Interaction between an electron beam and the ionized medium it creates

Experimental study of the plasma type oscillations excited by the interaction of an electron beam and the ionized medium which it creates. An electron beam with Pierce focus enters an ionization chamber at pressures ranging from 0.000002 to 0.001 torr. Electron energy varies from 500 to 4000 eV and beam intensity ranges from several milliamperes to 100 mA. As pressure is increased, a drastic jump in ion current yield takes place at 0.00016 torr, while if pressure is then decreased from high values, an analogous drastic drop is observed, but displaced below 0.00016 torr. The region between these two sharp changes in ion current yield is the site of oscillations on the order of megahertz. The frequency is proportional to the square root of the intensity of the electron beam and inversely proportional to the square root of the atomic mass of the ionized gas. The oscillations are interpreted as being due to the interaction of the ionizing beam and the reflected beam.