Discovery of electrostatic sound wave modes in a plasma in klystron oscillators

During tests of two-cavity klystron CW oscillators which are spacecharge focused, unusual FM sidebands were discovered. These modulation sidebands could only be detected with a sensitive discriminator and were found to result from video frequency oscillations. The video signals have been shown to be electrostatic sound wave modes of a beam-generated plasma and are similar to those investigated by Crawford (1961). Although they were discovered in the RF spectrum, they were also observable in the collector current.