Experimental investigations on electron interaction

Abstract

Certain observations in low-pressure gas discharges and in magnetrons appear to indicate an electron interaction effect several orders of magnitude in excess of the theoretical values based on binary encounters only. As the Coulomb interaction in low-pressure plasmas and in electron clouds usually extends over a range which contains at least several thousand electrons, it was not clear whether these observations indicated a breakdown of the theory, or some new effects, other than electron interaction. New experiments were therefore designed to establish an upper limit for the magnitude of the electron interaction effect. Both plasmas in thermal equilibrium and pure electronic space charges were investigated. The basic experimental method in each case was shooting a very carefully collimated electron beam through the charge cloud, and measuring the scattering by direct observation on a fluorescent screen. The apparatus was designed to resolve angles down to 10$^{-4}$. The density of the equilibrium plasma was inferred from capacity measurements on a thin wire probe in the plasma. The density of the electron cloud was determined by measuring the electron-optical effects produced. It is concluded that both for the plasma as for the electron cloud the scattering is less than five times the value predicted by the theories neglecting simultaneous multiple collisions. The interaction is therefore so small that it cannot be expected to account for the observed anomalies either in low-pressure gas discharges or in other electronic devices.