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5077 results in Electronic, optoelectronic devices, and nanotechnology

Page 67 of 254

  • 5 - Thermionic Diodes

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 190-223

  • Summary

    The basis of nearly all types of vacuum tube is the diode in which the electrons emitted by thermionic emission from a cathode are attracted to a positively charged anode. The current through the diode increases as the emitted current increases until it is limited by the negative space-charge of the electrons. The properties of planar, cylindrical and spherical space-charge limited diodes are discussed including the effects of relativity and of the initial thermal velocities of the electrons. When the voltage applied to the anode varies rapidly with time a static solution is no longer valid and transit-time effects must be considered. The properties of diodes in which the electrons are injected with a finite velocity and those in which the electron flow is two-dimensional are reviewed.

  • 15 - Magnetrons

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 565-628

  • Summary

    In a magnetron oscillator electrons emitted from a cylindrical cathode interact with the π-mode resonance of a cylindrical, re-entrant, slow-wave structure on the anode. The electrons move under the influence of a radial static electric field and an axial magnetic field. Rotating spokes of charge synchronous with the wave on the anode are formed on the surface of a space-charge hub. As the electrons drift outwards along the spokes the change in potential energy is converted to r.f. energy with high efficiency. Because a magnetron is an oscillator its steady-state operation is always non-linear. Care must be taken in the design of the anode and in the operation of the tube to avoid excitation of modes other than the π-mode. The design of the anode is considered in detail. The performance of magnetrons is reviewed including the effects of the external match on frequency, power output and efficiency. Useful understanding of the properties of a magnetron can be gained from a model which assumes a fixed hub, determined by the theory of the cut-off magnetron diode, and rigid spokes. This model reproduces all the main features of the performance of a magnetron.

  • 17 - Fast-Wave Devices

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 659-693

  • Summary

    Fast-wave devices employ interactions between modes of propagation in a smooth waveguide whose phase velocity is greater than the velocity of light and electron beams which are periodic so that synchronous interaction is possible. The most important type of tube is the gyrotron (electron cyclotron maser) in which the electrons moving in orbits under the influence of a longitudinal magnetic field interact with a transverse electric (TE) waveguide mode. The small-signal theory of gyrotrons reveals the synchronous conditions for the gyrotron oscillator, the gyro-klystron, -TWT and -BWO and the cyclotron auto-resonance maser (CARM). A simple non-linear model is described which can reproduce the main features of the performance of a gyrotron oscillator. The principles of the design of gyrotron oscillators and amplifiers are reviewed. There is a brief discussion of the principles of operation of the peniotron and the ubitron (free electron laser).

  • 9 - Electron Guns

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 317-351

  • Summary

    A linear electron beam is formed by an electron gun which is a diode in which the electrons pass through an aperture in the anode without interception. The Pierce gun for generating a cylindrical beam is based on a spherical space-charge limited diode. A non-emitting focusing electrode, which is a continuation of the cathode, and the anode nose are so shaped that the electrons flow in a convergent cone. The angle of convergence is reduced by the lens action of the anode aperture and by space-charge forces. The design of Pierce guns is discussed in detail including the effects of thermal velocities and the shaping of the magnetic field as the beam enters a magnetic focusing system. Pierce guns can also be designed for hollow beams and sheet beams. The current in the beam can be controlled by a grid or a modulating anode. In a Kino gun a planar, cut-off, magnetron diode is used for injecting a sheet beam into a crossed-field focusing system. The magnetron injection gun, based on a cut-off magnetron diode in which the cathode is a truncated cone, forms a hollow electron beam in the axial direction for injection into a longitudinal magnetic field.

  • 12 - Gridded Tubes

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 433-465

  • Summary

    Gridded tubes use a control grid close to the cathode to modulate the electron current. In triodes and tetrodes the modulated current is collected by the anode and passes through the output resonant circuit. The amplification is class A if current flows at all times, class B is it flows for half the r.f. cycle and class C if less than half a cycle. The gain is reduced and the efficiency increases as the conduction time decreases. The design and construction of triodes and tetrodes and of amplifiers incorporating them is discussed. In an inductive output tube (IOT) a bunched electron beam is formed by a gridded electron gun. The bunched beam is passed through a gap in a cavity resonator which extracts r.f. power from the bunches. The design of IOTs and examples of their application are discussed.

  • 10 - Electron Collectors and Cooling

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 352-374

  • Summary

    The spent electron beam in an electron tube carries power which must be dissipated safely as heat. In gridded tubes, magnetrons and most CFAs the power is dissipated on the anode. In linear-beam tubes and gyrotrons the beam is allowed to expand to give a reduced power density on a separate collector electrode. In every case the surface temperature must be low enough to avoid physical damage or out-gassing of the collector surface. The collecting electrode may be cooled by conduction, air or liquid flow or liquid boiling. The power dissipated in the collector of a linear-beam tube can be reduced by using a multi-element depressed collector in which the collecting electrodes are held at negative potentials with respect to the body of the tube. In these collectors it is necessary to ensure that the performance is not reduced by the emission of secondary electrons. The design of collectors is discussed including the effect of the distribution of velocities in the spent electron beam.

