travelling-wave tube

noun
  1. an electronic tube in which an electron beam interacts with a distributed high-frequency magnetic field so that energy is transferred from the beam to the field
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012