Dielectric-lined Waveguide Research Articles

Excitation of microwave by an annular electron beam in a plasma-filled dielectric lined waveguide

An axial relativistic electron beam passing through a slow wave structure is unstable to an electromagnetic perturbation whose phase velocity equals the velocity of the beam. This phenomenon of Cherenkov emission is the basis of all traveling wave tubes. In this paper an excitation of Cherenkov radiation by a thin annular relativistic electron beam in a plasma-filled dielectric-lined waveguide is analysed by use of the self-consistent linear theory. The effect of the thin annular electron beam on the beam-wave interaction is completely described by a jump condition. The dispersion equation and the simultaneous condition of the beam-wave interaction are derived. Finally, the growth rate of the wave is obtained, and the effect of the background plasma density and the electron beam radius on the growth rate of the wave are presented.

High-power multi-electron-beam Cherenkov free-electron lasers at mm wavelengths

A multi-electron-beam Cherenkov free-electron laser can use a multi-dielectric waveguide to generate high-power coherent radiation at mm wavelength. Presented is a mode analysis of the device, allowing a detailed description of the dispersion relations and field distributions of the waveguide for the important modes and the start current for each mode. The microwave loss of the waveguide and the mode competition problem are also discussed. The device is shown capable of producing radiation at millimeter wavelengths with power an order higher than that uses a dielectric-lined circular waveguide.

Optical propagation of the HE11 mode and Gaussian beams in hollow circular waveguides

The propagation of the HE11 mode and Gaussian beams in hollow oversized circular waveguides is analyzed using optical theories. Different types of waveguides are considered : hollow dielectric or conducting waveguides, dielectric-lined waveguides, corrugated waveguides. General formulas are derived which give the power transmission through these different guides. The best wall materials and structures are determined from a comparison of the waveguide transmissions, at the infrared and millimeter wavelengths. The question of the coupling between the HE11 mode and Gaussian beams is discussed and from a review of coupling coefficients derived before, an optimum value is pointed out. The problem of matching a Gaussian beam into circular waveguides in order to achieve the maximum power transmission is analyzed. These results are of interest for infrared lasers or waveguide applications and for Electron Cyclotron Wave (ECW) systems at the millimeter wavelength.

Stimulated Cherenkov emission in dielectric-lined waveguides.

Particle-in-cell simulations of a dielectric Cherenkov maser experiment are used to analyze the emissive characteristics of an annular relativistic beam in a dielectric-lined waveguide. Good agreement with recent experimental results is obtained. The large gain in the device follows linear theory until saturation, when particle bunching and trapping is observed. The beam density becomes highly modulated with pulse length \ensuremath{\sim}\ensuremath{\lambda}/2, where \ensuremath{\lambda} is the guide wavelength. The performance of the device in the high-current regime is highly nonlinear. A simple nonlinear theory, confirmed by numerical results, shows that the field saturates at a value corresponding to ${\mathrm{\ensuremath{\omega}}}_{\mathit{b}}$=\ensuremath{\Delta}\ensuremath{\omega}/2, where ${\mathrm{\ensuremath{\omega}}}_{\mathit{b}}$ is the electron bounce frequency in the wave and \ensuremath{\Delta}\ensuremath{\omega} is the detuning parameter.

A frequency-locked, high power, X-band dielectric Cerenkov maser

A high-power frequency-locked dielectric Cerenkov maser oscillator is presented. The device consists of an annular electron beam traveling down a dielectric-lined waveguide (wall radius 1.74 cm, liner thickness 2-3 mm, linear in =10). A 100 kW input signal is injected into the drift tube between the diode and liner, through a rectangular TE/sub 10/ to cylindrical TM/sub 01/ mode converter. When operated with a beam current of 500 A and a total output power of 13 MW, the device displays no RF quenching through the full width of the 100 ns beam pulse. Two higher current cases with power as high as 280 MW but severe RF quenching are presented. The results are compared with linear theory, and quenching mechanisms are discussed. >

X-band dielectric Cerenkov maser amplifier experiment

An X-band dielectric Cerenkov maser amplifier experiment is reported. The amplifier system consisted of a solid, thermionically generated electron beam propagating through a cylindrical waveguide partially filled with an annular, dielectric liner. The input signal was provided by a tunable (9-10.3 GHz) magnetron with power up to 10 kW. Electron beam voltages and currents of up to 250 kV and 100 A could be generated for 1 mu s pulse durations. The system was configured to operate in the TM/sub 01/ mode of the dielectric-lined waveguide. In this experiment the gain of the system with respect to the length of the dielectric liner was studied at a fixed input frequency of 10.3 GHz. At electron beam parameters of 160 kV and 60 A, a power gain of 24 dB over 56 cm of interaction length was measured for an input power of 4.5 kW, corresponding to a maximum RF amplified power of 1.15 MW and 12% efficiency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear theory of slow-wave ubitrons/free electron lasers

