travelling wave tube

Short definition from Wiktionary

Noun

1. 1
   Alternative spelling of travelling-wave tube.
2. 2
   A type of vacuum tube which amplifies electromagnetic signals, where the electron gun fires down the tube with an unmodulated electron beam, that passes through an interaction array, where the EM-signal is fed, which interacts with the electron beam, bunching the electrons, and transferring power to the signal.