Plasma Dynamics

Contains reports on six research projects.National Science Foundation (Grant ENG79-07047)U.S. Air Force - Office of Scientific Research (Grant AFOSR77-3143D)U.S. Air Force - Office of Scientific Research (Contract AFOSR82-0063)U.S. Department of Energy (Contract DE-ACO2-78-ET-51013)U.S. Department of Energy (Contract DE-AC02-78ET-53073.A002

Two-dimensional periodic surface lattice high power millimetre wave experiment

Millimetre wave radiation generation from a free electron maser based on a two-dimensional (2D) periodic surface lattice has been demonstrated. Second harmonic Gyrotron Backward Wave Oscillator (BWO) interaction with a two dimensional periodic surface lattice (PSL) has been observed. The major achievements in the thesis are the 1) design, 2) simulation, 3) construction and 4) operation of the experiment.;Two different methods were used to manufacture two different types of 2D periodic surface lattice. The first method used 1) electrochemcial deposition of copper on an aluminium former with the alumium subsequently removed by dissolving in strong alkali solution. The second method used a 2) 3D additive manufacturing technique resulting in a silver 2D PSL.;For construction method 1): four different copper 2D PSLs with the same inner diameter of 20 mm and the same number of longitudinal corrugations (7), period (3.0 mm) and azimuthal variations (20) but with different peak to peak amplitudes of 0.5 mm; 0.6 mm; 1 mm and 1.6 mm were manufactured. A W-band (75GHz to 110GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the 20 mm inner diameter copper 2D PSLs in order to investigate the effect of the amplitude of the perturbations (0.5 mm; 0.6 mm; 1 mm; 1.6 mm) has on the coupling of the surface and volume fields.;By comparing the measured transmission of millimetre waves through the four copper 2D PSLs with different amplitude of corrugations it was shown that an increase in the peak-to-peak perturbation amplitude resulted in excitation of surface fields by an obliquely incident wave and the excitation of an eigenmode of the PSL.;For the 3D additive construction method 2): a silver high contrast structure defined as having a peak-to-peak amplitude of corrugation of 1.6 mm (from top to bottom of the grating) that is larger than a quarter of the operating wavelength was manufactured. A G-band (140GHz to 220GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the silver 2D PSL of 7.2 mm inner diameter having 16 longitudinal periods each period of length 1.6 mm having a perturbation amplitude of 1.6 mm (peak to peak) and an azimuthal period of 3.5mm. In VNA millimetre wave measurements a resonance of the 1st harmonic at 171.1 GHz due to electromagnetic wave interaction with the 3.5 mm azimuthal period which corresponded to a W-band resonance at zero axial wavenumbers of ~85 GHz was observed.;Theoretical, numerical and experimental investigation of a proof of principle 2nd harmonic gyrotron BWO based on the silver 2D PSL was carried out. An electron gun that used a velvet cathode was designed and constructed. Experiments were conducted using the velvet cathode electron gun with the electron accelerating voltage produced by a cable Blumlein generator. The electron beam formed and transported through the 7.2 mm inner diameter silver 2D PSL beam-wave interaction region within an 18 mm bore 1.8 T solenoid was measured.;An 80 kV, 100 A electron beam with a beam outer diameter of 4 mm and inner diameter of 2mm which was approximately 1.8 mm away from the inner surface of the 2D PSL corrugation was measured. Numerical simulations predicted an electron beam of longitudinal velocity of 0.46c which excited an electromagnetic wave on the 2D PSL with a longitudinal (period 1.6 mm) and azimuthal (3.5 mm period) corrugations. Propagating the electron beam through the 2D PSL a possible 2nd harmonic gyrotron BWO was identified at a frequency of ~80GHz from measurements of the frequency using high pass cut off filters and the mode pattern as compared to numerical simulation and the electron beam wave dispersion calculations.;Millimetre wave radiation at a frequency of ~80GHz at an output power of 134 ± 5 kW corresponding to an operating efficiency of ~1.7 % was measured. Measurements of the frequency and mode pattern indicate that individual scatterers from the 2D PSL may have been synchronized by the lattice resulting in different parts of the electron beam interacting with the coherent Surface Field (SF) via the 2nd harmonic gyrotron Backward Wave Oscillator instability. Evidence indicates possible electron beam excitation of a cavity eigenmode consisting of the superposition between a TM0,2 incident Volume Field (VF) and a structurally induced TE5,1 Surface Field (SF).;Alternative interactions include a 2nd harmonic gyrotron Backward Wave Oscillator with a pure TE5.1 mode, 2nd harmonic gyrotron Backward Wave Oscillator with a pure TM0,2 mode (~75GHz) and a BWO interaction operating with the first spatial harmonic of the TM0,2 volume mode (~75GHz) or a combination of all four. For for a high contrast periodic surface lattice (corrugation depth of 1.6 mm is greater than λ/4) a hybrid EH type mode can be excited within the structure. To conclusively prove experimentally which interaction is dominating a more accurate 1) frequency measuremnet using a heterodyne frequency diagnostic and 2) mode pattern measurement achieved by automating the scan are required and is part of the future work.;The results of the experiment have been used to inform the design of a Cherenkov Maser which couples a TM0 ,6 volume field with a HE16,1 surface field on a 20 mm diameter 2D PSL structure with 20 longitudinal corrugations of period 3.2 mm and 16 azimuthal variations of peak to peak amplitude of 1 mm. The beam-wave interaction in the 20 mm inner diameter 2D PSL was modelled using the particle in cell code Magic 3D. In the simulations an axial magnetic field of 6 T was used to guide a 1.6 kA, 300 kV annular electron beam with a mean radius of 8.5 mm and beam thickness of 1 mm through the 2D PSL where the beam was located 0.5 mm away from the inner diameter of the 