Frequency-Tunable Reflective Gyrotron Backward-Wave Oscillator with Non-Adiabatic Electron Source PDF Download

Author: 蔡政宏
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Languages : en
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Author: Liang Zhang
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Languages : en
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This thesis is based on the research project of a W-band gyrotron backward wave oscillator (gyro-BWO) using a helically corrugated waveguide which is currently being built and upgraded in the University of Strathclyde. The gyro-BWO was optimally designed through numerical simulations to achieve an output maximum power of ~ 10 kW with a -3 dB frequency tuning range of 84 - 104 GHz. To increase the overall efficiency of the W-band gyro-BWO, an energy recovery system of four-stage depressed collector was designed, numerically optimized and fabricated on the gyro-BWO. Microwave components including the Bragg reflectors, the side-wall coupler, the three-layer microwave window and the pillbox window were designed, simulated and measured to facilitate the practical use of the energy recovery system. This thesis includes the analytically calculated results, the numerical simulations as well as the experimental results of the said components and system. A 14-section Bragg reflector together with the side-wall coupler located at the upstream of the helically corrugated interaction cavity was used to couple the microwave radiation out. This allowed the installation of the depressed collector at the downstream side of the gyro-BWO. The transmission coefficient of the coupler was numerically optimized to achieve -1.0 dB over the frequency tuning range, from 84 - 104 GHz. The Bragg reflector measurement agrees well with the simulation. The input coupler achieves an average -13 dB reflection over the frequency in the measurement. Theoretical analysis of the pillbox type window and multi-layer window based on mode-matching method was carried out. The simulation and optimization of the pillbox window achieved a reflection of less than -15 dB in the whole operating frequency range of 84 - 104 GHz. The three-layer window can achieve less than 30 dB reflection in the frequency range of 84 - 104 GHz in the simulation. A three-layer window and a pillbox window which particularly optimized in frequency range of 90 - 100 GHz (the operating frequency range of the gyro- TW A that shares the same experimental setup as the gyro-BWO) were fabricated. With manufacturing constraints the design of the three-layer window achieved an average -10 dB measured reflection in 84 - 104 GHz and better than -15 dB in 90 - 100 GHz. In the downstream side of the gyro-BWO, another 18-section Bragg reflector was used to reflect the radiation back into the upstream interaction cavity. And the transmission coefficient of -30 dB was obtained in the microwave measurements using a VNA, which means the microwave power leakage was less than 1%. The measurement results agreed well with the simulations. A four-stage depressed collector was designed to recover the energy from the spent electrons. The 3D PlC code MAGIC and a genetic algorithm were used to simulate and optimize the geometry of the electrodes. Secondary electron emissions were simulated and a few emission models were compared to investigate their effects on the overall recovery efficiency and the backstreaming rate for the multistage collector. The optimization of the shape and dimensions of each stage of the collector using a genetic algorithm achieved an overall recovery efficiency of about 70%, with a minimized backstreaming rate of 4.9%. The heat distribution on the collector was calculated and the maximum heat density on the electrodes was 240W/cm2 and the generation of "hot spots" could be avoided. The electric field distribution inside the depressed collector was calculated and the geometries of these electrodes were properly shaped to avoid the voltage breakdown in vacuum.

Author: THOMAS ALLEN SPENCER
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Category :
Languages : en
Pages : 340

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the existence of the gyro-backward-wave, as well as show the magnetic tunability of the gyrotron-backward-wave. In the solid beam case (1-2 kA), about 300-800 kW of extracted microwave power was detected in the waveguide detection system, implying that approximately 3-8 MW (efficiency $\sim$1-2%) of power is generated from the gyro-BWO device. The pulselengths for the solid beam case were from 300-600 ns (essentially the total flat-top voltage pulselength) over a frequency range of 4.5-6 GHz. The

Author: Antonio Carlos Torrezan de Sousa
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Languages : en
Pages : 185

