Low-voltage mercury-alkali metal arc

Measurements of the arc voltage and electron temperature were made on low-pressure arcs utilizing a mercury-pool cathode containing various concentrations of alkali metals. The arc-burning voltage was substantially lower than the pure mercury arc and the minimum arc voltage depended on the concentration of the alkali metal and ambient tube temperature. The measurements suggest that there are three processes which collectively are responsible for the arc behavior. These are a lower anode work function, a Penning two-stage ionization process of alkali metal atoms by mercury metastables and an improved electron emission by reliable anchoring of the cathode spot. With the addition of small amounts of alkali metal the cathode spot emission zone was stable on a molybdenum anchor at currents as high as 200 a. The cathode fall potential was the order of 4 v. It is shown that the voltage reduction is mainly in the cathode fall region and that it is lower for rubidium than cesium, potassium or sodium in the preceding order.