SCATTERING OF ELECTRONS IN IONIZED GASES

Abstract

Velocity distribution of electrons in low pressure discharges from hot cathodes.—Analysis of the current received by a collector placed opposite a hot cathode indicates that there are three groups of electrons present: (1) primary electrons which retain practically all the momentum they acquired in passing through the positive ion sheath around the cathode; (2) secondary electrons moving in random directions with a Maxwellian velocity distribution corresponding to a temperature roughly proportional to the energy of the primaries (200,000° for 100 volt primaries) and approximately independent of the nature or pressure of the gas or the current density; (3) ultimate electrons having a Maxwellian distribution of velocities corresponding to a much lower temperature than that of the secondaries, a temperature which is independent of the current density or the voltage of the primary electrons and which varies with the gas used and decreases slowly as the pressure is raised. The number of ultimate electrons is roughly 1000 times that of the primaries and secondaries for mercury vapor, although relatively less numerous in the case of hydrogen. In the uniform positive column of arcs only ultimate electrons are present.