Velocity of neutral atoms emanating from the cathode of a steady-state low-pressure mercury arc

Abstract

The velocity of the neutral atoms emanating from the cathode of a steady-state low-pressure mercury-arc discharge has been studied by a gravimetric-calorimetric method. The results can be interpreted in a consistent way by attributing a superthermal rms velocity of (7.5±0.6)×103 m sec−1 to a first neutral flux component arising from cathode-spot action (and having a particle flux proportional to the arc current), and a thermal rms velocity of (2–10)×102 m sec−1 to a second neutral flux component comprising particles evaporated from the spot-free mercury surface. A model is proposed and discussed which explains the origin of the fast neutrals: It is suggested that the primary particles emerging in connection with the cathode spots are essentially all ions which have gained superthermal velocities in the vicinity of the cathode spots, and that a fraction of these ions subsequently becomes neutralized retaining most or all of its energy while, on the other hand, the primary particles evaporated from the spot-free mercury surface are all neutrals. It is shown that charge-exchange collisions were sufficiently frequent under the employed experimental conditions to account for an equilibrium between the various flux components; the existence of such an equilibrium is borne out by the experiments. While the experimental method employed did not permit a measurement of the velocity distribution, it did provide sufficient evidence to conclude that the stated neutral velocities are characteristic for the two neutral fluxes in the immediate vicinity of the cathode, and that the degree of velocity equilibration between them at distances on the order of a centimeter from the cathode should depend on various experimental parameters. The two limiting cases are discussed: that either complete velocity equilibration takes place between the two neutral flux components, or none at all.