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

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

Design of a high-power 48GHz gyroklystron amplifier for accelerator applications

As the technology of radiofrequency linear accelerators (RF linacs) continues to improve, higher frequency acceleration systems become of interest as the achievable acceleration gradient has a dependence on frequency. Using a high driving frequency requires the consideration of many technological challenges. One such challenge is mitigating the effect of nonlinearities introduced during the electron acceleration and bunching process. To counteract the nonlinearity, an additional cavity at a harmonic of the main driving frequency can be included. This technique is known as harmonic linearisation. In existing C-band systems, harmonic linearisation can be achieved with an X-band structure, but if the main frequency is X-band, the lineariser must be Ka-band or higher. Linear klystrons are a well-developed technology and can reliably deliver tens of MW at X-band, but they are subject to a steep drop-off in achievable output power toward the Ka-band. The different interaction mechanism in a gyroklystron, based on phase-modulation of a helical beam, allows it to deliver multi-MW output power at significantly higher frequencies. The gyroklystron is therefore a strong candidate for delivering power to high-frequency linearising cavities. The international collaboration, CompactLight, is developing a design for a sophisticated X-ray Free Electron Laser (XFEL) with wide ranging research applications [1, 2]. The project required the consideration of both a 36GHz and 48GHz lineariser options. In each case, the development of new amplifiers was required to deliver sufficient power for the application. This thesis presents the design and analysis of a gyroklystron appropriate to drive a 48GHz linearising cavity. While the research presented in this thesis was performed with direct consideration of the CompactLight XFEL, its relevance is not exclusive to this project. With the performance of the microwave amplifier presented in this thesis, a lineariser at 48GHz could be a viable option for other C-band or X-band accelerator applications. Gyroklystron research was historically focused on radar applications. Since 48GHz lies in a frequency band unfavourable for atmospheric transmission, the development of components in this band has been lacking. The design presented in this thesis is the first published work on a MW-level amplifier at 48GHz and marks a step toward this frequency becoming a desirable choice for linearisation systems in future linacs. A gyroklystron design, including the electron source, vacuum windows, and input coupler has been designed through detailed simulation work. A triode-type magnetron injection gun compatible with a 2.02T axial guide magnetic field was designed and simulated. Applying -140kV to the cathode and -107.5kV to the modulating anode resulted in a gyrating electron beam with a current of 37A, guiding centre radius of 1.77mm, and velocity ratio spread of 8.9%. This resulted in a predicted gyroklystron output power of 2.0MW with a gain of 35dB at an efficiency of 38.6%.As the technology of radiofrequency linear accelerators (RF linacs) continues to improve, higher frequency acceleration systems become of interest as the achievable acceleration gradient has a dependence on frequency. Using a high driving frequency requires the consideration of many technological challenges. One such challenge is mitigating the effect of nonlinearities introduced during the electron acceleration and bunching process. To counteract the nonlinearity, an additional cavity at a harmonic of the main driving frequency can be included. This technique is known as harmonic linearisation. In existing C-band systems, harmonic linearisation can be achieved with an X-band structure, but if the main frequency is X-band, the lineariser must be Ka-band or higher. Linear klystrons are a well-developed technology and can reliably deliver tens of MW at X-band, but they are subject to a steep drop-off in achievable output power toward the Ka-band. The different interaction mechanism in a gyroklystron, based on phase-modulation of a helical beam, allows it to deliver multi-MW output power at significantly higher frequencies. The gyroklystron is therefore a strong candidate for delivering power to high-frequency linearising cavities. The international collaboration, CompactLight, is developing a design for a sophisticated X-ray Free Electron Laser (XFEL) with wide ranging research applications [1, 2]. The project required the consideration of both a 36GHz and 48GHz lineariser options. In each case, the development of new amplifiers was required to deliver sufficient power for the application. This thesis presents the design and analysis of a gyroklystron appropriate to drive a 48GHz linearising cavity. While the research presented in this thesis was performed with direct consideration of the CompactLight XFEL, its relevance is not exclusive to this project. With the performance of the microwave amplifier presented in this thesis, a lineariser at 48GHz could be a viable option for other C-band or X-band accelerator applications. Gyroklystron research was historically focused on radar applications. Since 48GHz lies in a frequency band unfavourable for atmospheric transmission, the development of components in this band has been lacking. The design presented in this thesis is the first published work on a MW-level amplifier at 48GHz and marks a step toward this frequency becoming a desirable choice for linearisation systems in future linacs. A gyroklystron design, including the electron source, vacuum windows, and input coupler has been designed through detailed simulation work. A triode-type magnetron injection gun compatible with a 2.02T axial guide magnetic field was designed and simulated. Applying -140kV to the cathode and -107.5kV to the modulating anode resulted in a gyrating electron beam with a current of 37A, guiding centre radius of 1.77mm, and velocity ratio spread of 8.9%. This resulted in a predicted gyroklystron output power of 2.0MW with a gain of 35dB at an efficiency of 38.6%

