High-Voltage breakdown and conditioning of carbon and molybdenum electrodes

Carbon and molybdenum electrodes employed in high-voltage devices such as ion sources and traveling wave tubes can be easily damaged by electrical breakdown and arcing events. Modification of the electrode surfaces due to these events can impact the voltage hold-off capability of the surfaces, which could lead to additional arcing, further damage, and the potential for device failure. Power supplies driving these devices are usually rated for their damage potential by the amount of stored energy. However, many systems that use arc discharges characterize the lifetime of the device for voltage hold-off and the amount of material removed from the surfaces by the amount of current that passes through the arc, or the "Coulomb-transfer rating." The results of a series of tests that were performed on the voltage hold-off capability and damage to several different carbon and molybdenum surfaces due to induced arcing are presented. Damage to the surfaces was characterized by the field emission threshold after arcing events, by the amount of Coulomb-transfer in the arc, and by scanning electron microscope photographs of the subsequent surfaces. Both conditioning and damage to the surfaces were observed, and are related to the characteristics of the materials and the electrical breakdowns.