J. Puech

Multipactor in rectangular waveguides

Multipactor inside a rectangular waveguide is studied using both an analytical approach and numerical simulations. Particular attention is given to an analysis of the role of such effects as the velocity spread of secondary emitted electrons and the action of the rf magnetic field on the electron motion. Conventional resonance theory is shown to give correct predictions for the multipactor threshold in cases where the height of the waveguide is very small and first order resonance multipactor dominates. In cases of higher order resonances, an accurate prediction of the multipactor threshold requires that the spread of the normal component of the electron emission velocity is taken into account. Furthermore, the spread of the tangential component of the electron emission velocity and the action of the rf magnetic field are shown to be very important when the waveguide height exceeds a certain critical value, which depends on the waveguide width. A new theory is developed for predicting the multipactor thre...

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.

Hybrid resonant modes of two-sided multipactor and transition to the polyphase regime

A plane parallel model of multipactor is studied in detail using both an analytical approach and numerical simulations. The analytical analysis is carried out within a widely exploited approximation, which assumes a fixed value for the initial velocity of secondary electrons. It is shown that the commonly accepted picture of the multipactor zones is not quite complete and should be modified by taking into account the existence of hybrid resonance modes and the important consequences of a secondary emission yield that significantly exceeds unity. Numerical simulations demonstrate that the chart of the multipactor zones is also very sensitive to a spread of the initial velocity of the electrons. In particular, the full effect of initial electron velocities cannot accurately be modeled by using a single fixed value only.

Investigations of time delays in microwave breakdown initiation

A detailed theoretical as well as experimental investigation has been made of the statistical properties of rf corona breakdown thresholds, relevant to situations where no artificial electron seeding is used and the electron breakdown avalanche is initiated from the weak naturally occurring electron seeding, primarily due to cosmic radiation. Good qualitative as well as quantitative understanding has been obtained concerning the statistical properties of the breakdown initiation process and its consequences for the observed breakdown threshold. Comparisons between theoretical predictions and experimental results show a good agreement.

Multipactor discharge on a dielectric surface: Statistical theory and simulation results

This paper presents a novel theory for describing the initial stage of a single-surface multipactor discharge on a dielectric surface in the presence of a dc electric field, which returns secondary emitted electrons to the surface. The calculations employ a statistical method based on an exact analytical solution for the probability density of the arrival times of the secondary electrons. A general integral equation determining the steady-state distribution of the emission phases of the secondary electrons and the threshold of the multipactor growth is formulated. A computer program has been developed to implement this theory for realistic secondary yield curves and arbitrary, nonuniform, distributions for velocities and angles of emitted electrons. Susceptibility diagrams, applicable to a wide range of materials, are obtained in terms of the rf and dc electric fields and are found to be relatively independent of the emission distribution of the electrons.

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

Effect of emission velocity spread of secondary electrons in two-sided multipactor

Detailed numerical simulations of the two-sided multipactor have been carried out within a plane-parallel model. The main aim of the simulations is to clarify the uncertainty that still exists in the literature concerning the overlapping of multipactor zones. Three different codes (Monte Carlo, particle-in-cell, and statistical) were used to calculate the multipactor charts within a wide range of parameters such as spread of initial velocities of the secondary electrons and maximum value of the secondary emission yield. It was found that an increase in the spread of initial velocities results in overlapping of the multipactor zones, provided the secondary emission is high enough. In the opposite case, an increase in the spread of initial velocities leads to a suppression of the higher multipactor zones.

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.

On the effective diffusion length for microwave breakdown

The concept of effective diffusion length, as a means to characterize diffusion losses in calculations of microwave breakdown in radio frequency (RF) devices with inhomogeneous electric fields, is discussed in detail with emphasis on geometrical situations where the inhomogeneity of the microwave electric field plays an important role for the breakdown threshold. In particular, good analytical approximations are found for the microwave breakdown threshold field, in a number of different geometrical situations. Finally, an explicit experimental example demonstrates how the effective diffusion length in a coaxial resonant filter structure can be inferred from a set of breakdown threshold data obtained for different pressures.