for the iris is found in terms of the conventional parameters: Applied rf voltage, product of rf frequency and iris height, and iris length to height ratio. Numerical simulations are also carried out using the exact analytical description of the quasistatic rf field and taking into account a spread of electron emission velocities.

In this paper, a general and systematic scheme is formulated for finding approximate solutions of two-way diffusion equations. This expansion scheme is valid for arbitrary mean-free path and can be carried out to any desired accuracy. Its potential is demonstrated by constructing approximate solutions for two problems concerning the kinetics of an electron beam, and the accuracy is found to be very good even when only a few terms are included in the expansion. The approximate solutions found are compared with numerical calculations and previous analytical work in the literature.

av a% azoStability analysis of lower-hybrid cones.