Graphene travelling wave amplifier for integrated millimeter-wave/terahertz systems

Terahertz (THz) technology offers exciting possibilities for various applications, including high resolution biomedical imaging, long-wavelength spectroscopy, security monitoring, communications, quality control, and process monitoring. However, the lack of efficient high power easy-to-integrate sources and highly sensitive detectors has created a bottleneck in developing THz technology. In an attempt to address this issue, this dissertation proposes a new type of graphene-based solid state travelling wave amplifier (TWA). Inspired by the unique properties of electrons in graphene two-dimensional (2D) fluid, the author proposes a new type of TWA in which graphene acts as the sheet electron beam. These properties include higher mobility and drift velocity at room temperature, zero effective mass, relativistic behavior, and a truly 2D configuration. Since the plasma properties of 2D electron fluid become more pronounced as the effective mass of electrons decreases and electron mobility increases, THz devices based on graphene with massless quasiparticles significantly outperform those made of relatively standard semiconductor heterostructures. Another significant advantage of graphene over semiconductors is that while the high drift velocity and electron mobility of semiconductors 2D electron gas (2DEG) are achieved only at very low temperatures, graphene has high mobility and drift velocity at room temperature. This thesis describes the theoretical and practical methods developed for the analysis, design, and fabrication of a graphene-based THz TWA. It investigates the interaction between the electromagnetic wave and the drifting plasma wave in graphene by two methods. In the first approach, electrons in graphene are modelled as a 2D Fermi liquid, and the hydrodynamic model derived from a relativistic fluid approach is used to find the conductivity. In the second approach, the travelling wave interaction is analyzed using a quantum mechanical model. The drifting Fermi distribution function is applied to the linear conductivity response function of graphene obtained from random phase approximation. The conductivity of graphene is obtained as a function of frequency, wave number, chemical potential, and drift velocity. The result is consistent with the hydrodynamic approach. Both methods show that negative conductivity, and thus gain, is obtained when the drift velocity is slightly greater than the phase velocity. It is shown that the two methods produce comparable results. In the next step, a slow-wave grating structure is designed and an estimate of the actual gain is obtained for the proposed graphene TWA structures. The Floquet mode analysis of top grated slab and rectangular silicon waveguides is presented. Here, a new theoretical method is developed to accurately estimate the field distribution of the first order space harmonic of a hybrid mode inside a periodic top-grated rectangular dielectric waveguide. This method gives explicit expressions for the interaction impedance of the slow wave grating structures that are then used to design the waveguide and the grating. To verify the proposed approximation method, the results obtained with this approach are compared with the simulation results. Finally, a prototype structure is fabricated. The recipes developed for different parts of the structure are presented. These parts include: a nanometer size grating, a sub-millimeter dielectric waveguide, and biasing contacts on top of the graphene layer. The developed recipes ensure reliable fabrication processes for large-area graphene devices. In addition, two different methods used to fabricate long uniform gratings are compared. This work ends by showing the measurement results obtained for the fabricated devices

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

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

Quantum dynamics of atomic bright solitons under splitting and re-collision, and implications for interferometry

We numerically study the classical and quantum dynamics of an atomic bright soliton in a highly-elongated one-dimensional harmonic trap with a Gaussian barrier. In the regime of the recent experiment by Dyke {\it et al.}, the system realizes a coherent nonlinear soliton beam-splitter and interferometer whose accuracy we analyze. In the case of tighter radial trap confinement and enhanced quantum fluctuations, a split soliton can represent a spin-squeezed, or alternatively, a fragmented condensate with reduced phase-coherence that can be measured by interfering the split soliton by the barrier. We also find large quantum mechanical uncertainties in the soliton's position and momentum due to nonlinear interaction with the barrier.Comment: 30 pages, 9 figures. Modified in response to referees' comments, and with additional simulation result

