Ignition of a Thermionic Cathode Arc

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Abstract
Ignition of a thermionic cathode discharge in mercury vapor at the low applied potential of 2.4 V is shown to be the result of a potential minimum in space 0.8 V below the cathode, and of a favorable contact potential difference. Good emitters require more voltage to fire than high work function cathodes. Using an argon buffer gas at about 3.5 mm Hg pressure, ignition is easiest with an added Hg vapor pressure of 1.85 mm. Adequate pressure is required because of the two‐stage ionization which produces ignition. This process applies also to the noble gases, and requires that ignition cannot occur with electron energy less than half the ionization potential.