  • 4 - Slow-Wave Structures

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 134-189

  • Summary

    Wide bandwidth vacuum tubes employ the interaction between a travelling electromagnetic wave and a stream of electrons. The electromagnetic structures must support electromagnetic waves whose phase velocity is less than the velocity of light. The essential properties of uniform slow-wave structures are derived from Maxwell’s equations. Virtually all practical slow-wave structures are periodic in space and their properties can be investigated using equivalent circuits. The r.f. electric field of the structure which interacts with the electrons can be expanded as a set of space harmonics using Fourier analysis. The properties of planar slow-wave structures in the form of meander and ladder lines can be explored using equivalent circuits whose parameters have been calculated from static field solutions. The properties of helix slow-wave structures can be understood using solutions of Maxwell’s equations but it is difficult to get accurate numerical results in this way. A simple alternative, which yields useful results, is to use equivalent circuit analysis similar to that for planar structures. This method can also be applied to the ring-bar structure. High power structures based on folded waveguides and coupled cavities can be modelled using equivalent circuits. Methods of measuring the properties of slow-wave structures are discussed.

  • 14 - Travelling-Wave Tubes

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 507-564

  • Summary

    In a travelling-wave tube amplifier (TWT) a linear electron beam interacts with the longitudinal r.f. electric field of a slow-wave structure. Approximate synchronism between the electrons and the wave on the structure is maintained over a wide band of frequencies in low power tubes using helix slow-wave structures. High power tubes using coupled-cavity structures have narrower bandwidths. The structure is divided into two or more sections by severs to prevent feedback oscillations in high gain tubes. The initial velocity of the electrons is slightly greater than the phase velocity of the wave on the structure. The mean velocity of the electrons decreases as some of their kinetic energy is transferred to the wave and the beam becomes bunched. The power output saturates when the velocity of the bunches is equal to the velocity of the wave. The saturated output power can be increased by tapering the output end structure to reduce its phase velocity. Small- and large-signal modelling gives understanding of the principles of operation of a TWT including the factors affecting efficiency. The design of TWTs for octave bandwidth, multi-octave bandwidth, high efficiency, millimetre wave, and high power operation is considered in detail. Hybrid tubes are considered briefly.

  • 18 - Emission and Breakdown Phenomena

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 694-734

  • Summary

    Electron and X-ray emission and voltage breakdown caused by static, pulsed, and radio-frequency electric fields are important for the operation of vacuum tubes and also set limits to their performance. The physical principles of thermionic, field enhance, field, photo-electric and secondary electron emission from solid surfaces and the generation of X-rays are reviewed. The construction and properties of the principal types of thermionic cathode are described together with a brief review of cold cathodes employing field emission. Voltage breakdown may occur in vacuum tubes both inside and outside the vacuum envelope and through insulators. The physics of each type of electrostatic breakdown are reviewed. The theory of r.f. vacuum breakdown (multipactor) is discussed in detail both with and without the presence of a static magnetic field revealing the conditions under which a discharge may occur.

  • 3 - Resonators

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 93-133

  • Summary

    Cavity resonators form an essential part of the r.f. structure in many vacuum tubes. A resonator provides a strong r.f. electric field which can interact with a stream of electrons. The basic theory of resonators and coupled resonators is reviewed using a lumped element model and the description of a resonator in terms of its frequency, shunt impedance and R/Q is explained. The effects of external loading are considered together with the excitation of a resonator by an external source. At microwave frequencies cavity resonators are used in place of lumped-element circuits. The calculation of the properties of circular and rectangular cavities is described including the effects of surface roughness on the shunt impedance. Re-entrant cavities are important in microwave tubes because they provide a strong r.f. electric field in a short interaction gap. Their properties can be calculated approximately using an equivalent circuit and, with high precision, using the method of moments. An external transmission line can be coupled to a cavity resonator using either loop or iris coupling. The calculation of the properties of both methods of coupling, including large irises, is described. The methods of measuring the properties of cavity resonators are discussed.

  • 8 - Electron Flow in Crossed Fields

  • Richard G. Carter, Lancaster University
  • Book:
    Microwave and RF Vacuum Electronic Power Sources
    Published online:
    27 April 2018
    Print publication:
    12 April 2018, pp 287-316

  • Summary

    Uniform electron flow in the presence of space-charge can be obtained by using static electric and magnetic fields which are perpendicular to each other and to the direction of electron motion. The electrodes providing the electric field may be parallel planes or concentric cylinders and electrons may be emitted from the whole of the surface of the negative electrode. If the magnetic field is increased from zero a magnetron diode conducts initially but becomes cut-off above a certain field strength. Theoretical steady state solutions for the electron trajectories can be found in both cases. Alternatively a sheet electron beam may be injected into the space between the electrodes. The physics of magnetron diodes has been the subject of much debate and is difficult to simulate because of the effects of secondary electron emission from the cathode caused by back-bombardment. The experimental evidence of the behaviour of cut-off magnetron diodes shows that a steady state solution does not exist. It appears that the electron cloud is subject to collective oscillations which enable electrons to reach the anode in contradiction to simple theory.

Page 67 of 254