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

Demonstration of a high power broadband mm-wave gyro-TWA

Design and experimental results of a broadband, high power, millimetre-wave gyrotron traveling wave amplifier (gyro-TWA) operating in the 75-110 GHz frequency band and based on a helically corrugated interaction region (HCIR) and cusp electron beam source are presented. The second harmonic cyclotron mode of the electron beam was used to match the dispersion of an eigenwave in the HCIR, achieving energy transfer from the electrons to waves over a large frequency range. The gyro-TWA was measured to generate a maximum power of a few kWs with an unsaturated gain of 36-38 dB in the driving frequency band of 91-96.5 GHz

Simulation of a four-stage depressed collector for a W-band gyro-BWO

To improve the overall efficiency of the W-band gyrotron backward wave oscillator (gyro-BWO) currently being built in the University of Strathclyde, an energy recovery system using a four-stage depressed collector was simulated and designed. The spent beam information was exported from the simulation of the gyro-BWO using the 3D PIC code MAGIC. The geometry of the depressed collector was optimized using a genetic algorithm to achieve the optimum overall recovery efficiency for specific parameters of the spent beam. Secondary electron emissions were simulated to investigate the effects of the secondary electrons on the overall recovery efficiency and the backstreaming of the electrons from the collector region

Numerical Simulation of a Gyro-BWO with a Helically Corrugated Interaction Region, Cusp Electron Gun and Depressed Collector

The gyrotron backward wave oscillator (gyro-BWO) is an efficient source of frequency-tunable high-power coherent radiation in the microwave to the terahertz range. It has attracted significant research interest recently due to its potential applications in many areas such as remote sensing, medical imaging, plasma heating and spectroscopy. A gyro-BWO using a helically corrugated interaction region (HCIR) has achieved an even wider frequency tuning range and higher efficiency compared with a conventional gyro-BWO with a smooth-bore cavity. This is due to the existence of an “ideal”eigenwave in the HCIR with a large and constant group velocity when the axial wave number is small

W-band Brewster window for a wideband gyro-TWA

This paper presents the design of a Brewster window for a W-band gyrotron travelling wave amplifier (gyro-TWA). To maintain the Gaussian-like HE11 mode from the corrugated horn, a corrugated waveguide was optimized to host the Brewster window. The Brewster window was simulated and measured to have a lower than -20 dB reflection over the frequency band 85-101 GHz

A broadband corrugated horn for a W-band gyro-TWA

A quasi-optical mode converter in the form of a corrugated horn has been constructed for a W-band gyrotron traveling wave amplifier (gyro-TWA). The prototype was designed and optimized through analytical and numerical simulations. The horn converts a cylindrical TE 11 mode into a free-space TEM 00 mode in a frequency band of 90 GHz to 100 GHz. The optimized simulation predicts a return loss better than -35 dB and a Gaussian coupling efficiency of 97.8% and the measured prototype demonstrates a close agreement with these figures

A high-power Ka-band free electron maser, defined by a 2D – 1D Bragg lasing cavity

One of the on-going research programs, at the University of Strathclyde, involves the development of high-power, pulsed, Free-Electron Masers (FEMs) with the lasing cavity defined using periodic corrugations on the drift-tube walls 1-4 . These corrugations form 1D and 2D Bragg resonators, whose reflection bands determine the dominant resonance of the maser 5 . Proper selection of the FEM undulator magnetic field strength, allows for efficient extraction of energy from a mildly relativistic (400 - 500 keV) electron beam at the resonant frequency of the lasing cavity, leading to monochromatic output at power levels of several tens of megawatts and pulse durations of ~150ns (determined primarily by the pulse duration of the driving power supply of ~250ns)

Design of a gridded cusp gun for a W-band gyro-TWA

This paper reports the design and optimization of a gridded cusp electron gun for a W-band gyrotron traveling wave amplifier. By applying positive or negative biasing potentials to additional electrodes that are placed in front of the emitter, the electron beam can be switched on and off quickly and easily. In simulations, an optimal velocity ratio (alpha) of 1.12 with an alpha spread of ~10.7% was achieved when the gridded-type gun was operated at a beam voltage of 40 kV and a current of 1.7 A

Cusp electron gun with modulation electrode for a THz gyro-amplifier

A terahertz gyrotron traveling wave amplifier (gyro-TWA) centered at 370GHz is under development for the electron paramagnetic resonance (EPR) imaging application. This paper reports the investigation of a triode-type cusp electron gun for the terahertz gyro-TWA. The simulation results showed that at the beam alpha (the ratio of transverse to axial velocity) center of 1.07, an optimal alpha spread of ∼ 10\% was achieved, when it was operated at a beam voltage of 50 kV and a beam current of 0.35 A

Design of a TE10-to-TE61 mode coupler for a 372 GHz gyrotron travelling wave amplifier

This paper presents the design of a TE10-to-TE61 mode coupler for a 372 GHz gyrotron travelling wave amplifier. The optimized coupler was able to achieve an -1 dB transmission over the frequency band of 360-384 GHz. The transmission decreases by 1.2 dB when the copper conductivity of 0.5E7 S/m was used in the simulation. The designed coupler was scaled to W-band for manufacture and measurement. The measured transmission and phase response confirmed the mode coupling between the input TE10 mode and the desired TE61 mode