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This thesis reports the design and experimental demonstration of frequency-tunable submillimeter-wave gyrotrons operating in continuous wave (CW) at the second harmonic of the electron cyclotron frequency. An unprecedented continuous frequency tuning range of more than 1 GHz has been achieved in both a 330- and a 460-GHz gyrotron via magnetic field tuning or voltage tuning. The 330-GHz gyrotron has generated 19 W of power in a cylindrical TE4,3,q mode from a 13-kV 190-mA electron beam. The minimum start current was measured to be 21 mA, where good agreement was verified between the measured start current values and the calculation from linear theory for the first six axial modes, q = 1 through 6. A continuous tuning range of 1.2 GHz with a minimum output power of 1 W has been achieved experimentally via magnetic or beam voltage tuning. The output stability of the gyrotron running under a computerized control system was assessed to be ±0.4% in power and ±3 ppm in frequency during a 110-hour uninterrupted CW test. Evaluation of the gyrotron microwave output beam using a pyroelectric camera indicated a Gaussian-like mode content of 91%. Measurements were also carried out in microsecond pulse operation at a higher beam current (610 mA), yielding a minimum output power of 20 W over a tuning range of 1.2 GHz obtained by means of cyclotron frequency tuning and thermal tuning. The 330-GHz gyrotron will be used as a source for 500 MHz nuclear magnetic resonance (NMR) experiments with sensitivity enhanced by dynamic nuclear polarization (DNP). In addition to the 330-GHz gyrotron, the design and CW operation of a tunable second-harmonic 460-GHz gyrotron are described. The 460-GHz gyrotron operates in the whispering gallery mode TE1 1,2 and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high order axial modes TE1,2,q. The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz with a minimum output power of 2 W has been obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 hours, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering gallery mode to a Gaussian-like beam was also verified. Based on images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%. The 460-GHz gyrotron is intended to be used as a submillimeter-wave source in a 700-MHz DNP/NMR spectrometer.

Author: Alexej Grudiev
Publisher: Cuvillier Verlag
ISBN: 3736906331
Category : Technology & Engineering
Languages : de
Pages : 164

