Floquet's theorem in three dimensions and its application to the dispersion characteristics of circular waveguides with screw periodicity for use with gyrotron backward-wave oscillators

Floquet's theorem for three dimensions is proved for each of the two mathematical conditions of periodicity: the differential equation and the boundary condition. The theorem applied to circular waveguides with screw periodicity provides the dispersion characteristics of the uncoupled modes. These characteristics are compared to those of a normal circular waveguide to show how screw periodic RF structures shift in phase the waveguide cutoff point and how such a shift is advantageous to gyrotron backward-wave oscillators.

     

Sperm morphology as diagnosed by strict criteria: probing the impact of teratozoospermia on fertilization rate and pregnancy outcome in a large in vitro fertilization population

OBJECTIVE: To investigate the predictive value of sperm morphology assessed by strict criteria on IVF outcome. DESIGN: Retrospective analysis of all IVF cycles (January 1987 to December 1992). MAIN OUTCOME MEASURES: All patients were assigned to one of three groups based on sperm morphology: P-pattern (< 4% normal forms), G-pattern (4% to 14% normal forms), and N-pattern (> 14% normal forms). Morphology pattern was related to other semen characteristics and IVF outcome. RESULTS: Despite corrective measures at oocyte insemination, the fertilization rate was significantly different among the three morphology groups, P < G < N. N-pattern sperm produced a mean fertilization rate over 85% regardless of low motility or concentration. In a cohort study, P-pattern cycles produced a lower implantation rate and lower ongoing pregnancy rate, independent of the lower fertilization rate. CONCLUSIONS: Strict morphology is an excellent biomarker of sperm fertilizing capacity, independent of motility and concentration. P-pattern sperm may denote a poorer prognosis for establishing a pregnancy, even after a satisfactory fertilization rate is achieved.     

The effect of liquid gallium on the strengths of stainless steel and thermoplastics

We have investigated the effect of liquid gallium on type 316L stainless steel (as a candidate for the P–V–T pressure vessel), and four thermoplastics: two semicrystalline (high-density polyethylene and polypropylene) and two amorphous (polystyrene and poly(methyl methacrylate)). Specimens were coated with gallium and held at elevated temperatures and reduced pressure for extended periods. Measurements conducted on the plastics include weight change analyses, tensile tests and particle diffusion analysis using dispersion X-ray spectroscopy. For the stainless steel specimens, tensile and corrosion tests were conducted. Scanning electron microscopy was used to determine the level of corrosion. The results are compared with specimens heat treated identically but without gallium contact. After 3 months, the gallium corroded the surface of the steel to a depth of only 12 m. No penetration path of the gallium into the steel has been observed. The gallium was also found to cause no change in the mechanical properties of the polymers tested, nor was it found to have caused any weight change in the specimens.     

Hydrogen passivation in n- and p-type 6H-SiC

Hydrogen passivation effects are found to be much more prevalent in p-type 6H-SiC relative to n-type material. Reactivation of passivated B acceptors occurs at ~700°C, corresponding to a reactivation energy of ~3.3 eV. This is much higher than for passivated acceptors in Si, where reactivation occurs at ≤200°C. The incorporation depth of ²H from a plasma at 200°C is ≤0.1 μm in 30 min, corresponding to a diffusivity approximately two orders of magnitude lower than in Si at the same temperature. The average energy of ions in the ²H plasma has an influence on the peak concentration of incorporated deuterium and on its diffusion depth.     

TRANSIENT HEAT CONDUCTION EXTERIOR TO A CYLINDRICAL SURFACE

An analytical expression is obtained for the temperature surrounding a cylindrical surface of finite length. The boundary condition is that of constant heat flux and the method of solution involves Laplace and Fourier transforms.     

Beam mode compositions on passage through a Fabry-Perot interferometer

A determination is made on the quality of a millimetre-wave beam on passage through a Fabry-Perot interferometer by decomposing the beam cross-section into modes. The modes are then added at the output plane, where each mode is weighted by a constant dependent on the Fresnel number.     

Propagation losses at 10.6 µm in hollow-core rectangular waveguides for distributed feedback applications

Eigenvalue equations for the TE and TM modes propagating in a rectangular hollow-core waveguide are derived. The solutions to the eigenvalue equation are used to determine the theoretical losses for the lowest even order mode propagating in a rectangular waveguide whose cross-sectional dimensions are suitable for distributed feedback at 10.6 μm. Waveguide materials such as gold, BeO, glass, and germanium are analyzed. It is shown that by using gold for the top and bottom waveguide regions with BeO for the side walls, loss as small as 1.19 dB/m can be obtained with a cross-sectional dimension 0.1 mm × 0.8 mm. Using external discharge pumping the authors have created an active medium with gain in excess of 17 dB/m in a 0.08 mm hollow-core slab waveguide. Loss calculations indicate the feasibility of succesfully constructing a conventional electric discharge waveguide laser for DFB operation at 10.6 μm. Using a BeO-glass combination with cross-sectional dimensions 0.1 mm × 1 mm, a loss of 2.75 dB/m is calculated. It is also shown that a TEA waveguide laser could be built using the side walls for electrodes. This would result in a DFB waveguide laser at 10.6 μm with 2.75 dB/m loss and a cross section 0.1 mm × 2 mm. These results indicate that with good fabrication techniques and with the application of distributed feedback, it may be possible to construct a CO2waveguide laser with a significantly reduced waveguide cross section.     

An investigation into density-wave propagation within high-temperature superconducting plasmas for use in creating traveling-wave devices

This paper theoretically investigates a novel application of high-temperature superconductors where the superconductor serves as the active component in a microwave or millimeter traveling-wave amplifier. A guided electromagnetic wave interacts with a dc superconducting electron current to set up charge-density gradients within the superconducting electron "plasma." The electromagnetic wave gradually extracts energy from the superconducting electrons by traveling in phase synchronism with these charge gradients. The interaction mechanism is similar to that of a conventional traveling-wave tube amplifier or oscillator. We have modeled the wave behavior of superconducting electrons using the London equations and a two-fluid approach. Our model includes dissipation within the superconductor, and it shows that traveling-wave devices may be possible using high-quality thin-film superconductors in which dissipation is kept low.     

Backward-wave interaction using step periodic structures

Periodicities introduced into hollow waveguides with step and turn symmetry are shown to have characteristics useful for backward-wave interaction in microwave electron tubes. Using Floquet's theorem, it is shown that the modes of a hollow waveguide can be shifted in phase over one period of the structure. Interaction with the shifted backward-wave mode is then possible over a range of frequencies. A gyrotron backward-wave oscillator is particularly suited to this structure; the interaction could be magnetically tuned over a range of frequencies, while a low phase constant is maintained. A lower phase constant allows the electron beam to interact coherently with the backward wave over longer distances, since any velocity spread in the electron beam forces some of the electrons to be out of step with the backward wave, thus reducing efficiency. A solid waveguide has the added advantages of being easily built for high frequencies and rugged for high temperatures.