Nonequilibrium spin phenomena in quantum dots induced by periodic optical excitation

The coherent control of a charge carrier spin that is localized in a semiconductor quantum dot and the generation of long-lived states for information storage are of particular interest for quantum information processing. This spin interacts predominantly with the surrounding nuclear spins in the quantum dot, which can be described by the central spin model. The periodic application of circularly polarized laser pulses induces nonequilibrium spin dynamics in the quantum dot, giving rise to various phenomena that can be observed in experiments. In this thesis, models and semiclassical approaches are developed to simulate the driven spin dynamics in this system under experimental conditions. For the case where a transverse magnetic field is applied, it is found that the part of the spin mode locking effect stemming from nuclei-induced frequency focusing depends nonmonotonically on the strength of the magnetic field, with strong similarities to experimental observations. The complex behavior is related to various nuclear magnetic resonances with respect to the repetition rate of the laser pulses, which can be exploited for novel kind of nuclear magnetic resonance spectroscopy of the emerging nonequilibrium steady states. For the case where a longitudinal magnetic field is applied, the influence of the pump pulse power on the spin inertia and on the polarization recovery effect is analyzed. With the help of the developed model, the related experiments can be understood and described quantitatively. In this context, a novel effect termed resonant spin amplification in Faraday geometry is predicted, which enables the direct measurement of the longitudinal g factor of the resident charge carriers. Model calculations are used to find the optimal conditions for its detection and ways to improve its visibility are pointed out. The comparison with recent experiments that demonstrate the realization of the effect shows a remarkable agreement.Die kohärente Kontrolle eines Ladungsträgerspins, der in einem Halbleiterquantenpunkt lokalisiert ist, sowie die Erzeugung langlebiger Zustände zur Informationsspeicherung sind von besonderem Interesse für die Quanteninformationsverarbeitung. Solch ein Spin wechselwirkt hauptsächlich mit den ihn umgebenden Kernspins im Quantenpunkt, was durch das Zentralspinmodell beschrieben werden kann. Durch periodische Anregung mit zirkular polarisierten Laserpulsen lässt sich die Spindynamik in Quantenpunkten in ein Nichtgleichgewicht treiben, wodurch verschiedene Phänomene auftreten können, die sich in Experimenten beobachten lassen. In dieser Arbeit werden theoretische Modelle und semiklassische Methoden entwickelt, um die getriebene Spindynamik unter experimentellen Bedingungen zu simulieren. Im Falle eines angelegten transversalen Magnetfelds zeigt sich, dass der Teil des „Spin Mode Locking“ Effekts (Synchronisation von Spin-Moden), welcher aufgrund einer durch die Kernspins induzierten Frequenzfokussierung entsteht, eine nicht-monotone Abhängigkeit von der Stärke des Magnetfelds aufweist, mit starken Parallelen zu experimentellen Beobachtungen. Verantwortlich für das komplexe Verhalten sind verschiedene Kernspinresonanzen bezogen auf die Wiederholungsrate der Laserpulse. Hieraus ergibt sich eine neue Art von Kernspinresonanzspektroskopie, durch welche die langlebigen Nichtgleichgewichtszustände untersucht werden können. Im Falle eines angelegten longitudinalen Magnetfelds wird der Einfluss der Pulsleistung auf den „Spin Inertia“ (Spinträgheit) und den „Polarization Recovery“ (Wiederherstellung der Spinpolarisation) Effekt untersucht. Die zugehörigen Experimente lassen sich durch das entwickelte Modell quantitativ verstehen und beschreiben. In diesem Zusammenhang wird ein neuer Effekt vorhergesagt, welcher als „Resonant Spin Amplification in Faraday Geometry“ (Resonante Spinverstärkung in Faraday Geometrie) bezeichnet wird und die direkte Bestimmung des longitudinalen g-Faktors der Ladungsträger ermöglicht. Optimale Bedingungen für dessen Beobachtung und Möglichkeiten zu Verbesserung seiner Sichtbarkeit werden aufgezeigt. Der Vergleich mit kürzlich durchgeführten Experimenten, welche die Existenz des Effekts bestätigen, zeigt eine hervorragende Übereinstimmung