Cathode Technologies for Terahertz Source

There is a requirement of free electrons in any kind of vacuum electron device. The source of free electrons is called the cathode, which generates the required amount of electron emission when external energy in the form of either high field or thermal energy is supplied. The term "cathode" is the Latinized form of the Greek word kathodos, which means "way down" or "descent" and refers to the setting sun. The name was suggested by William Whewell in 1834, who had been consulted by English chemist and physicist Michael Faraday while writing his research paper related to electrolysis where free electrons move through liquid. The invention of cathode rays by Julius Plücker and Johann Wilhelm Hittorf and a demonstration by Arthur Schuster in 1890 prove an electron can be moved and can be deflected under vacuum by a proper application of the electric field. The discovery of X-ray tubes in 1895 by the German physicist Wilhelm Conrad Röntgen ignited the research on cathode technology. Later on, the demand for cathode technology was enhanced drastically after the invention of various kinds of high-power high-frequency vacuum electron devices. This chapter is devoted to various cathode technologies aimed toward making vacuum electron sources, particularly for vacuum electron terahertz devices. Before going into the technologies involved, the basic emission physics and the classification of cathodes are presented for a better understanding and to get an overview.