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

Wideband gyro-amplifiers

Gyro-amplifiers using helically corrugated waveguides have shown exceptional gain, power, bandwidth, and efficiency performance at cm and mm wavelengths. The performance of a long pulse (and therefore high vacuum) system is strongly influenced by factors other than the intrinsic bandwidth of the interaction. We shall discuss these and other challenges, along with their mitigation in high average power wideband amplifiers

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 Braggtype 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

Experimental study of microwave pulse compression using a five-fold helically corrugated waveguide

This paper presents the experimental study of microwave pulse compression using a five-fold helically corrugated waveguide. In the experiment, the maximum power compression ratio of 25.2 was achieved by compressing an input microwave pulse of 80 ns duration and 9.65 GHz to 9.05 GHz frequency swept range into a 1.6 ns Gaussian-envelope pulse. For an average input power of 5.8 kW generated by a conventional traveling wave tube, a peak pulse output power of 144.8 kW was measured corresponding to an energy efficiency of 66.3%

New Type of sub-THz Oscillator and Amplifier Systems Based on Helical-Type Gyro-TWTs

This work presents the development and systematic investigation of a new sub-THz source for the generation of trains of coherent high-power ultra-short pulses at 263 GHz via passive mode-locking of two coupled helical gyrotron traveling wave tubes (helical gyro-TWT). The frequency of 263 GHz is an established figure for continuous wave (CW) DNP-NMR application and, therefore, the investigated source will allow the development of novel spectroscopy methods such as time-domain DNP-NMR for which powerful sub-THz pulses with highest coherency are required. For the first time, it is shown that the operation of the passive mode-locked helical gyro-TWTs in the hard excitation regime is of particular importance to reach the optimal coherency of the generated pulses. To enable the operation in the hard excitation regime, a new extended passive mode-locked oscillator is proposed. The extended passive mode-locked oscillator will furthermore enable the generation of specific pulse sequences in addition to the generation of pulses with constant repetition frequency. This could be of particular interest for some time-domain DNP-NMR methods where well-defined pulse sequences are required

New Type of sub-THz Oscillator and Amplifier Systems Based on Helical-Type Gyro-TWTs

This work presents the development of a new sub-THz source for the generation of trains of coherent high-power ultra-short pulses at 263 GHz via passive mode-locking of two coupled helical gyro-TWTs. For the first time, it is shown that the operation of such passive mode-locked helical gyro-TWTs in the hard excitation regime is of particular importance to reach the optimal coherency of the generated pulses. This could be of particular interest for some new time-domain DNP-NMR methods

A W-band gyrotron backward wave oscillator with helically corrugated waveguide

This thesis was previously held under moratorium from 9th May 2011 until 9th May 2013.This thesis presents the results of a successful W-band gyrotron backward wave oscillator experiment. Three major achievements presented in this thesis are: 1) The design, simulation, construction and operation of a cusp electron gun; 2) The design, simulation, optimisation, construction and experimental measurement of a W-band helically corrugated waveguide and 3) the operation of the world's first W-band gyro-BWO using both a helically corrugated waveguide and a cusp electron gun. Gyro-BWO interaction with a 2nd cyclotron harmonic axis-encircling annular electron beam was observed. The interaction region was constructed through an accurate electroplating method while the designed dispersion characteristics agreed well to the experimental measurements. The loss through the optimised construction method was low, recorded around 1dB through the frequency range of interest. The following work presents the analytical, numerical and experimental investigation of a proof of principle gyro-BWO experiment. The design, simulation and optimisation of a thermionic cusp electron gun that can generate a 1.5A, 40kV axisencircling electron beam are discussed. Simulations showed a high quality electron beam with ~8% velocity spread and ~10% alpha spread. Experiments were conducted using this electron gun and the accelerating voltage pulse, diode current, transported beam current are presented. The electron beam profile was recorded showing a clear axis-encircling beam image from which the electron beam diameter and alpha values can be measured. Microwave radiation was measured over a frequency range of ~91-100GHz with a approximate maximum power of ~0.37kW. Operating over the magnetic field range 1.79T to 1.9T and measured over a range of alpha values this result was very impressive and proved the successful operation of the gyro-BWO.This thesis presents the results of a successful W-band gyrotron backward wave oscillator experiment. Three major achievements presented in this thesis are: 1) The design, simulation, construction and operation of a cusp electron gun; 2) The design, simulation, optimisation, construction and experimental measurement of a W-band helically corrugated waveguide and 3) the operation of the world's first W-band gyro-BWO using both a helically corrugated waveguide and a cusp electron gun. Gyro-BWO interaction with a 2nd cyclotron harmonic axis-encircling annular electron beam was observed. The interaction region was constructed through an accurate electroplating method while the designed dispersion characteristics agreed well to the experimental measurements. The loss through the optimised construction method was low, recorded around 1dB through the frequency range of interest. The following work presents the analytical, numerical and experimental investigation of a proof of principle gyro-BWO experiment. The design, simulation and optimisation of a thermionic cusp electron gun that can generate a 1.5A, 40kV axisencircling electron beam are discussed. Simulations showed a high quality electron beam with ~8% velocity spread and ~10% alpha spread. Experiments were conducted using this electron gun and the accelerating voltage pulse, diode current, transported beam current are presented. The electron beam profile was recorded showing a clear axis-encircling beam image from which the electron beam diameter and alpha values can be measured. Microwave radiation was measured over a frequency range of ~91-100GHz with a approximate maximum power of ~0.37kW. Operating over the magnetic field range 1.79T to 1.9T and measured over a range of alpha values this result was very impressive and proved the successful operation of the gyro-BWO