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The possible applications of gyro-oscillators span a wide range of technologies [61 – 63]. In the field of magnetic confinement fusion studies, such applications as lower hybrid current drive, electron cyclotron resonance heating (ECRH) and current drive, plasma production for different processes, and active plasma diagnostic measurements have been demonstrated. For these applications, it is necessary to develop CW gyromonotrons that operate at both higher frequencies and higher output power. Hence single-mode 110-170 GHz CW gyromonotrons with a conventional cylindrical cavity, capable of high average power (0.5-1 MW per tube), and 2 MW coaxial cavity gyromonotrons are currently under development. Gyromonotrons are also successfully used in materials processing (e.g. in advanced ceramic sintering, surface hardening, or dielectric coating of metals and alloys) as well as in plasma chemistry. The use of gyromonotrons for such technological applications appears to be of interest if one can realize a relatively simple, low cost device which is easy in service. Hence gyromonotrons with low magnetic field (operating at the 2nd harmonic of the electron cyclotron frequency), low anode voltage, high efficiency, and long lifetime are under development. Potential applications of gyro-BWOs as a tunable millimeter-wave source including position-selective heating of fusion plasma, spectroscopy, materials processing, and drivers for ultra-high power amplifiers also motivate further theoretical and experimental investigations. In many cases, the time dependence of the electromagnetic field in gyro-oscillators shows nonharmonic or multi-frequency behavior. This can be a transient process during a start-up phase (which generally causes th operating frequency to change with time), during a short-pulse operation, or during mode competition. In the latter case, the modes interacting at different cyclotron harmonics and/or in different regimes (gyromonotron or gyro-BWO) at the same time often show quite different operating frequencies. This can also be a nonstationary regime caused by operating the device at parameters significantly exceeding the start oscillation condition. Such operation regime can occur not only when increasing one of the operating parameters, for example, the beam current, but also when keeping the operating parameters constant and decreasing the start oscillation condition parameters, for example, by introducing reflections of the output power back from the load into the interaction space. Hence, in order to accurately simulate nonstationary phenomena in gyro-oscillators, a time-dependent multimode code which is capable to self-consistently model the electromagnetic field within broad spectral bandwidth is necessary. In this work, the development of a self-consistent time-dependent analysis for gyro-oscillator simulation, which is believed to be as rigorous as the PIC code approach, but which is much more efficient especially in the case of an azimuthally non-homogeneous electromagnetic field has been presented. An accurate representation of the electromagnetic field is obtained by expanding the field components in terms of the solenoidal and the irrotational eigenfunctions of the equivalent completely closed cavity. The use of the eigenfunction expansion method reduces the boundary value problem for the field components to that of solving a linear system of ordinary differential equations (ODE) for the expansion coefficients. A convolution technique together with time domain analytic expressions for the characteristic admittances of the waveguide modes and for the reflection coefficients at the apertures of the cavity are applied to accurately formulate the time-dependent boundary conditions. The presented formulation is valid for describing a field having a broad frequency spectrum. The electron beam is represented by an ensemble of particles. The relativistic equations of motion associated with the particles are solved self-consistently with the ODE for the expansion coefficients by means of a multi-step integration scheme. After code validation, it has been used to attack a number of problems. First, it has been demonstrated that the developed time-dependent self-consistent multimode code is capable to model the interaction between the modes operating at the fundamental and/or at higher harmonics of the cyclotron frequency. The numerical results for the fundamental modes agree well with the experimental results published in [144]. Moreover, it is shown that the use of the surface impedance model [132] for the calculation of modes operating at harmonics higher than the fundamental leads to wrong results in high-power coaxial cavity gyromonotrons. Hence the use of this model must always be checked if the radiation wavelength is less than about 5 times the width of the corrugation grooves. The influence of reflections on the operation of an 1 MW, 140 GHz, TE22,6 mode gyromonotron developed at Forschungszentrum Karlsruhe has been studied by means of the described self-consistent time-dependent multimode analysis. It has been found that for the gyromonotron with operating parameters close to their optimum, the onset of an unstable operation regime occurs for reflection coefficients R ≥ 0.4, and the relative width of the spectrum of the output radiation increases up to 12%. The results of the numerical investigation on output power variation and frequency pulling due to a variation of the reflection parameters agree well with the available experimental data and demonstrate a strong dependence on the reflections. Moreover, our analysis of the gyromonotron operating in the presence of reflections has shown several new features of ist nonstationary behavior. First, it has been shown that the traditionally accepted scenario of the transition from the single-frequency stationary operation regime to the chaotic nonstationary regime when increasing the reflection coefficient is not correct. In contrast, an increase of the reflection coefficient above a certain threshold value (R ≥ 0.4) leads to the sequential excitation of a number of modes of the mismatched gyromonotron cavity. Then the gyromonotron shows a very complicated periodic or quasiperiodic behavior of a regular nature. No signs of chaotic behavior have been found. Second, for higher reflection coefficients (R ≥ 0.85), the main mode as well as most of the other modes of the mismatched cavity are suppressed by that mode which shows the highest eigenfrequency, and whose field profile shows its maximum in the vicinity of the left aperture. The eigenfrequency of this mode lies next to the cutoff frequency of the left aperture and limits the achievable bandwidth of the gyromonotron output radiation when one tries to increase it by means of reflections. Third, decreasing the distance to the reflecting load increases the frequency separation between the modes, what correspondingly reduces the number of excited modes and makes the spectrum of the gyromonotron output radiation less dense. Furthermore, the obtained results reveal important details of the saturated behavior of the gyro-BWO which have not been shown in the nonstationary analysis performed before. Although on one side, they agree with results obtained by using a stationary code [50] which employs a similar outgoing-wave boundary condition, and which also uses a self-consistent beam-field interaction model, on the other side, the results demonstrate the importance of timedependent calculations for a gyro-BWO stability analysis. Moreover, the results are in good agreement with recently available results obtained by another nonstationary code [51] as well as with the experiments [10]. Furthermore, it has been shown that tuning the external magnetic field leads to the excitation of the undesired taper mode which reduces the magnetic tuning bandwidth. Moreover, in an injection-locked gyro-BWO, significant modifications of the locking bandwidth curve have been observed when the magnetic field is tuned. The locking bandwidth curve which has been found in the simulations as well as its asymmetry agree well with the experiments [108].

