Potential Drop and Ionization at Mercury Arc Cathode

Abstract

By means of a movable Langmuir collector, the potential, ion concentration and electron temperature were measured at various distances from a stationary mercury cathode spot, at various arc currents. The results indicated a cathode drop of 10.0 volts, and a small negative potential gradient beyond the fall space which was more pronounced at the larger currents. The ion concentration varied between 2${\mathrm{}\left(10\right)\mathrm{}}^{13}$ and 3${\mathrm{}\left(10\right)\mathrm{}}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$ for distances between 0.4 cm and 1.7 cm and arc currents between 11 amp and 4.2 amp. The concentrations evidently greatly exceed these values very close to the cathode. The mean electron energies were about 1.4 volts near the cathode and fell to a little less than a volt at the greater distances. From the lack of saturation of currents to the collector at space potential, the coefficient of electron reflection at the amalgamated tungsten collector surface was found to be close to 0.5. It is shown that the thickness of the cathode fall space must be less than 1.76${\mathrm{}\left(10\right)\mathrm{}}^{-4\mathrm{}}$ cm and that the field at the cathode surface must exceed 7.6${\mathrm{}\left(10\right)\mathrm{}}^4$ volts·${\mathrm{cm}}^{-1\mathrm{}}$.