Abstract

This chapter contains new simulation results concerning the physical foundations of how microwave tubes operate based on Cherenkov's mechanism of radiation (interaction with slow electromagnetic wave) and some experimental results connected with improving the output characteristics of the magnetrons (a mm band surface wave magnetron and a magnetron with two RF outputs of energy), as well as results of computer modeling of a 320-GHz band traveling wave tube (TWT). The results of analytical calculations and computer modeling, a phase bunching process in the mm band surface wave magnetron, are considered. It is shown that the process of phase focusing has two features associated with a concentration of RF wave energy close to the vanes of an anode block and higher electron hub of a space charge as compared to the classical magnetrons. The features and examples of practical application of the magnetron with two RF outputs of energy are presented. It is shown that the main advantage of the magnetrons is its extended functionalities (for example, possibility of frequency tuning including electronic tuning of a frequency from a pulse to pulse). The presented materials will be of interest not only for starting researchers but also for those who have microwave tube experience.

Keywords: Cherenkov's radiation, electromagnetic field, electron beam, magnetron, TWT, frequency tuning, millimeter range, terahertz range

### 1. Introduction

The applications of electromagnetic fields are of great importance in such areas as radar and navigation, communications, information and communication technologies, industry and agriculture, medicine, etc.

It is the interaction of moving charged particles (for example, electrons) with the electromagnetic field that is the cornerstone of electromagnetic phenomena. It is known that the electron moving rectilinearly and evenly with constant speed does not radiate. Moreover, the electromagnetic field, which exists around particle, moves together with particle at the same speed, and its properties remain invariable. If the nature of movement of a particle and/or its speed changes, for example, the trajectory of movement becomes curvilinear or the electron begins to move unevenly (turns to be accelerated or slowed down), the state of its own electromagnetic field also changes. As a result, there arises a free electromagnetic field, i.e., electromagnetic radiation (EMR), which has wave nature and freely advances in the environment in the form of electromagnetic wave. Depending on the existing conditions of its propagation (or accumulation of electromagnetic energy), which are

determined by the properties of the medium (for example, features of the spatial configuration of periodic waveguide and resonant electrodynamic structures) as well as the character of movement trajectories of electrons in an electron beam, there might exist the EMR different types.

The following types of radiation are known: Cherenkov's radiation (interaction with slowed-down waves), transient and diffraction radiations (or Smith-Pascrell's radiation as applied to optical band), and bremsstrahlung and magnetodeceleration radiation (while having a magnetic field), as well as its versions: synchrotron or undulator radiation (for a relativistic case) and cyclotron radiation (for a case of the movement of nonrelativistic particles) [1–7]. The analysis reveals that various mechanisms of the radiations have both common and specific features of their occurrence. It is also necessary to note that the existence of one dominating type of radiation may result in the interference of several types of radiation under certain conditions [8]. As a result, the output parameters of a microwave source will depend on the fact how efficiently the conditions of electron interaction will be correlated with an electromagnetic field from the viewpoint of conversion efficiency of the energy, being reserved in an electronic flow, into electromagnetic energy, and how fully and correctly the requirements to the devices distributing and accumulating an electromagnetic field (periodic and resonator electrodynamic structures) are formulated.

Studying the features of physics of the electromagnetic radiations has led to producing various microwave vacuum tubes in the wide range of frequencies [9]. Considerable recent attention has been focused on creating microwave sources in a short-wave part of millimeter (0.1–0.3 THz) and terahertz (0.3–3.0 THz) ranges. It is known that the absolute advantage of terahertz range is the broad band of frequencies, as well as the ability to penetrate through opaque media, including metals, organic materials, etc. This property positively distinguishes it from ionizing radiation (for example, X-rays) while opening wide perspectives to diagnose a variety of diseases in medicine [10].

The scheme of practical mastering of a short-wave part of a spectrum of electromagnetic oscillations is shown in Figure 1.

It is evident nowadays that the lack of the microwave vacuum devices with continuous power output from a few watts to tens and more watts in this part of the electromagnetic spectrum restricts the opportunities of further development and improvement of technologies in such areas as spectroscopy, radio astronomy, space, and biochemical research, as well as producing a new generation of information and communication systems. Besides the development of this frequency range will allow intensifying safety control (search and detection of explosives, remote identification of chemical substances) to exercise production quality control of finished goods (checking packages of medical products, etc.). The application of relativistic tubes (for example, the relativistic magnetrons or the fast-wave devices like free-electron lasers or gyrotrons) for solving the above-mentioned

#### Figure 1.

The scale of electromagnetic waves and areas of their application.

Vacuum Microwave Sources of Electromagnetic Radiation DOI: http://dx.doi.org/10.5772/intechopen.83734

problems allows providing the needed levels of output power in this range. However, these devices possess large mass-dimensional characteristics and require for operating the high values of voltages and strong magnetic fields. In this regard, producing effective and compact microwave sources of electromagnetic oscillations in this frequency range seems to be much more attractive with the help of miniaturizing the constructions of the classical microwave tubes possessing Cherenkov's radiation mechanism.

The present chapter deals with vistas of developing the microwave sources of electromagnetic oscillations (the magnetron and the O-type TWT) whose operation is based on the interaction of an electron beam with the slowed-down electromagnetic wave of electrodynamic structure (νph , c, where νph is the phase velocity of electromagnetic wave and c is the light speed), i.e., there exists Cherenkov's radiation mechanism.
