ABSTRACT

This chapter introduces compact, high-power terahertz (THz), coherent radiation sources based on vacuum electronic devices (VEDs). The types and issues in THz VEDs are briefly explained. For THz VEDs, the technical issues and developments in the backward wave oscillator (BWO), klystron, traveling wave tube (TWT), gyrotron, spatial-harmonic magnetron (SHM), and free-electron laser (FEL) are presented. The electron beam sources for THz VEDs are also described. 8.1 Introduction

8.1.1 THz Radiation Using Vacuum Electron BeamsThere have been considerable demands on compact, high-power, coherent radiation sources up to the THz frequency range due

to their unique physical properties for various applications. THz radiation sources have been developed using electronic and photonic technologies such as vacuum electronic devices (VEDs), solid-state devices, and quantum cascade lasers. However, the development of more compact, low-cost, and high-power THz sources is still required for applications in the practical field. THz VEDs are one of the promising candidates because of their efficient interaction mechanism between an electron beam and an electromagnetic wave in vacuum. VEDs were widely used as a high-power radiation source for communication, various heating systems, diagnostics, and radar systems [1-7]. The origin of energy in coherent electromagnetic wave is from the conversion of kinetic or potential energy of linear or rotating electron beams. In principle, a charged particle that encounters a change of velocity radiates photons. Almost every coherent radiation using free electrons can be categorized into three types by the synchronization condition between an electron beam and an electromagnetic wave. These are Cerenkov radiation, Bremsstrahlung radiation, and scattering, as shown in Fig. 8.1.