Experimental study of the dipole instability of an electron beam in drift space

Experiments on electron-beam dipole instability with an azimuth mode m = 1 were performed at a gas pressure from 0.0006 to 0.01 torr, a beam current of 27 A, a beam electron energy of 300 keV, and a pulse duration up to 500 microsec. The observed instability in the developed stage has the form of a standing wave, with frequency varying with pressure and transport length in the 0.4-4 MHz range. It is shown that the mechanism for the formation of the instability may be connected with the asymmetric injection of the beam into the diaphragm of the beamguide, which leads to the appearance of a radial force associated with the induction of image currents.