Richard G. Carter

Accuracy of microwave cavity perturbation measurements

Techniques based on the perturbation of cavity resonators are commonly used to measure the permittivity and permeability of samples of dielectric and ferrite materials at microwave frequencies. They are also used to measure the local electric- and magnetic-field strengths in microwave structures including the shunt impedances of cavity resonators and the coupling impedances of slow-wave structures. This paper reexamines the assumptions made in the theory of these techniques and provides estimates of the errors of measurement arising from them.

A taxonomy of objectives for professional education

ABSTRACT A discussion of available ways of analysing and specifying educational objectives shows that none is adequate for considering professional education. A new taxonomy of educational objectives designed to meet this need is described. The classification is based on a description of personal attributes under the main headings: knowledge, skill and personal qualities. The application of the taxonomy to the analysis and design of curricula is discussed together with implications for learning experiences and assessment. It is suggested that the taxonomy could have applications in other areas of education as an aid to clarity of thought about educational objectives.

Design of helix slow-wave structures for high efficiency TWTs

TWTs for space applications commonly have a helix pitch profile which incorporates a section with increased phase velocity followed by a negative phase velocity taper. A simple method is described for the initial design of a helix slow-wave structure of this kind to achieve high overall efficiency. It is shown that the use of a section with increased phase velocity increases the beam efficiency of a TWT while reducing the generation of second harmonic power. The technique is illustrated by its application to a 70 W Ku-band TWT and the performance is shown to be comparable with that of an existing TWT.

Suppression of multipacting in rectangular coupler waveguides.

Although a rectangular waveguide coupler has the conceptual advantages of simplicity and capability of withstanding higher power, builders of modern superconducting accelerators are routinely choosing instead a coaxial coupler for its proven performance. Multipacting induced discharge has been found to be an operating mechanism that prevents a rectangular waveguide coupler from reaching its full potential. Earlier calculations predicted the existence of two-sided multipacting in a rectangular waveguide geometry. In the present study, special waveguide sections of CESR type were built and tested. Multipacting characteristics of the waveguide were identified. Two multipacting suppression methods, the slotted waveguide method and the DC magnetic bias method, were experimentally evaluated. The multipacting current is suppressed by a factor of more than 2 by opening a slot on the broad wall. Complete multipacting suppression can be realized by using the DC magnetic bias method.

Three-dimensional simulation of multistage depressed collectors on micro-computers

A three-dimensional (3-D) package for simulation of asymmetric and crossed-field multistage depressed collectors for microwave tubes has been developed. This package is based upon the 3-D finite-difference code KOBRA3-INP. The main features of the package are a user-friendly input interface, post-processors for collector analysis and calculation of secondary electron trajectories, and versatile output graphics. Both PC and mainframe versions of the package have been developed. The results of simple benchmark tests and those of simulation and analysis of asymmetric and crossed-field collectors including the effects of secondary electrons are presented. It is found that the asymmetric hyperbolic electric field collector shows very low backstreaming. It is shown that the representation of trajectories in energy space gives a better insight into the behavior of individual trajectories than plotting in coordinate space. The package will be useful for designing novel types of depressed collector. >

Frequency and phase modulation performance of an injection-locked CW magnetron

It is demonstrated that the output of a 2.45-GHz magnetron operated as a current-controlled oscillator through its pushing characteristic can lock to injection signals in times of the order of 100-500 ns depending on injection power, magnetron heater power, load impedance, and frequency offset of the injection frequency from the natural frequency of the magnetron. Accordingly, the magnetron can follow frequency and phase modulations of the injection signal, behaving as a narrow-band amplifier. The transmission of phase-shift-keyed data at 2 Mb/s has been achieved. Measurements of the frequency response and anode current after a switch of phase as a function of average anode current and heater power give new insight into the locking mechanisms and the noise characteristics of magnetrons

Calculation of the Properties of Reentrant Cylindrical Cavity Resonators

The lowest resonant frequencies of reentrant cylindrical cavity resonators are calculated using the method of moments to obtain upper and lower bounds. The accuracy and convergence of the results are investigated and the factors and shunt impedances calculated. A simple empirical assumption about the choice of basis functions leads to results that are of good accuracy and readily computed. The results obtained are compared with those of experiment and from calculations using MAFIA and Microwave Studio.

Optimization of Multistage Depressed Collectors

Random walk and genetic algorithm techniques have been implemented in a 3-D collector simulation code to automate the design optimization of multistage depressed collectors. An axisymmetric four-stage collector and an asymmetric two-stage collector have been optimized following both approaches. Procedures for the implementation of these methods in any suitable code and simulated performance of the collectors are demonstrated. The results show significant improvement in the collector performances due to optimization. A comparison between the optimum collector performances that were obtained using these methods has also been carried out.

A fast large-signal model for coupled-cavity TWTs

A fast and accurate one-dimensional large-signal model for coupled-cavity TWTs has been developed that can be used interactively for the optimization of a TWT design. Different modeling techniques for enhancing the speed of the large-signal model with no loss in accuracy are discussed. It has been shown that the speed of the model can be increased by (1) suitable selection of the basic integration parameters such as the number of electron discs per RF cycle and number of integration steps per cavity, (2) choosing a suitable method of integration of the relativistic equation of motion, (3) optimizing the iteration processes through each cavity and each section, and (4) efficient calculation of the space-charge forces. The model has been tested for the low-space-charge high-efficiency NASA CTS 200-W tube and for a high-space-charge high-gain tube. The characteristics of a 58-cavity tube for a single value of input power and frequency can be computed in less than 1.5 min of CPU time on a VAX 11/785 computer. This is at least 5 to 6 times faster than the previous models with the same accuracy. >

Optimization of Coaxial Couplers

A simple and accurate method has been developed to optimize the performance of coaxial couplers for traveling-wave tubes using a 3-D electromagnetic field simulation code. The new technique, which is a combination of numerical and analytical procedures, is presented with a comparison with conventional practice. By using this method, a number of couplers with Sub- Miniature A, Threaded Neill-Concelman, and waveguide output for different tubes were modeled, validated by the experimental cold test data, and optimized. The use of this technique can reduce the optimization time by up to 90% in comparison with the conventional numerical only approach.