Craig R. Donaldson

A cusp electron gun for millimeter wave gyrodevices

The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter wave harmonic gyrodevices, are reported in this paper. Using a “smooth” magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio α of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC.

Multi-Mode Coupling Wave Theory for Helically Corrugated Waveguide

Helically corrugated waveguide has been used in various applications such as gyro-backward wave oscillators, gyro-traveling wave amplifier and microwave pulse compressor. A fast prediction of the dispersion characteristic of the operating eigenwave is very important when designing a helically corrugated waveguide. In this paper, multi-mode coupling wave equations were developed based on the perturbation method. This method was then used to analyze a five-fold helically corrugated waveguide used for X-band microwave compression. The calculated result from this analysis was found to be in excellent agreement with the results from numerical simulation using CST Microwave Studio and vector network analyzer measurements.

Design and Numerical Optimization of a Cusp-Gun-Based Electron Beam for Millimeter-Wave Gyro-Devices

A novel thermionic cusp electron gun operating in the temperature-limited regime that produces a large-orbit electron beam through a nonadiabatic magnetic-field reversal was designed, analyzed, and optimized to give an electron-beam ideal for driving gyro-devices, particularly in the millimeter-to-submillimeter-wavelength range due to its small cross-sectional size. The annular-shaped axis-encircling electron beam had a beam current of 1.5 A at an acceleration potential of 40 kV, a tunable velocity ratio alpha (= vperp/vz) between one and three, an optimized axial velocity spread A.vx/vz of ~8%, and a relative alpha spread Deltakappa/alpha of ~10% at an alpha value of 1.65.

A high directivity broadband corrugated horn for W-band gyro-devices

A quasi-optical mode converter in the form of a corrugated horn has been designed through numerical simulations, manufactured and experimentally measured for application in W-band gyro-devices. This horn converts a cylindrical TE11 mode into a free space TEM00 mode in a frequency band of 84-104 GHz with a reflection better than 30 dB, a Gaussian coupling efficiency of 98% and directivity of 26.6 dB at 95 GHz. The small beam waist makes such a horn ideal for use with a depressed collector system. The measured results are in excellent agreement with the numerical simulations.

Design and Measurement of a Broadband Sidewall Coupler for a W-Band Gyro-TWA

The input coupler is an important component for a microwave amplifier. In this paper, a sidewall single-hole input coupler for a W-band gyrotron traveling-wave amplifier that operates at the frequency range of 90-100 GHz was designed and measured. Instead of using a cutoff waveguide, a broadband Bragg-type reflector with a small spread in phase was optimized for use as part of the input coupler. The minimum radius of the reflector was two times the size of a cutoff waveguide, which reduced the possibility for some of the beam electrons being collected in this section and lost to the amplifier interaction region.

Wide-band HE11 mode terahertz wave windows for gyro-amplifiers

Broadband HE11 mode output windows, based on the multilayer concept, are studied for high power gyro-amplifiers operating in the low terahertz region. As the wave power in the hybrid HE11 mode is concentrated in the center of the circular waveguide, smaller reflection and better coupling to the fundamental free space Gaussian mode can be achieved for the windows. Two windows are designed for optimized performance through simulations for operation in two frequency ranges of 360- 400 GHz and 90-100 GHz. The simulated performance, practical constraints in realization and manufacturing methods of the 90-100 GHz window is discussed. This window was constructed and microwave properties measured showing a lower than -27 dB reflection. This result agrees with simulation data which validates the simulation methodology and effectiveness of the design.

Design Study of a Fundamental Mode Input Coupler for a 372-GHz Gyro-TWA I: Rectangular-to-Circular Coupling Methods

The design of two fundamental mode rectangular-to-circular waveguide input couplers for a low-terahertz gyrotron-traveling wave amplifier (gyro-TWA) is presented. A T-junction input coupler with a Bragg reflector and a multiple-hole directional coupler were optimized for operation between 360 and 384 GHz, the proposed gyro-TWA bandwidth. The T-junction coupler and the multiple-hole coupler achieved the respective bandwidths of 10% and 35%. The benefits and potential limitations of the low-terahertz wave coupler topologies are discussed alongside the challenging manufacturing methods of the submillimeter-wave components.

W-band gyro-devices using helically corrugated waveguide and cusp gun : design, simulation and experiment

This paper presents the design and simulation of W-band Gyro-devices using helically corrugated waveguides as the beam-wave interaction region and a cusp gun as the electron beam source. The electron beam system and the beam-wave interaction were optimized through numerical simulations by using a particle-in-cell (PIC) code MAGIC to predict (calculate) the output power and frequency bandwidth. The beam cross sectional measurement using a scintillator plate confirmed that an axis encircling electron beam was achieved with the designed beam parameters of current 1.5 A and energy 40 keV. The W-band helically corrugated interaction region for the gyrotron backward wave oscillator (Gyro-BWO) was manufactured with a dispersion from 80 GHz to 110 GHz measured using a vector network analyser which was found to be in good agreement with simulations and theory. The Gyro-BWO achieved frequency-tuneable operation by adjusting the magnetic field in the interaction cavity. A -3 dB bandwidth of ~84-104 GHz and output power ~10 kW were simulated using the electron beam from the cusp gun. The gyrotron travelling wave amplifier (Gyro-TWA) is designed to have a -3 dB frequency bandwidth of 90-100 GHz, output power of 10 kW and saturated amplification gain of 40 dB.

Design and simulation of a ~390?GHz seventh harmonic gyrotron using a large orbit electron beam

A similar to 390 GHz harmonic gyrotron based on a cusp electron gun has been designed and numerically modelled. The gyrotron operates at the seventh harmonic of the electron cyclotron frequency with the beam interacting with a TE71 waveguide mode. Theoretical as well as numerical simulation results using the 3D particle-in-cell code MAGIC are presented. The cusp gun generated an axis-encircling, annular shaped electron beam of energy 40 keV, current 1.5 A with a velocity ratio alpha of 3. Smooth cylindrical waveguides have been studied as the interaction cavities and their cavity Q optimized for 390 GHz operation. In the simulations similar to 600W of output power at the design frequency has been demonstrated.

Design Study of a 372-GHz Higher Order Mode Input Coupler

The design of a higher order mode (HOM) input coupler for a low-terahertz gyrotron travelling wave amplifier is presented. A two-branch waveguide coupler based on the even distribution of incident power is designed to couple the rectangular TE10 mode to the circular TE61 mode. The optimised tapered waveguide input coupler achieved an operating frequency range of 359-385 GHz, equating to a bandwidth of 7%. A prototype waveguide coupler scaled to W-band (75-110 GHz) was manufactured. The vector network analyser measured return loss of the HOM coupler is shown to be <;10 dB at 90-96 GHz. A transmission and phase measurement is presented to analyse TE61 mode conversion in the coupler.