Pressure-induced Arc Transition and Optical Quenching

When the pressure of a molecular gas surrounding a carbon arc is reduced below a certain value a sudden contraction of the cathode spot is observed. This is interpreted as a change from the thermionic to the vapour cathode arc. A transition mechanism is suggested: on decreasing the molecular concentration of the ambient gas optical quenching is reduced and thus a decreasing number of molecules remove potential energy from excited vapour atoms. Hence an increasing number of excited atoms are available which transfer their potential energy to the electrons in the cathode substance, thereby releasing electrons from the cathode so that a vapour cathode spot is formed. A numerical estimate of the required quenching cross section (order 10 -15 cm 2 ) shows that it is in accord with the values found in optical quenching experiments. The implications of this approach in regard to tungsten, mercury and copper cathodes are discussed.