2DPSL. Numerical simulations predict excitation of the 2D PSL by this electron beam will generate 150MW of power at an operating frequency of 97 GHz corresponding to an electronic efficiency of 30 %.Millimetre wave radiation generation from a free electron maser based on a two-dimensional (2D) periodic surface lattice has been demonstrated. Second harmonic Gyrotron Backward Wave Oscillator (BWO) interaction with a two dimensional periodic surface lattice (PSL) has been observed. The major achievements in the thesis are the 1) design, 2) simulation, 3) construction and 4) operation of the experiment.;Two different methods were used to manufacture two different types of 2D periodic surface lattice. The first method used 1) electrochemcial deposition of copper on an aluminium former with the alumium subsequently removed by dissolving in strong alkali solution. The second method used a 2) 3D additive manufacturing technique resulting in a silver 2D PSL.;For construction method 1): four different copper 2D PSLs with the same inner diameter of 20 mm and the same number of longitudinal corrugations (7), period (3.0 mm) and azimuthal variations (20) but with different peak to peak amplitudes of 0.5 mm; 0.6 mm; 1 mm and 1.6 mm were manufactured. A W-band (75GHz to 110GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the 20 mm inner diameter copper 2D PSLs in order to investigate the effect of the amplitude of the perturbations (0.5 mm; 0.6 mm; 1 mm; 1.6 mm) has on the coupling of the surface and volume fields.;By comparing the measured transmission of millimetre waves through the four copper 2D PSLs with different amplitude of corrugations it was shown that an increase in the peak-to-peak perturbation amplitude resulted in excitation of surface fields by an obliquely incident wave and the excitation of an eigenmode of the PSL.;For the 3D additive construction method 2): a silver high contrast structure defined as having a peak-to-peak amplitude of corrugation of 1.6 mm (from top to bottom of the grating) that is larger than a quarter of the operating wavelength was manufactured. A G-band (140GHz to 220GHz) Vector Network Analyser was used to measure the transmission of millimetre waves through the silver 2D PSL of 7.2 mm inner diameter having 16 longitudinal periods each period of length 1.6 mm having a perturbation amplitude of 1.6 mm (peak to peak) and an azimuthal period of 3.5mm. In VNA millimetre wave measurements a resonance of the 1st harmonic at 171.1 GHz due to electromagnetic wave interaction with the 3.5 mm azimuthal period which corresponded to a W-band resonance at zero axial wavenumbers of ~85 GHz was observed.;Theoretical, numerical and experimental investigation of a proof of principle 2nd harmonic gyrotron BWO based on the silver 2D PSL was carried out. An electron gun that used a velvet cathode was designed and constructed. Experiments were conducted using the velvet cathode electron gun with the electron accelerating voltage produced by a cable Blumlein generator. The electron beam formed and transported through the 7.2 mm inner diameter silver 2D PSL beam-wave interaction region within an 18 mm bore 1.8 T solenoid was measured.;An 80 kV, 100 A electron beam with a beam outer diameter of 4 mm and inner diameter of 2mm which was approximately 1.8 mm away from the inner surface of the 2D PSL corrugation was measured. Numerical simulations predicted an electron beam of longitudinal velocity of 0.46c which excited an electromagnetic wave on the 2D PSL with a longitudinal (period 1.6 mm) and azimuthal (3.5 mm period) corrugations. Propagating the electron beam through the 2D PSL a possible 2nd harmonic gyrotron BWO was identified at a frequency of ~80GHz from measurements of the frequency using high pass cut off filters and the mode pattern as compared to numerical simulation and the electron beam wave dispersion calculations.;Millimetre wave radiation at a frequency of ~80GHz at an output power of 134 ± 5 kW corresponding to an operating efficiency of ~1.7 % was measured. Measurements of the frequency and mode pattern indicate that individual scatterers from the 2D PSL may have been synchronized by the lattice resulting in different parts of the electron beam interacting with the coherent Surface Field (SF) via the 2nd harmonic gyrotron Backward Wave Oscillator instability. Evidence indicates possible electron beam excitation of a cavity eigenmode consisting of the superposition between a TM0,2 incident Volume Field (VF) and a structurally induced TE5,1 Surface Field (SF).;Alternative interactions include a 2nd harmonic gyrotron Backward Wave Oscillator with a pure TE5.1 mode, 2nd harmonic gyrotron Backward Wave Oscillator with a pure TM0,2 mode (~75GHz) and a BWO interaction operating with the first spatial harmonic of the TM0,2 volume mode (~75GHz) or a combination of all four. For for a high contrast periodic surface lattice (corrugation depth of 1.6 mm is greater than λ/4) a hybrid EH type mode can be excited within the structure. To conclusively prove experimentally which interaction is dominating a more accurate 1) frequency measuremnet using a heterodyne frequency diagnostic and 2) mode pattern measurement achieved by automating the scan are required and is part of the future work.;The results of the experiment have been used to inform the design of a Cherenkov Maser which couples a TM0 ,6 volume field with a HE16,1 surface field on a 20 mm diameter 2D PSL structure with 20 longitudinal corrugations of period 3.2 mm and 16 azimuthal variations of peak to peak amplitude of 1 mm. The beam-wave interaction in the 20 mm inner diameter 2D PSL was modelled using the particle in cell code Magic 3D. In the simulations an axial magnetic field of 6 T was used to guide a 1.6 kA, 300 kV annular electron beam with a mean radius of 8.5 mm and beam thickness of 1 mm through the 2D PSL where the beam was located 0.5 mm away from the inner diameter of the 2DPSL. Numerical simulations predict excitation of the 2D PSL by this electron beam will generate 150MW of power at an operating frequency of 97 GHz corresponding to an electronic efficiency of 30 %

