Electrical Breakdown in High Vacuum

TLDR

Field magnification depends on electrode separation below 4 × 10⁻⁴ cm, and explaining low‑voltage breakdown requires an unusually high electron yield per ion in the gap. The high electron yield is explained by positive ion space charge that boosts field emission, raising ion current density until breakdown occurs. Measurements show that field‑emission currents, following Fowler–Nordheim behavior, precede breakdown, evaporate anode metal, and increase by only 65 % at breakdown, with ion current density far lower than electron current density and no direct ionization enhancement.

Abstract

Currents preceding breakdown have been measured between closely spaced tungsten electrodes in high vacuum. It is found that field emission currents sufficient to evaporate anode metal flow before breakdown. These currents follow the Fowler-Nordheim equation when field magnification due to surface irregularities on the cathode is taken into account. The field magnification is a function of distance at electrode separations less than 4×10−4 cm. Explanation of the observed breakdown at low voltage and small spacing requires an unusually high yield of electrons at the cathode per ion formed in the gap. Furthermore there is no measurable direct enhancement of the current by ionization even at higher voltages. The high electron yield must therefore exist over the entire observed range of breakdown voltages. This high yield is satisfactorily accounted for by the increase in field emission due to the positive ion space charge, which in turn increases the positive ion current density until there is breakdown. It is shown that breakdown occurs when the field emission current is increased by only 65 percent. This condition is reached with the ion current density much smaller than the electron current density.