An evacuated metal tube in which electrons pass through a series of small gaps (usually)
in the form of cavity resonators in the high frequency range) so arranged and spaced
that, at a specific excitation frequency, the stream of electrons on passing through
successive gaps gains additional energy from the electric field in each gap.
Source:
PAC, 1982, 54, 1533
(Glossary of terms used in nuclear analytical chemistry (Provisional))
on page 1545