Introduction to the Physics of Gyrotrons

As unique sources of coherent high-power, microwave, and millimeter-wave radiation, gyrotrons are an essential part of the hunt for controlled fusion. Presently, gyrotrons are actively used for electron cyclotron resonance plasma heating and current drive in various controlled fusion reactors. These sources have been under development in many countries for more than forty years. In spite of their widespread use, however, there is as yet no single book to introduce non-specialists to this vital field.Now Gregory S. Nusinovich, an early pioneer of the gyrotron and widely regarded today as the world's leading authority on the subject, explains the fundamental physical principles upon which gyrotrons and related devices operate. Nusinovich first sets forth some "rules of thumb" that allow readers to understand gyrotron operation in simple terms. He then explores the fundamentals of the general theory of gyrotrons and offers an overview of the various types of gyro-devices, including gyromonotrons, gyroklystrons, gyro-traveling-wave tubes, and gyrotwystrons. He explains not only the theory, linear and nonlinear, but also the practical challenges that users of such devices face. This book will be of interest to undergraduate and graduate students as well as to those who develop gyrotrons or who use them in various applications. It should also appeal to plasma physicists interested in charged-particle dynamics, as well as to applied physicists needing to know more about micro- and millimeter-wave technologies

Maser-beam instability of Bernstein waves

The present study constitutes a continuation and improvement of the preceding work by Yoon et al. [J. Geophys. Res. 104, 19801 (1999)]. In the present discussion, an instability of Bernstein waves excited by a beam of energetic electrons is investigated. Special attention is paid to the regime where the ratio of plasma frequency, vpe , to electron gyrofrequency, Ve , is sufficiently higher than unity. An approximate but fairly accurate scheme is introduced to deal with the situation dictated by the condition, vpe 2 /Ve 2e1. The present investigation is motivated by the research in solar radiophysics. However, in this article the emphasis is placed on basic properties of the instability rather than its application

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

Dissipation and amplification of waves in space and laboratory plasmas

This thesis is concerned with theoretical investigations into the dissipation and amplification of waves in laboratory and space plasmas. Three specific examples of current interest are examined. The first is the dissipation of a magnetosonic wave by resonant mode conversion into a shear Alfven wave and the subsequent collisionless damping of the shear Alfven wave by resonant particle interactions. The other two involve the amplification of electromagnetic and whist— ler waves by electron cyclotron instabilities

