A microwave tube which uses the interaction between an electron beam and the RF energy on microwave cavities to provide signal amplification. The klystron operates on principles of velocity modulation very similar to those in a TWT except that klystron interaction takes place at discrete locations along the electron beam. Common types of klystrons are the reflex klystron (an oscillator having only one cavity), two-cavity klystron amplifiers and oscillators, and multi-cavity klystron amplifiers.