Electron energy distributions in plasmas III. The cathode regions in helium, neon and argon

Abstract

The Druyvesteyn method of measuring electron energy distributions in low-pressure plasmas has been applied to the cathode region of hot cathode discharges in helium, neon and argon at a pressure of 0·05 mm Hg. In all gases the column was striated. In helium and neon, measurements in the striation nearest the cathode show two well separated groups of electrons, the more energetic or primary electrons arising from the Langmuir double space-charge sheath at the filament; the lower-energy group or secondary electrons being produced by the primary electrons in inelastic collisions. The concentrations of both groups fall off with increasing distance from the filament, but their separation and energy spread is constant throughout the striation indicating the absence of any energy exchanging process capable of appreciably modifying the energy distribution. It would appear therefore that the energy-exchanging processes observed by Langmuir (1925), by Merrill & Webb (1939), and by Gabor, Ash & Dracott (1955) were inoperative in the discharges employed in the present experiments. In the absence of an energy-exchanging process a uniform column cannot arise at the pressures employed, since in order to give rise to a truly uniform column the axial field must extend over many mean free paths, a condition which is not fulfilled in these discharges, since the entire discharge tube is less than 100 mean free paths long and thus on the average an electron makes fewer than 1000 collisions between cathode and anode. The application of an axial field simply augments the energy of the electrons without modifying the broad features of the energy distribution. If therefore the energy distribution consists of two separate groups of electrons a uniform field can maintain the discharge while primary electrons remain, but when these are exhausted, because of the distinct separation between the primary and secondary groups, new primary electrons can only be created by a step of potential. Under these conditions the striated column will exist and a transition to a stable uniform column is only to be expected when a mechanism exists for modifying the energy distribution in a few mean free paths. Measurements at higher currents in helium and neon show a definite tendency for the trough between the two groups to be eliminated; this is presumably due to secondary processes. The presence of a high metastable concentration enables both primary and secondary groups to be modified by secondary collisions thus eliminating the sharp demarcation between the two groups which was evident at lower currents. It seems possible also that the disposition of the first critical potential in a gas may influence its ability to develop a uniform column, since if the first excitation potential is appreciably less than half the ionization potential the primary and secondary groups will overlap. This might be effective in the metal vapours. The probable influence of impurities on the appearance of the discharge and the energy distribution are considered on the basis of the data available from measurements in hydrogen, helium and neon. Measurements in the negative regions in argon, while showing features characteristic of the striated column, were quite different in character from the other rare gases in that a fairly strong but decreasing field existed in the tube. Measurements here show very clearly how a uniform field augments the energy of the electrons without modifying the broad features of the distribution.