J.M. Baird

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. >

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.

Backward-wave oscillators for the frequency range from 600 GHz to 1800 GHz

The authors describe an effort to develop O-type backward-wave oscillators for the frequency range from 600-1800 GHz for local oscillators in spectroscopy measurements in space applications. Recent interdigital O-type backward-wave oscillators have yielded RF output in the frequency range from 200-800 GHz. One of these oscillators used aluminium metallization on a type-2A diamond substrate. A second interdigital circuit was built to gain more experience with this type of circuit at a lower frequency. The circuit was fabricated with aluminium metallization on a crystalline quartz substrate, but with only the metallization etched to form the interdigital circuit and no etching of the substrate. A third tube utilizes a tapered slot antenna to radiate the RF output to a hemispherical lens. The 44- mu m circuit used aluminium metallization on crystalline quartz, with no etching of the quartz.<<ETX>>

Comparative TE modal analysis and extended parameter calculations of magnetron-wall waveguide for gyro-peniotron applications

Accurate determination of the magnetron-wall waveguide modal content is approached by examining the waveguide transverse cutoff constant (for which no closed-form analytic solution is known). The method of solution involves dividing the waveguide into two regions, an inner (circular) region and an outer (slot) region, assuming a constant electric field at the common interface, and equating the impedance functions at the interface. Computer calculations include large ratios of outer-to-inner wall radii (20:1), normalized slot angles between 0.01 and 0.99, a variable number of vanes (2-30), and higher-order modes that have not been previously reported. Mode numbering of the higher-order modes is also considered. The TE mode spectrum is plotted as a function of waveguide geometry. A waveguide model was built, and frequency resonances were measured. Resonances were correlated using a scanning filter computer program and used to calculate the modal cutoff frequencies. Experimental data agree with calculated values to within 1.25, 0.57, 0.49, and 2.48% for the four modes detected thus far. Results for lowest-order modes agree with those of other investigators to within about 5% or better in most instances. >

Investigation of techniques for minimizing resistivity of thin metallic films at submillimeter wavelengths

Experimental investigations of the substrate deposition temperature and annealing temperature influence on aluminum films deposited on diamond substrates were conducted. Tests were performed at direct current and at 101.55 GHz. Minimum resistivity levels, near theoretical predictions, occurred for deposition temperatures in the range of 50–160°C and for peak annealing temperatures of 100–120°C. Both colder and hotter substrate temperatures resulted in larger resistivity levels.

Ring-resonant cavity for the harmonic auto-resonant peniotron (HARP)

The need to eliminate the backward traveling wave in HARP (harmonic auto-resonant peniotron) interactions is demonstrated. The theory and design of a ring-type resonant cavity to achieve this are described. It is shown that this cavity can support high-efficiency HARP interactions.<<ETX>>

Fourth harmonic gyromagnetron development

Gyromagnetron devices (gyrotrons using magnetron types of circuits to operate on cyclotron harmonics) are being investigated as approaches for high-power millimeter-wave production. A low-power oscillator has just been made operational to experimentally investigate several possible modes of operation. Preliminary results have been obtained with RF oscillation observed at 83.4 GHz with operation near the fourth cyclotron harmonic.

Self-Consistent Transmission Line Analysis Of The Gyrotron Traveling-Wave Amplifier

A self-consistent analysis of the gyrotron traveling-wave amplifier is performed using a single particle theory and an exact transmission-line field analysis to yield a nonlinear large signal theory able to describe the effects of tapered, nonideal wall waveguides and tapered dc magnetic fields. Guiding center drift is considered, as well as efficiency as a function of input power, RF detuning, de detuning, current, and beam size.