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Languages : en
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A non-linear self-consistent computer simulation code is used to analyze the saturated output of the Gyrotron Backward Wave Oscillator (Gyro BWO) which can be used as a tunable driver for a 250 GHz FEL amplifier. Simulations show that the Gyrotron BWO using a Pierce/Wiggler gun configuration can produce at least 10 kW of microwave power over the range 249 GHz to 265 GHz by varying beam voltage alone.

Author: Craig Ross Donaldson
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Category :
Languages : en
Pages : 0

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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.

Author:
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Category :
Languages : en
Pages : 47

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Experimental and theoretical analyses, including particle-in-cell computer code simulations, are presented for the Gyrotron Backward Wave oscillator (Gyro-BWO) highpower microwave device. The Gyro-BWO has been designed, constructed, and initially tested as a frequency tunable device. The design has concentrated on a TE01, 4- to 6- GHz annular beam device. The annular beam is produced by the RAMBO pulser, which has a diode voltage of -300 to -800 kV and diode current of 1 to 20 kA. Initial results have shown that the device is operating in the backward wave mode. Initial results have also demonstrated that the device is magnetically tunable; that is, the frequency is tunable by adjusting the magnetic field. A Vlasov-type antenna is used for the extraction of the microwave signal on the diode end of the experiment, with initial power extraction of up to 2 MW ... High power microwave, HPM, Gyro-BWO, Gyrotron, Gyrotron backward wave oscillator.

Author: Ralph Bly Miller
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Category :
Languages : en
Pages : 74

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Author: David Samuel Tax
Publisher:
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Category :
Languages : en
Pages : 205

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The gyrotron is a source capable of producing megawatt power levels at millimeter-wave frequencies for many important applications, including electron cyclotron heating and current drive in magnetic fusion devices. It is important that the gyrotron operates with high efficiency and provides a high quality output beam to minimize system size, maximize reliability and avoid additional losses in external systems. This thesis presents the experimental study of such a gyrotron designed to operate at MW power levels and whose initial 110 GHz operation was expanded to include operation at 124.5 GHz. To this end, a new set of components, including a cavity, mode converter, and output window were designed for operation at both frequencies. The cavity was designed using the code MAGY and the Q factors of 830 for the TE22,6,1mode at 110 GHz and 1060 for the TE24,7,1 mode at 124.5 GHz would be suitable for CW operation in an industrial gyrotron. The mode converter consisting of a dimpled-wall launcher and 4 phasecorrecting mirrors could theoretically produce an output beam with 99 % Gaussian beam content at each frequency while a single-disc window was implemented with over 99.5 % power transmission at both frequencies. The achieved output power in experiment was 1.1 MW at 110 GHz and 850 kW at 124.5 GHz for the design parameters of 96 kV and 40 A. At 98 kV and 42 A, the gyrotron achieved 1.25 MW and 1 MW at 110 and 124.5 GHz, respectively. Mode competition is typically a major limitation in such gyrotrons, and stable single-mode operation was demonstrated at both frequencies. At 110 GHz, the output beam had 98.8 % Gaussian beam content, while at 124.5 GHz, the output beam quality was 94.4 %. Another experiment within this thesis demonstrated the implementation of a mode converter with smooth mirrors that would be less susceptible to machining and misalignment errors. A Gaussian beam content of 96 % was measured in that experiment. In addition, a thorough study of the gyrotron start-up scenario was performed, for which experimental work had been lacking in the literature. The start-up scenario is the sequence of modes that are excited during the rise of the voltage pulse and is essential for the gyrotron to operate in its most efficient regime known as the hard self-excitation regime. This gyrotron operates nominally in the TE22,6,1 mode near the 110 GHz cutoff frequency with an axial field profile that is approximately Gaussian at the steady-state peak voltage. In experiments performed in the smooth mirror mode converter configuration, lower frequency modes were observed at lower voltages as opposed to higher frequency modes as predicted by theory. Analysis of these modes showed that they are backward-wave modes far from their cutoff frequency which have higher order axial field profiles, i.e. TE21,6,3 and TE21,6,4 modes at frequencies of 108-109 GHz. The excitation of these modes was investigated and shown to be possible by using theory and single-mode simulations with the code MAGY. This discovery was important as these modes were not included in past code runs, and thus future improvements can be made to incorporate this effect.