To many of us, perhaps the primary introduction to vacuum tubes dates back to the childhood stage when peeping through a radio receiver from the pierced shield behind used to be greatly exciting, owing to the existing lights inside! Another note of attraction used to be the magic eye in many of those receivers indicating the level of tuning (of the receiver) with relay stations. Malfunctioning of those massive radio receivers generally used to happen due to the failure of operation of some vacuum tubes inside the cabinet – this much only we could grasp during those childhood days, upon gifted (with cautionary notes) with a few such non-functioning tube units to play with!
Finding the operational features of the primitive version of those thermionic vacuum tubes in higher school level physics books generated great interest in the subject. Nevertheless, it was that time a general (or perhaps conceptually wrong!) thought that vacuum tubes may not survive for long, so far as the applications are concerned, due the rapid replacement of giant tube-based radio receivers with the much smaller solid-state-based ones. However, physics courses in further higher studies essentially involved varieties of application-oriented vacuum tubes working in certain regions of frequencies.
Apart from just the radio receivers, various forms of vacuum tubes were of great demand for specific applications. Among those, the most easily understandable usage would be the cathode ray tubes in television receivers. Some other forms of vacuum tubes were vital in developing voltage regulators and sophisticated switching systems in electronic computers. Some miniaturized specific versions of vacuum tubes were commonly used in digital calculators and other switching-based instruments (for example, multi-vibrators).
Quoting some of the still-existing applications of vacuum tubes, the use of X-rays remains indispensable in medical sciences as well as characterizing samples in scientific investigations; indeed, highly specialized vacuum tubes are used to devise such instruments. Nuclear physicists use high-power radio wave tubes in particle accelerators. Household microwave ovens exploit the features of high-power magnetrons. Klystrons and traveling wave tubes are indispensable in military applications, because the amount of microwave power (about hundreds of watts) generated by these are not achievable by the use of solid-state devices. Certain high-power vacuum tubes, used in defense systems, can generate signals in the range of 10–100 MW power.
The aforementioned examples essentially reflect, though superficially, the importance of vacuum tubes, or the microwave tubes in particular, in the present day societal needs. The background of current research scenario and the R&D thrust in the area gave us the illuminating thought to bring out a Special Issue of the Journal of Electromagnetic Waves and Applications (JEMWA) – Microwave Tubes and Applications – dedicated to the recent research advancements. We invited (Retd.) Prof. B. N. Basu, who used to lead the Center of Research in Microwave Tubes at the Institute of Technology (presently the Indian Institute of Technology, Banaras Hindu University, India), to guest-edit the Issue. Prof. Basu has been a highly acclaimed researcher who contributed significantly in the developments of microwave tubes, and won several awards on his credentials. Also, his being one of the esteemed members of the editorial board of JEMWA gave us the best choice to invite him as the Guest Editor. Dr S. K. Datta, a leading scientist in the Defense Research and Development Organization (India), actively joined Prof. B. N. Basu through helping in various stages to realize the Special Issue. Dr Datta’s research focus has also been on microwave tubes.
This Special Issue incorporates papers contributed by leading researchers in the area. Some of the included papers are of the pedagogical type, whereas some are highly technical. The Editors-in-Chief are hopeful that the readers will find the Issue interesting and useful in making the relevant R&D community abreast of the recent developments in microwave tubes research. Also, novice researchers would possibly find the materials important in formulating their own research problems.
Joint Editors-in-Chief