The numerical and experimental investigation of gyro-multiplier configurations

This thesis examines the feasibility of two different configurations of gyromultiplier, both of which operate at the fourth harmonic of the electron cyclotron frequency. The full numerical modelling and design of components for the testing of a novel, single cavity gyro-multiplier experiment has been documented. In addition, numerical simulations of a configuration featuring three distinct cavity sections have also been conducted. The introduction of an eight-fold azimuthal corrugation into the walls of a cylindrical cavity allows for the realisation of a single cavity gyro-multiplier arrangement, with generation of 2nd harmonic, TE2,2, and 4th harmonic, TE4,3, resonances, at frequencies of 37.5 GHz and 75 GHz, respectively. The interaction region is of mean radius, 8 mm, with a corrugation depth, 0.7 mm, and is 39 mm in length. The idealised electron beam utilised is of voltage, 60 kV, with current between 5 A and 10 A, confined in a magnetic field of ~0.7 T. Separation of the two emission frequencies was intended through the use of a 6 mm length cut-off taper; however, mode conversion to two above cut-off modes has been numerically demonstrated. The power contained in the 4th harmonic has been estimated at ~10-50 W. Extension of the output taper has proven to be sufficient to reduce the mode converted signals by an order of magnitude, while not impinging on the propagation of the 4th harmonic signal. The design and simulation of a knife-edge electron gun and kicker system has also been performed, with the final beam predicted to have a velocity spread of ~19%. In order to demonstrate the “cold” response of the interaction region to the 2nd harmonic signal, the design, construction and testing of several additional components are also documented. Novel slotted wall mode converters, capable of generating TEm,1 modes from a rectangular TE1,0 input signal, have demonstrated high spectral purity and large, ~10% bandwidth. A set of TE2,1 launchers, of 3.98 mm radius, operating between 37-41 GHz have demonstrated ~56% conversion efficiency, while a similar set for the TE4,1 mode, of 3.78 mm radius, demonstrated 20% conversion efficiency, between 70-80 GHz. A set of ripple wall mode converters, of maximum radius, 8.7 mm, featuring a 20 period, axial sinusoidal ripple, of depth 0.30 mm, designed to convert the TE2,1 mode to a TE2,2, have also been demonstrated. These converters display ~20 MHz bandwidth, at ~38 GHz. Using these couplers demonstrated the corrugated interaction region dispersion was insensitive to the polarisation of incident quadrupole modes, in keeping with theory. By examining a gyro-multiplier setup with three distinct cavity sections, it has been demonstrated that by operating the first and third cavities at the fundamental harmonic, effective generation of a 4th harmonic signal can be realised from a second cavity of radius slightly larger than that of the initial cavity. The interaction regions examined were of radius 0.7 mm, 0.783 mm, and 1.5 mm, and of lengths 2.4 mm, 2.4mm and 3.6 mm, respectively. By using an idealised electron beam of voltage, 80 kV, beam current of 0.7 A, and pitch factor of 1.4, generation of the TE1,2 and TE1,3 modes at a fundamental frequency of 342.5 GHz, and 4th harmonic, polarised in the TE4,6 mode at a frequency of 1.37 THz has been predicted, for a modest confining magnetic field of ~14.15 T. Although sensitive to the magnitude of the applied field, the maximum power contained in the 4th harmonic signal has been estimated to be 120 W

DEVELOPMENT OF A FOUR CAVITY SECOND-HARMONIC GYROKLYSTRON AS DRIVER FOR A LINEAR ACCELERATOR

Gyroklystrons are microwave amplifiers that combine the multi-cavity configuration of a klystron with the energy extraction mechanism of the cyclotron maser instability. These devices have been studied at the University of Maryland for several years. This work is focused on the development of a 17.14 GHz four-cavity frequency-doubling gyroklystron circuit. This device was designed specifically to drive a high gradient linear accelerator recently developed by the Haimson Corporation. The gyroklystron was designed using the code MAGYKL, yielding a predicted output power of 87 MW for an input drive power of 250 W, with a velocity pitch ratio (alpha) of 1.4. The tube was later fabricated, and underwent a series of experimental tests to evaluate its performance. The highest peak power observed was 18.5 ± 1.7 MW, corresponding to an efficiency of 7.0 % and a gain of 24.0 dB. This result fell short of the theoretical design, yet it was consistent with the low value of the velocity pitch ratio (alpha=0.85) realized in the experiments. This limitation on alpha was linked to the onset of instabilities in the input cavity. The ultimate cause of these instabilities was the thermal non-uniformity in the emitter of our electron gun, which led to a significant variation (approximately 50 %) of the current density across the beam. In order to remedy this problem, we have radically redesigned the input cavity, changing both its geometry and Q factor. These measures should dramatically reduce the probability of instabilities, thus allowing us to remove the experimental limitations imposed on alpha. This new design is presented here. We also describe advanced designs of an output cavity with radial power extraction, and a compact circular to rectangular mode converter. A detailed description of the present experimental setup is given, along with an overview of the power transport system necessary to feed the accelerator with output power from the gyroklystron