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Principles and capabilities of high-power microwave generators.

A number of basic principles apply to almost all high-power microwave tubes. These principles not only govern and constrain the operation of the devices, but also provide a means for classifying them.

ELECTRON RADIATION AND CLASSES OF MICROWAVE TUBES

The operation of microwave tubes is based on coherent radiation of electromagnetic waves by electron beams. Such radiation can be treated as a result of the self-organization of the electron's individual (spontaneous) emission. Initially, electrons radiate electromagnetic waves individually with random phases. Then, in the process of spontaneous emission, if the parameters of a device are chosen properly, electrons can be gathered in microbunches capable of radiating in phase. This effect leads to the production of coherent radiation.

There are three basic kinds of electromagnetic radiation by charged particles: Cherenkov or Smith-Purcell radiation, transition radiation and bremsstrahlung radiation.

Cherenkov radiation occurs when electrons move in a medium with a speed greater than the phase speed of electromagnetic waves in the medium. The radiation of electrons passing through or near periodic structures, known as Smith-Purcell radiation, may also be understood as a form of Cherenkov radiation. Microwave tubes based on this kind of radiation, including TWTs, BWOs, orotrons and others, are often termed Cherenkov devices.

Transition radiation occurs when electrons pass through the boundary between two media with different refractive indexes or pass-through perturbations in a medium such as conducting grids, plates or gaps between conducting surfaces. Such gaps or cavities in which the microwave field is localized are used in klystrons and monotrons, the devices based on coherent transition radiation.

Since the microwave field in such cavities is axially localized, this field may also be represented as a superposition of electromagnetic waves propagating with different axial wavenumbers.

Bremsstrahlung radiation occurs when electrons move with a varying velocity in external electric and/or magnetic fields. Typically, this motion is oscillatory. The electrons then radiate waves whose Doppler-shifted frequencies coincide with the frequency of the electrons' oscillations or one of their harmonics. These waves can be either fast or slow. However, in order to supply powerful radiation in the millimeter and submillimeter range of wavelengths, it is preferable to radiate fast waves, which need not be localized near the walls of a structure or dielectric material.

The family of microwave sources based on coherent bremsstrahlung includes many devices in which electron oscillations may be induced in either constant or periodic fields. The most common tubes based on radiation of electrons gyrating in a constant magnetic field are cyclotron resonance masers (CRMs). Similarly, the most common tubes based on the radiation from electrons oscillating in periodic external fields are free electron lasers (FELs). Finally, there are tubes such as vircators, orbitrons, strophotrons and others in which electrons radiate while oscillating in constant static electric fields.

COHERENT CHERENKOV AND TRANSITION SOURCES

A) O-Type Cherenkov Devices: In O-type Cherenkov devices, the electrons move along linear trajectories with, in the absence of radiation, constant velocity. They can radiate moving through or past dielectrics or in periodic slow-wave structures. The latter case is more appropriate for high-power microwave generation because restrictions caused by heating and breakdown problems are not so severe.

B) Klystrons: Klystrons are the most familiar microwave devices based on coherent transition radiation of electrons. In general, transition radiation may occur when electrons pass through a boundary between two media with different refractive indexes or through perturbations in the medium such as conducting grids and plates. In RF tubes, these perturbations are grids. In klystrons, they are short gaps formed by pairs of grids or some other elements, like cavities, where the microwave field is localized. A typical klystron configuration consists of two or more cavities. All cavities except the last one are used to properly form electron bunches from an initially steady electron flow. The last cavity is used to produce coherent transition radiation from the prepared electron bunches.

C) Klystron Alternatives: To avoid the principal restrictions considered above, numerous modifications of the klystron have been suggested. Basically, all of these modifications can be divided into two classes. The first has to do with overcoming the constraints caused by the limitation on electron transit time. The second class of klystron modifications consists of devices aimed at increasing the optimal perveance of the electron beams that allows for efficient, high-power operation. This class includes multibeam klystrons and klystrons in which the electron beam is extended in one transverse dimension (sheet and hollow annular beam configurations).