A three-dimensional nonlinear formulation of a slow-wave ubitron/free electron laser is presented. The configuration is that of an electron beam propagating through a dielectric-lined rectangular waveguide in the presence of a planar wiggler field. The wiggler field model describes parabolic pole faces for enhanced beam focussing. The electromagnetic field is described in terms of a multimode ensemble of the normal modes of the vacuum waveguide. In this case, the dielectric liner is imposed along the y-axis of the guide (which is also the principal orientation of the wiggler) and there are no orthogonal TE or TM modes. Instead, we deal with modes which are transverse electric (LSE) or magnetic (LSM) to the y-axis. Equations are derived which describe the evolution of the amplitude and phase of these modes in terms of the microscopic particle currents. These equations are solved in conjunction with the complete Lorentz force equations for the electron trajectories. The essential purpose of employing a slow-wave structure is to reach shorter wavelengths at relatively low beam energies, and numerical results will be presented to demonstrate the potentialities of this concept.

High efficiency operation of Cherenkov masers

A nonlinear simulation of the Cherenkov maser amplifier is presented for a configuration in which an electron beam propagates through a dielectric-lined cylindrical waveguide. The parameters used correspond to an experiment at General Dynamics which measured a total efficiency of 11.5% at 8.6 GHz. The simulation is in agreement with this but indicates that the system was too short to reach saturation and that an efficiency of 30% would have been possible for a longer system, and the performance is not significantly degraded by thermal spreads up to 20%.

Cerenkov free-electron laser with a dielectric-lined double-slab waveguide

The dispersion relations of dielectric-lined double-slab waveguides used in Cerenkov free-electron lasers are derived and the existence of a group of odd modes, which can be simultaneously excited with the previously considered even modes, is reported. A discussion on the dependence of the radiation frequency and single-pass gain on the system parameters, the dielectric thickness, the beam energy and the beam channel width, is presented. The effects of the odd modes on the Cerenkov free-electron laser operation are investigated.

Nonlinear analysis of high-power Cherenkov masers.

A nonlinear simulation of the Cherenkov maser amplifier is presented for a configuration in which an electron beam propagates through a dielectric-lined cylindrical waveguide. The parameters used correspond to an experiment at General Dynamics which measured a total efficiency of 11.5% at 8.6 GHz. The simulation is in agreement with this but indicates that the system was too short to reach saturation and that an efficiency of 30% would have been possible for a longer system, and the performance is not significantly degraded by thermal spreads up to 20%.

Nonlinear analysis of the Cerenkov maser

The nonlinear evolution of the Cerenkov maser amplifier is investigated numerically for a configuration that consists of an energetic electron beam propagating through a dielectric-lined cylindrical waveguide. An axial guide magnetic field is included in the formulation in order to improve beam confinement. A set of coupled nonlinear differential equations is derived in three dimensions that governs the evolution of both the electromagnetic field and the trajectories of an ensemble of electrons. The system is assumed to be azimuthally symmetric, and the electromagnetic field is represented as a superposition of the TM0n modes of the vacuum waveguide. The initial conditions are chosen to model the simultaneous injection of either a solid or annular electron beam, and an electromagnetic wave of arbitrary input power. Thermal effects are treated under the assumption that the beam is initially monoenergetic but exhibits a pitch angle spread; however, the subsequent evolution of the beam is treated in a self-consistent manner. This class of distribution is appropriate to the treatment of diode-produced beams and describes a beam with an initial axial energy spread. This is the crucial determinant in the efficiency, since saturation occurs by means of an axial bunching mechanism that results in the phase trapping of the beam. The specific parameters used in the numerical analysis correspond to experiments conducted at Dartmouth College [J. Appl. Phys. 58, 627 (1985)], and good agreement is found between theory and experiment.

Three-dimensional nonlinear analysis of the Cherenkov Maser

The nonlinear evolution of the Cherenkov Maser amplifier is investigated numerically for a configuration which consists of an energetic electron beam propagating through a dielectric-lined cylindrical waveguide. An axial guide magnetic field is included in the formulation in order to improve beam confinement. A set of coupled nonlinear differential equations is derived in three dimensions which governs the evolution of both the electromagnetic field and the trajectories of an ensemble of electrons. The system is assumed to be azimuthally symmetric, and the electromagnetic field is represented as a superposition of the TM0n modes of the vacuum waveguide. The initial conditions are chosen to model the simultaneous injection of the electron beam, which may have either a solid or annular cross section, and an electromagnetic wave of arbitrary input power. In order to simplify the analysis, thermal effects are treated under the assumption that the beam is initially monoenergetic but exhibits a pitch-abgle spread. Although the beam is initially monoenergetic, the subsequent evolution of the beam is treated in a self-consistent manner. The specific parameters used in the numerical solution of the coupled wave-particle equations correspond to experiments conducted at Dartmouth College, NH, USA, and good agreement is found between the results of the simulation and the experimental observations.

