Ulf Jordan

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.

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.

Microwave breakdown of the TE11 mode in a circular waveguide

This paper investigates the microwave breakdown threshold in a circular waveguide excited in the lowest order (TE11) mode, where the electric field strength depends on both the radius and the azimuthal angle. This analysis complements and extends previous investigations of breakdown in cylindrical waveguides and resonators that have been restricted to circularly symmetric waveguide modes so far. A simple analytical approximation of the breakdown threshold of the TE11 mode is found, using a direct variational approach. The predictions are compared with the results of numerical calculations and the agreement is found to be very good.

Microwave breakdown in slots

The properties of microwave-induced breakdown of air in narrow metallic slots are investigated, both theoretically and experimentally, with emphasis on factors important for protection against transmission of incident high-power microwave radiation. The key factors investigated are breakdown power threshold, breakdown time, peak-leakage power, and total transmitted energy, as functions of incident pulse shape and power density. The theoretical investigation includes estimates of the electric field intensification in narrow slots and basic breakdown plasma modeling. New results important for application to the high-power microwave field, such as the influence of pulse shape on breakdown time and peak-leakage power, are presented. The experimental investigation comprises a set of slot breakdown experiments at atmospheric pressure, which are analyzed to extract key parameters, such as transmission cross section, breakdown time, peak leakage power, and transmitted energy. The experimental data is compared and shown to be in good agreement with results obtained in the theoretical investigation.

Microwave Corona Breakdown Around Metal Corners and Wedges

This paper presents an analytical, numerical, and experimental analysis of the breakdown strength of microwave gas-filled RF devices containing sharp corners and wedges. For the idealized case of a wedge-shaped geometry, it is shown that only under certain physical circumstances does the singularity and the concomitant strongly enhanced microwave field determine the breakdown strength. In particular, when diffusion is the dominating loss mechanism for the electron density, breakdown is a volumetric process, and the field singularity does not determine the breakdown threshold. In such situations, excessive accuracy in numerical calculations is not required. Conditions for volumetric and localized breakdown, respectively, are established analytically, and the validity is demonstrated by numerical simulations. Finally, the analysis is extrapolated and compared with experimentally observed breakdown thresholds in commercially available resonators of nonidealized geometry. Good agreement between theoretical predictions and experimental results is demonstrated.

Microwave Breakdown in RF Devices Containing Sharp Corners

The present work reports on an analytical, numerical, and experimental analysis of the importance of electric field singularities around sharp corners for the determination of the breakdown strength of microwave RF devices. It is shown that only under certain physical circumstances, does the singularity and the concomitant strongly enhanced microwave field determine the breakdown strength. In particular, in situations where diffusion is the dominating loss mechanism for the electron density, it is shown that breakdown is a volumetric process and that the field singularity does not determine the breakdown threshold. Conditions for volumetric and localized breakdown respectively are established analytically and the validity is demonstrated by numerical simulations. Finally an experimental investigation is made which confirms the predicted behavior and demonstrates the accuracy which is possible to obtain for the determination of the breakdown threshold

Microwave breakdown in air for multi-carrier, modulated or stochastically time varying RF fields

An investigation is made of the threshold for microwave breakdown in air in situations where the microwave power is strongly modulated or stochastically varying in time as, e.g. in communication systems based on multi-carrier operation where interference between the carriers may cause occasional high power peaks in the microwave power. Thresholds are established for the scenario of coherent and co-phased carriers as well as for breakdown in an electric field with a stochastically varying amplitude.

Multi-carrier microwave breakdown in air-filled components

An investigation is made of the threshold for microwave breakdown in mobile telephone communication systems based on multi-carrier operation where interference between the carriers may cause occasional high power peaks in the transmission system. Thresholds are established for the worst case scenario of coherent and co-phased carriers as well as for the more realistic case of modulated carriers when the signal is considered as a stochastic signal.