Cathode emission was studied at over 1 atmosphere pressure into He and A plasmas at 100‐ to 500‐A total current. The cathode spot temperature and current density were measured by a high speed photometric method developed for this purpose. A low vacuum thermionic work function was found to be a most important criterion for long cathode life. Emission current densities of the order of 100 to 200 A/mm2were found to be independent of total current; surface temperature, typically, increased 300°C for the 5‐fold increase in current and was approximately 2500°C in argon for a thoriated W cathode. Calcium‐barium‐aluminate‐impregnated W cathodes operated well in such an arc, remaining at very low temperatures, confirming the results obtained with the thoriated W.