Secondary Electron Emission by Photoelectric Action and Ion Bombardment at the Cathode in Corona Breakdown of Argon

Differences between the secondary cathode mechanisms active in coaxial cylindrical argon-filled counter tubes with positive central wire at various gas pressures observed by E. J. Lauer at Berkeley and the authors at Milan, have been resolved as a result of recent work by the authors at Berkeley. It appears that at pressures ranging from 1000 to about 150 mm Hg, with spectroscopically pure argon, the dominant mechanism is a photoelectric liberation from the cathode caused by photons with some microseconds of time delay and of energy about 10 ev. These lead to a corona threshold with γₚ ranging from 1 to 5×{}10^-3, uncorrected for electron back diffusion. At pressures between 25 and 100 mm Hg an electron liberation by impact of A⁺ and A₂⁺ ions on the Ni cathode was confirmed to be important in the corona current, in agreement with Lauer's findings. This effect seems to be very dependent on the conditions of the surface and was found to be active only after a degassing at 900^∘{}C for hours.