D) M-Type Devices: M-Type devices are microwave tubes in which electrons move in crossed external electric and magnetic fields. Most people are familiar with cylindrical and planar magnetrons. The family of M-type devices consists of a large number of magnetrons:

* Magnetrons with different geometry of the anode structure, such as hole-and-slot blocks, rising sun blocks, trapezoidal blocks with straps for better mode frequency selection;

* Coaxial magnetrons, in which regions of the anode structure are tightly coupled through slots with a single high-Q resonator surrounding the anode block. Such devices operate more stably at high power levels than conventional magnetrons with the strap-vane structure;

* Inverted magnetrons (inverted Coaxial magnetrons are also used), in which the surface of the outer cathode and, correspondingly, the beam current can be much larger than in conventional magnetrons;

* Frequency-tunable (mechanically and electrically) magnetrons;

* Nigotrons, in which both the cathode and the anode contain resonator blocks that increase the strength of the alternating electric field near the cathode and improve mode selectivity of the device;

* Magnetron amplifiers.

SOURCES BASED ON BREMSSTRAHLUNG RADIATION

Among microwave tubes based on electron bremsstrahlung radiation in a constant magnetic field, the most common are CRMs or gyrodevices. Among microwave tubes with periodic external fields, the most common are FELs, previously known as ubitrons. Among microwave sources based on electron oscillations in a potential well formed by an electrostatic field, the most powerful are vircators.

A) Cyclotron Resonance Masers: In CRMs, the main effect leading to electron bunching and to coherent bremsstrahlung radiation is the relativistic dependence of electron cyclotron frequency on its energy. The instability resulting from this effect is known as the cyclotron maser instability. The most surprising feature of this phenomenon is the fact that this relativistic effect is important even for a weakly relativistic electron. Therefore, when electrons interact with an HF field with a small axial wavenumber (such a field can easily be excited in open resonators), all the electrons' kinetic energy may be withdrawn without significantly disturbing the cyclotron resonance.

B) Free-Electron Lasers: FELs are capable of producing powerful coherent radiation over a range extending from the microwave wavelengths to optical wavelengths. Today there are a large number of experimental devices driven by electron beams produced by a variety of accelerators spanning a range of energies from hundreds of kilovolts to hundreds of megavolts. Many of the basic features of these devices are the same.

C) Vircators: Vircators are microwave sources based on the bremsstrahlung radiation of relativistic electrons oscillating axially in electrostatic fields. Such oscillations in linear relativistic electron beams may be caused either by the formation of virtual cathodes (when an injected beam current exceeds the limiting current defined by space-charge effects) or by low-potential electrodes reflecting streaming particles. Electrons passing through an anode foil enter a resonator whose walls are at the anode potential. Here, due to voltage depression caused by space-charge effects, some electrons stop forming the virtual cathode at a certain distance from the anode. These particles then start to move back towards the anode and, as a result, oscillate in the region between real and virtual cathodes. The frequency of these oscillations defines the frequency of the microwave radiation produced in the cavity.

CONCLUSION

We have considered the basic principles of operation of high-power microwave sources. Further progress in the development of high-power microwave sources will depend on the solution of a number of physical and technical problems. Among these, the most important are the production of intense electron beams with a small spread in electron energies and velocities, as well as the development of highly selective oversized microwave structures (wave-guides and resonators). Impressive results have already been obtained on the way to a solution of these problems. Further steps in this direction may lead to significant enhancement in the power and frequency of microwave sources, as well as to improvement of the radiation coherency. This progress can make new sources of high-power microwaves attractive for novel applications.

Gregory S. Nusinovich and Thomas M. Antonsen, Jr., are affiliated with the University of Maryland, College Park, MD. Vladimir L. Bratman and Naum S. Ginzburg are affiliated with the Institute of Applied Physics, Russian Academy of Science, Nizhny Novgorod, Russia.
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Author:Nusinovich, Gregory S.; Antonsen, Thomas M., Jr.; Bratman, Vladimir L.; Ginzburg, Naum S.
Publication:Journal of Electronic Defense
Date:Jan 1, 1995
Words:1436
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