Coaxial configuration of the dielectric Cherenkov maser

The linearized Lorentz force, continuity equation, and Maxwell's equations are used to calculate the system dispersion relation for a coaxial configuration of the dielectric Cherenkov maser. The system consists of two coaxial conductors lined with dielectric and an annular relativistic electron beam, which propagates between the two liners. The dispersion relation for the beam and dielectric-lined coaxial waveguide structure and the no-beam system that describes the dependence of the generated frequency on the coaxial waveguide parameters are presented. Using the linearized dispersion relation, the growth rate for the beam-TM/sub 0n/ waveguide mode instability is calculated in the strong-coupling tenuous beam limit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Plasma-filled dielectric Cherenkov maser

The effect of a background plasma on a dielectric Cherenkov maser is examined by solving the linearized, beam-plasma, dielectric-lined waveguide dispersion relation. The results indicate that introduction of the background plasma can produce a higher spatial growth rate for the beam-waveguide instability and can reduce the electric field at the dielectric surface when compared to the system with no background plasma present. It is also found that for some sets of waveguide parameters, the TM/sub 0n/ electromagnetic modes can propagate frequencies that are below the background plasma frequency in the system. These modes have a finite off frequency and are different from the Trivelpiece-Gould modes for the system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A slow wave free-electron laser

A simple calculation of a free-electron laser in the Compton regime that uses a dielectric-lined waveguide is presented. The introduction of a dielectric lining in a free-electron laser considerably reduces the requirements on beam voltage for generating a given frequency omega /sub 1/=k/sub 0/c/(1 - v/sub b/ eta /c), where k/sub 0/ is the wiggler wave period eta is the effective index of refraction (1 >

HYBRID MODE HORN ANTENNAS

ABSTRACT Hybrid mode horns are of considerable practical importance, especially as feeds for lens and reflector antennas. In this tutorial review the boundary conditions necessary to support hybrid modes in round waveguide are derived. Expressions for the field components in HE1n and EH1n modes are given and from these the transverse field over the open mouth of the guide is found. The Fourier transform of this field is then used to calculate far field radiation patterns. Solutions to the characteristic equation for hybrid modes are indicated and the dispersion characteristics for balanced modes are presented graphically. Finally, a brief discussion of dielectric-lined waveguide is given and asymptotic solutions are developed to show that nearly balanced HE11 and EH11 modes can exist over a limited frequency range when the dielectric is lossless, but over an unlimited range when the dielectric is highly lossy.

Dc Electric-Field Enhancement of Cherenkov-Radiation Output Power

The Cherenkov radiation in dielectric-lined waveguide with transverse magnetic (TM) mode coupling has been investigated by using a newly developed relativistic electromagnetic particle model. The parameters used in the simulation are relevant to a recent laboratory experiments. With a 12-A and 220-kV electron beams, the results show that the excited wave is at ω≈ 26 GHz with 310-kW output power. The increase in electron momentum spread is observed to reduce the tightness of electron bunches and to enhance the energy spread of the electrons trapped in the potential well, which results in smaller gain per unit length and reduced nonlinear efficiency. It has also been demonstrated by computer simulations that the output power can be improved by an order of magnitude if in an amplifier configuration the interaction length without phase focusing is doubled and an axial dc electric field (30V/cm) is imposed right before the electron bunches reach the bottom of the potential well.

The excitation of microwaves by a relativistic electron beam in a dielectric-lined waveguide

The dispersive properties of a relativistic electron beam in a dielectric-loaded waveguide are investigated. The linearized fluid and Maxwell’s equations are used to derive the dispersion relation and to study the eigenmodes for the system. The Čerenkov microwave radiation mechanism is investigated.

High-power Cerenkov maser oscillator

An electron beam driven dielectric-lined waveguide has produced 30–100 kW of coherent radiation over an octave band on the TM01 mode of the waveguide. Operation of the TM02 mode has been realized as well. Impedance mismatches at the ends of the liner section provide a reflected signal, which undergoes amplification during successive passes.

Propagation in a dielectric-lined circular waveguide

A model for dielectric-lined circular waveguide is developed for use in a feasibility study of trunk waveguide communication systems. The model is designed so that a rapid calculation of the propagation coefficients may be made over a wide range of typical waveguide parameters using an interactive minicomputer program. The theory, which is more general than previous theories for dielectric-lined circular waveguides, is derived making use of complex cutoff and propagation coefficients, and may be applied to most circularly symmetric problems having two concentric dielectric materials. An error analysis of the model confirms its usefulness in waveguide-system studies.