New Type of sub-THz Oscillator and Amplifier Systems Based on Helical-Type Gyro-TWTs

This work presents the development of a new sub-THz source for the generation of trains of coherent high-power ultra-short pulses at 263 GHz via passive mode-locking of two coupled helical gyro-TWTs. For the first time, it is shown that the operation of such passive mode-locked helical gyro-TWTs in the hard excitation regime is of particular importance to reach the optimal coherency of the generated pulses. This could be of particular interest for some new time-domain DNP-NMR methods

A w-band quasi-optical mode converter and gyro-BWO experiment

High power coherent microwave sources at shorter wavelengths (mm and sub-mm) are in great demand, especially in the fields of plasma physics, remote sensing and imaging and for electron spin resonance spectroscopy. Gyro-devices are by their nature particularly suited to this type of application due to the fast-wave cyclotron resonance maser instability, which is capable of producing high power radiation at frequencies that prove challenging for other sources. A W-band gyro-device based on a cusp electron beam source with a helically corrugated interaction region is currently under development to provide a continuously tuneable source over the range between 90 GHz to 100 GHz with a CW power output of ~10 kW. The work presented herein encompasses the design, construction and measurement of a prototype output launcher for this gyro-device. A corrugated mode converting horn was designed to act as a quasi-optical mode converter that converts the fundamental operating mode within the gyro-TWA (TE11) to a hybrid mode, which is closely coupled to the fundamental free space Gaussian mode (TEM00). This free space mode allows the possibility for the inclusion of an energy recovery system that can recover a percentage of the energy from the spent electron beam and is predicted to increase overall efficiency by up to 40%. For this scheme the electron beam must be decoupled from the radiation, which can pass through the collector system and vacuum window unperturbed while the electrons are collected at the energy recovery system. This type of corrugated mode converting horn was chosen due to the advantages of a greater bandwidth and the capability to provide a source that is continuously tuneable over this bandwidth. The results of the design and integration of this corrugated mode converting horn with the gyro-device are presented. The prototype operates over a continuously tuneable bandwidth of 90 to 100 GHz with a return loss better than -35 dB and a Gaussian coupling efficiency of 97.8%. The far field radiation pattern shows a highly symmetrical structure with 99.9% of the power radiated within a cone with a half angle of less than 19° and a cross-polar level less than -40 dB.High power coherent microwave sources at shorter wavelengths (mm and sub-mm) are in great demand, especially in the fields of plasma physics, remote sensing and imaging and for electron spin resonance spectroscopy. Gyro-devices are by their nature particularly suited to this type of application due to the fast-wave cyclotron resonance maser instability, which is capable of producing high power radiation at frequencies that prove challenging for other sources. A W-band gyro-device based on a cusp electron beam source with a helically corrugated interaction region is currently under development to provide a continuously tuneable source over the range between 90 GHz to 100 GHz with a CW power output of ~10 kW. The work presented herein encompasses the design, construction and measurement of a prototype output launcher for this gyro-device. A corrugated mode converting horn was designed to act as a quasi-optical mode converter that converts the fundamental operating mode within the gyro-TWA (TE11) to a hybrid mode, which is closely coupled to the fundamental free space Gaussian mode (TEM00). This free space mode allows the possibility for the inclusion of an energy recovery system that can recover a percentage of the energy from the spent electron beam and is predicted to increase overall efficiency by up to 40%. For this scheme the electron beam must be decoupled from the radiation, which can pass through the collector system and vacuum window unperturbed while the electrons are collected at the energy recovery system. This type of corrugated mode converting horn was chosen due to the advantages of a greater bandwidth and the capability to provide a source that is continuously tuneable over this bandwidth. The results of the design and integration of this corrugated mode converting horn with the gyro-device are presented. The prototype operates over a continuously tuneable bandwidth of 90 to 100 GHz with a return loss better than -35 dB and a Gaussian coupling efficiency of 97.8%. The far field radiation pattern shows a highly symmetrical structure with 99.9% of the power radiated within a cone with a half angle of less than 19° and a cross-polar level less than -40 dB

The development of broadband millimeter-wave and terahertz gyro-TWA

The gyrotron travelling wave tube amplifiers (gyro-TWAs) presented in this paper can operate with high efficiency (30%), huge powers and wide bandwidths at high frequencies that no other amplifier can provide. In principle, this is a technology that can be scaled to >1 THz and operate with 20% bandwidths. Resonant coupling of two dispersive waveguide modes in a helically corrugated interaction region (HCIR) can give rise to a non-dispersive eigenwave over a wide frequency band. The synchronism between the ideal wave and an electron cyclotron mode, either fundamental or harmonic, of a large orbit electron beam contributes to the broadband amplification. An electron beam of 55 keV, 1.5 A with a velocity pitch angle of ~1 generated by a thermionic cusp gun is used in our 100 GHz gyro-TWA experiment, which achieves an unsaturated output power of 3.4 kW and gain of 36–38 dB. The design and experimental results of the many components making the gyro-TWA will be presented individually and then the whole system will be introduced. The amplification of a swept signal by the W-band gyro-TWA is demonstrated showing its capabilities in the field of telecommunications. Furthermore, the design studies of a cusp electron gun in the triode configuration and the realization of a 3-fold HCIR operating at 372 GHz will also be displayed