Joel Rasch

Reduction of the Multipactor Threshold Due to Electron Cyclotron Resonance

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 is equal to 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.

Microwave Multipactor Breakdown Between Two Cylinders

An analysis has been made of the microwave breakdown threshold for multipactor in an open structure comprising two parallel cylinders, approximating, e.g., parts of a helix antenna. The electron motion in the corresponding electromagnetic field is analyzed by separating the motion into a slowly varying drift velocity (driven by the ponderomotive force due to the electric field inhomogeneity) and a rapidly oscillating part (driven by the oscillating electric field). Furthermore, the curvature of the cylindrical surfaces of emission is shown to give rise to a new effect that implies a loss of electrons. This leads to a more stringent multipactor breakdown condition for the two-wire structure than for the classical situation corresponding to the case of two plane parallel infinite plates. The importance of this effect is determined by the ratio of the cylinder radii and the distance between the cylinders, and it is shown that when this ratio is small, multipactor can only occur for surfaces having very large secondary emission coefficients. A detailed analysis is also made to determine the lowest voltage between the cylinders at which multipactor becomes possible.

Microwave corona breakdown in a gas-filled rectangular resonator cavity

The threshold for microwave corona breakdown in a rectangular cavity is investigated using a direct variational approach. Good accuracy of the results is verified by a comparison with a full numerical solution of the relevant eigenvalue problem determining the breakdown threshold equation. The importance of the interplay between diffusion and attachment in setting the breakdown threshold is discussed. A comparison with experimental results shows very good agreement.

Simulations of Multipactor Breakdown Between Two Cylinders

Simulations have been performed to determine the multipactor breakdown threshold in a microwave structure composed 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 2-D 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 a 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.

Non-resonant multipactor-A statistical model

High power microwave systems operating in vacuum or near vacuum run the risk of multipactor breakdown. In order to avoid multipactor, it is necessary to make theoretical predictions of critical parameter combinations. These treatments are generally based on the assumption of electrons moving in resonance with the electric field while traversing the gap between critical surfaces. Through comparison with experiments, it has been found that only for small system dimensions will the resonant approach give correct predictions. Apparently, the resonance is destroyed due to the statistical spread in electron emission velocity, and for a more valid description it is necessary to resort to rather complicated statistical treatments of the electron population, and extensive simulations. However, in the limit where resonance is completely destroyed it is possible to use a much simpler treatment, here called non-resonant theory. In this paper, we develop the formalism for this theory, use it to calculate universal curves for the existence of multipactor, and compare with previous results. Two important effects that leads to an increase in the multipactor threshold in comparison with the resonant prediction are identified. These are the statistical spread of impact speed, which leads to a lower average electron impact speed, and the impact of electrons in phase regions where the secondary electrons are immediately reabsorbed, leading to an effective removal of electrons from the discharge.

Gas breakdown in inhomogeneous microwave electric fields

The main physical properties of gas breakdown in inhomogeneous high frequency microwave electric fields are investigated using both analytical and numerical analysis. In particular, the interplay between diffusion and attachment in redistributing electrons from high field regions to low field regions and the concomitant effect on the breakdown threshold is studied using three different examples of ionization profiles. The results give a clear physical picture of the dependence of breakdown electric field on pressure showing a high pressure branch determined by conditions in the high field region, a low pressure branch determined by the properties of the low field region only, and a more or less smooth transition region for intermediate pressures. Simple estimates for the extension of the transition region are given in good agreement with the numerical simulations, and a comparison of the theoretical breakdown threshold with previous experimental results also shows good agreement.

General Study of Multipactor Between Curved Metal Surfaces

An analytical study of the electron trajectories between two opposite electrodes having curved surfaces is undertaken for the case when the electron transit time exceeds the RF period. The analysis is based on a statistical approach, which makes it possible to calculate the width of an electron bunch after a number of electron transits taking into account the spread of electron emission velocity, and the spatial nonuniformity of the RF field using the concept of the ponderomotive force. The results are used to estimate the multipactor threshold in terms of a value of the secondary emission yield, which is necessary to balance electron losses. Based on the model, it is predicted that multipactor is impossible inside the realistic configuration of a helix antenna where four electrodes are placed on the same cylindrical surface.

Galerkin approach to approximate solutions of some nonlinear oscillator equations

An analysis based on the Galerkin method is given of some nonlinear oscillator equations that have been analyzed by several other methods, including harmonic balance and direct variational methods. The present analysis is shown to provide simple yet accurate approximate solutions of these nonlinear equations and illustrates the usefulness and the power of the Galerkin method

Microwave Corona Breakdown in rf Devices

The main physical properties of microwave corona breakdown in gases in the pres- ence of inhomogeneous electric flelds are investigated using numerical calculations of the conti- nuity equation for the density of free electrons. In particular, the interplay between difiusion and attachment in redistributing electrons from high fleld regions to low fleld regions and the concomitant efiect on the breakdown threshold is studied for difierent examples of the varia- tion of the electric fleld strength. The results give a clear physical picture of the dependence of breakdown electric fleld on pressure with two complementary limits; a high pressure limit with a localized breakdown plasma conflned to the high fleld region and a low pressure limit determined by the properties of the low fleld region only and a breakdown plasma extending over the entire volume of the rf device. A comparison between results for the critical microwave breakdown fleld as obtained by numerical calculations and experimental results in a microwave cavity show very good agreement.

Simple model of the rf noise generated by multipacting electrons

A simplified analytical model is developed to predict the spectrum of electric current induced by the multipacting electrons between two parallel electrodes exposed to an rf voltage of fixed amplitude. The model is based on the resonant multipactor theory and makes it possible to calculate the relative spectral amplitudes of electric current at different harmonics and sub-harmonics of the applied rf frequency. The theoretical predictions are confirmed by numerical simulations of multipactor inside a rectangular waveguide. Specifically it is seen that the relative height of the spectral peaks decreases with increasing gap height.