An experimental investigation of the microwave radiation emitted by a high-current relativistic electron beam

The development of a high-frequency instability in a relativistic electron beam, propagating through a drift space, has been studied from measurements of the microwave emission from the beam. The results show that a part of the radiation is emitted from the region close to the beam front and that the intensity of this emission grows in time as it is convected along with the beam. The growth rate is of the order of 108s−1 and depends on the macroscopic flow parameters. Experiments in different gases show that the growth rates are approximately proportional to the ionization probability of the gas. Precursor radiation is also monitored at power levels comparable to those found at the beam front.Possible explanations of the radiation are proffered and the physical importance of the emission (∼100 kW/m length of drift tube) on beam neutralization and structure are indicated.