Vacuum electronics at the dawn of the twenty-first century

In this introduction to the Special Issue on vacuum electronics, the history, operating principles, and recent technological trends of vacuum electronic devices are reviewed. The development of microwave power tubes is described and improvements in these devices over the past quarter of a century are highlighted. These improvements have been substantial and have been driven by modern high-power applications and by advances in materials and computational science. The second part of this paper describes the advent of a new class of vacuum electronics generator as involving relativistic electron beams and interaction with fast waves (e.g., gyrotrons, free-electron lasers). These new types of generators are opening the electromagnetic spectrum beyond the microwave region (i.e., millimeter-wave, infrared, ultraviolet, and even X-ray) for applications of high-power, coherent generators of electromagnetic radiation.