R.W. Grow

Thermal analysis of PPM-focused rod-supported TWT helix structures

A thermal analysis, which can be implemented on a small computer system, has been developed for PPM (periodic permanent-magnet)-focused rod-supported TWT (traveling-wave tube) helix structures. The method fully accounts for the variable thermal conductivity of several of the materials used in helix assemblies and considers the nonuniform geometry of the various elements of the structure along the direction of heat flow. Including the effects of the thermal interface resistances, accurate predictions can thus be made of the thermal behavior of the circuit for a given power dissipation on the helix, and the influence on the helix temperature rise of the thermal and geometrical parameters of the various elements of the structure can be easily examined. An experimental test shows good agreement with the computed results and indicates that the method can be used to estimate the thermal interface resistance of experimental helix assemblies. >

Starting conditions for backward-wave oscillators with large loss and large space charge

Starting conditions of backward-wave oscillators are obtained both analytically and numerically for large loss and large space charge. Analytical results are obtained for zero loss and space charge up to infinity and for zero space charge and loss up to infinity. Computer results are carried out for circuit loss up to 225 dB and space charge up to QC = 25. An analytical equation is developed for computing starting conditions for large loss approaching infinite values and for large space charge approaching infinite values. Since the starting C is proportional to the loss per wavelength, large values of loss in a backward-wave oscillator can prevent oscillation from occurring unless the current can be made sufficiently large; i.e., the starting C is independent of length.

The effects of dielectric and metal loading on the dispersion characteristics for contrawound helix circuits used in high-power traveling-wave tubes

The theory developed by M. Chodorow and E.L. Chu (Stanford Univ. Tech. Rep. TR249, 1954) is expanded to include the problem of a contrawound helix interposed between two dielectric regions, symmetrically oriented inside a conducting cylinder. The single helix oriented in an identical fashion is simultaneously analyzed. Numerical results are presented in the form of dispersion diagrams over a wide range of parameters. Interesting behavior found within these diagrams is discussed and, whenever possible, comparable to experimental results. >

Self-consistent simulation of harmonic gyrotron and peniotron oscillators operating in a magnetron-type cavity

A self-consistent, large-signal computer simulation code is developed to study the interaction between the RF wave and a large helical-orbit. axis-encircling electron beam in a magnetron-type open cavity. The theory and the results of simulations of different modes of oscillations operating in an eight-vane cavity are given and discussed. A novel type of peniotron interaction that gives high efficiency for realistic thick beams is revealed. >

New multimedia modules in CAEME CD-ROM series: Wave motion and mechanics

This paper presents two modules of multimedia lessons developed for the center of excellence for multimedia education (CAEME) CD‐ROM series. The first module is a lesson on vibration and wave motion, and the other is concerned with lessons of mechanics. The developed lessons contain several multimedia assets, i.e., text, graphics, audio, video, movie, and animation. In developing these modules, emphasis has been placed on increasing the interactivity, and hence is expected to be of broader use by seniors in high schools and freshman in engineering.

Terahertz Backward-wave Oscillators with Photonic Crystal Waveguides

We are developing backward wave oscillators (BWOs) for use above 1 THz. The devices employ photonic crystal waveguide (PCW) circuits for the slow-wave circuits, micromachined into silicon substrates and coated with gold to form the metallic photonic crystals. These circuits are being tested with an electron beam gun in a BWO tube previously used with an interdigitated circuit that tuned over an octave, up to 270 GHz, and gave approximately 0.5 mW over this range (Barnett, 1989). The PCW circuits with E-plane bends allow for a much greater coupling of the electron beam to the circuit, enabling higher frequencies and powers. The main design ideas include using a serpentine PCW formed with a series of photonic waveguide bends have been designed for 1.5 THz operation. Straight waveguide models were matched to waveguide bends with three dimentional calculations. The entire circuit was simulated with a two dimentional approximation. The photonic crystals allow a gap to be opened between the circuit halves, which allows for an electron sheet beam that couples well with the circuit. Three-dimensional waveguide simulations with a finite element software package have indicated that a gap on the order of half the free-space wavelength may be accommodated, with a calculation of the electric field intensity and characterized by its scattering parameters. These calculations have also been used to design the metallic post dimensions that define the photonic crystal. The resulting mode gives an electric field component parallel to the electron beam path thus allowing for better coupling between the electron beam and the circuit, compared to an interdigitated circuit. The circuits are being microfabricated with deep reactive ion etching, gold evaporation, and substrate bonding. Photonic crystal circuits fabricated at the University of Utah which also include horn antennas formed with anisotropic etching of the silicon substrate, designed for 2.5 THz operation was shown (Vuppala, 2006)

Raman scattering as a diagnostic technique for cathode characterization

The feasibility of using the optical technique of Raman scattering to determine the chemical composition of cathode surfaces has been investigated. Reference Raman spectra have been obtained for many of the chemical compounds expected to be present at the surface of dispenser cathodes. In addition, spectra obtained from 5-3-2 impregnant and several cathode surfaces are presented. The potential advantages of the approach are discussed.

Submillimeter-wave BWO's

It is the goal of this study to explore concepts of making milliwatt output level backward-wave oscillators for frequencies in the range of 600 GHz up to at least 2,000 GHz. Such sources would find many scientific uses, particularly as local oscillators in receivers for submillimeter spectroscopy. We are currently studying designs using ion beam assisted etching to produce small slow-wave circuits on diamond substrates. The interdigital line is a fundamental backward-wave circuit and has a much higher coupled impedance than the traditional vane circuit, but the much smaller size is not a limitation with modern photolithography and ion beam etching technology. Low frequency scaled cold circuit tests showed that the coupled impedance is reasonably high and the tuneable bandwidth can be 40 percent or more. With the higher coupled impedance, lower beam density (and cathode density) can be used, and magnetic fields of only a few kilogauss are required. Hence, it appears that low voltage, wideband, long life, and lightweight submillimeter BWO sources are feasible. An experiment with an 18 µm pitch interdigital line etched on diamond, designed to oscillate at 600 GHz with a 4 kV beam and 3 kG magnetic field, is being constructed.

A mathematical technique for an exact small-signal field analysis of multiple-stream interaction in a finite longitudinal magnetic field

A mathematical technique for solving Maxwells equations and the Lorentz force equation with no approximations except the small-signal approximation is presented. A finite dc magnetic field parallel to the dc velocity of the charges is included. Polarization variables are used, and the boundary conditions include ac surface charge density and surface current density. The advantages of the method are that both fast waves and slow waves are included without a quasi-static approximation, and only the determinantal equation requires computer solution. The partial differential equations are solved directly and need not be solved by computer.

Use of extrapolation on wave expansions to force the satisfaction of the Rayleigh hypothesis

An extrapolation method is shown to force the convergence of a diverging wave expansion. This wave expansion will then satisfy the Rayleigh hypothesis over a much broader region. Only a single wave expansion then becomes necessary for field solutions in a particular region, making methods such as analytic continuation unnecessary.