Anode sputtering in high-voltage mercury arc valves

A method is described whereby the anode sputtering rate during recovery of a mercury arc valve can be estimated from its transient inverse current and voltage waveforms. Inverse currents have been measured in an experimental valve, from which corresponding anode sputtering rates are deduced for a variety of cathode coolant temperatures and commutation conditions. These measurements substantiate proposed methods of reducing anode sputtering in valves. The accuracy of the method has been confirmed by examining sputtered deposits found in an experimental valve after several hundred hours of rectifier operation. Major applications of this method are in the development and testing of valves for high-voltage direct current (hvdc) transmission. These include rapid assessment of the effect of design modifications on anode sputtering, and increasing the anode sputtering rate by a known factor through manipulation of the test conditions (e.g., cathode coolant temperature and dI/dt), thereby considerably shortening the time required for sputtering life tests. The latter application is particularly valuable in the case of single-gap valves where the effects of anode sputtering during recovery are likely to be the ultimate limitation on serviceable life. The saving in testing time is especially important because of the high operating cost of test plant at relevant levels, in our case 50 kV, 1000-ampere dc bridge rating.