Donor Concentration Changes in Oxide-Coated Cathodes Resulting from Changes in Electric Field

Large reproducible changes in work function of oxide cathodes have been observed following step-like changes in dc current drawn from, as well as to, the coating. One type, with a time constant in the range of 10−2 to 1 second, has been reported by others. It appears to be caused by a redistribution of donors by the field set up in the coating. We studied this phenomenon further as a function of temperature, dc current density, coating composition, and barium deposition. We also discovered a second type of transient which follows the first. It has a time constant, in the range from 100 to 104 seconds. The amplitude of this change is of the same order as that of the first. The time dependence is in most cases given by a simple exponential. Both amplitude and time constant are a function of the cathode itself as well as its environment. This transient is thought to be caused by a change of the total number of donors. A simple analysis accounting for the exponential form yields values for the donor loss rate.