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

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

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

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

Study of a 0.2 THz extended interaction oscillator driven by a pseudospark-sourced sheet electron beam

The study of a planar G-band extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS) sheet electron beam is presented. This enables the advantages of a planar interaction circuit combined with the merits of a PS sheet electron beam, including large beam cross section, high current density and the fact that a PS electron beam does not require the use of an external focusing magnetic field. Beam-wave interaction simulations for this planar EIO predicted a peak output power of 2.1 kW at ~0.2 THz. Investigations indicate that this planar EIO has a better tube performance with a higher radiation power compared with the PS pencil electron beam EIO

Design of a Ka-band MW-level high efficiency gyroklystron for accelerators

Design of a three-cavity Ka-band MW-level gyroklystron operated at the fundamental TE02 mode is presented in this paper. The initial design of the magnetron injection gun (MIG) and interaction circuit has been completed by using the PIC (Particle in cell) code MAGIC. The PIC simulation shows this gyroklystron can deliver an output power of more than 1.5 MW with a gain of > 35 dB at 36 GHz. The achieved efficiency exceeds 40 % when driven by a 95 kV, 45 A beam. The optimized MIG has a transverse velocity spread of less than 2.5% when the velocity ratio is around 1.3

Simulations of the self-focused pseudospark-sourced electron beam in a background ion channel

Using pseudospark discharge sourced electron beams for the generation of high-peak-power millimeter and terahertz radiation has attracted increasing research interest in recent years. However, one of the crucially important and hitherto unanswered questions is "what is the upper-frequency limit at which millimeter-wave devices can be driven by pseudospark discharge sourced electron beams?". In this paper, we studied this question from the perspective of beam transportation in a plasma background, more specifically an ion channel using particle-in-cell simulations to find the limitations. The parameter ranges of the beam transportation with small oscillations in the beam diameter were investigated and summarized, through simulations of beam propagation in a large diameter drift tube with different ion densities, plasma electron densities, beam density distributions, and beam energies. The beam transportation in a small diameter beam tunnel was also simulated. It showed the maximum beam current with a small velocity spread that can be transported in the beam tunnel was determined by the diameter of the beam tunnel and the ion density. High injected current will cause significant beam loss and reduce the overall efficiency. The simulation results indicate a minimum diameter of the beam tunnel in a millimeter-wave circuit that can be effectively driven by a pseudospark-sourced electron beam. The equivalent upper limit in the operating frequency is about 400 GHz

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