R. Udiljak

Multipactor in a coaxial transmission line. I. Analytical study

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.

Conformal mapping analysis of multipactor breakdown in waveguide irises

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.

Multipactor in a Waveguide Iris

Microwave discharge in vacuum in a waveguide iris is analyzed under the approximation of a uniform electric field over the finite width of the iris. The analysis shows that the random electron drift due to the initial velocity of the secondary electrons leads to a significantly increased multipactor threshold as compared to the result predicted by the (infinitely wide) parallel plate configuration. This increase is determined by the value of the iris height-to-length ratio. For each fixed value of this ratio, the multipactor susceptibility charts can be generated in the traditional engineering units. An increase in this ratio results in a similar shrinkage of each of the multipactor resonance zones. Within each particular zone, the effect is more pronounced for the lower RF frequencies due to the longer transit time between the walls of the iris

New method for detection of multipaction

A new global method for unambiguous detection of multipaction in microwave communication systems is presented. Theory as well as experiments demonstrate the potential and usefulness of the method. A short review of existing methods for multipaction detection is given as a background, in order to illustrate the advantages of the new method.

Multipactor breakdown in waveguide irises

A detailed analysis is made in order to establish the multipactor breakdown threshold in wave guide irises. The analysis involves an analytical investigation based on the conformal mapping technique and also extensive numerical simulations. Comparison with recent experimental results shows good agreement. The main goal of the work is to obtain multipactor charts for the iris, i.e. the breakdown voltage as a function of the frequency - gap product, for different height to width ratios of the iris.

Multipactor in a coaxial transmission line. II. Particle-in-cell simulations

The effects of nonuniform radiofrequency fields are analyzed for a cylindrical coaxial transmission line using a particle-in-cell code. The behavior predicted by the analytical analysis is confirmed by this study. In addition, by including an initial velocity spread and different maximum secondary electron yields, a more elaborate analysis is performed. It is found that in the case of high secondary emission, an increase in the spread of initial velocities results in an overlapping of the multipactor zones for both double and single sided multipactor. In the other case, when the secondary emission is low, a large enough initial velocity spread can lead to total suppression of the electron avalanche for the higher order modes. Comparison with available experimental data shows good agreement.

Improved model for multipactor in low pressure gas

The effect of electron-neutral collisions on the multipactor threshold in a low pressure gas is analyzed using a model that takes into account the friction force from the electron impacts with the neutrals, the thermalization of the electrons, as well as the contribution from impact ionization to the total number of electrons. It is found that the friction force is most important for first-order multipactor for materials with a low first cross-over energy and leads to an increased threshold. For higher-order modes with the same material, the thermalization effect dominates and results in a lowered threshold. The contribution to the total number of electrons from impact ionization for such materials is insignificant, but for materials with a high first cross-over energy this contribution dominates the behavior and a decreased threshold is obtained for all orders of multipactor.

Multipactor in low pressure gas

The effect of electron-neutral collisions on the multipactor threshold in a low pressure gas is analyzed in the case when the ionization energy of the gas particles is larger than the impact energy of the oscillating electrons. It is found that a higher microwave power is required to initiate multipactor, since the collisions tend to slow down the electrons. It is also found that the phase limits, within which multipactor can occur, are affected by the presence of the low density gas. Transition to the regime of corona discharges is also discussed.

Multipactor in Microwave Transmission Systems Using Quadrature Phase-Shift Keying

A numerical simulation analysis is made of the effect of signal transmission using the quadrature phase-shift keying of a constant-amplitude RF carrier wave on the initiation of multipactor breakdown in RF devices operating under close-to-vacuum conditions. It is found that this form of signal modulation has a suppressing effect on the growth of the multipactor electron avalanche that depends on the parameters of the system. However, for technically realistic values of signal and phase-shift frequencies, the effect is found to be small.