Observation and Analysis of Graphite Hollow Cathode after 45,000-Hour Life Test

Discharge characteristics during a 45,660 h life test and destructive analysis results after the life test of a discharge hollow cathode developed in the Japan Aerospace Exploration Agency (JAXA) are described. The discharge and keeper voltages had remained almost stable at about 30 and 8 V respectively throughout the life test and there were no gradual voltage increases due to cathode deterioration. The eroded depths of the keeper and orifice plate due to ion bombardment were approximately 0.2 mm, respectively. In addition, there was no chemical reaction between the graphite and insert materials. These results show that adopting graphite may be a good solution to solve the problem of erosion of the keeper disk and orifice plate. The insert inner surface was clean within 7 mm from the downstream edge, where many pores were found. Either no tungsten crystallite was deposited or no poisoning layer was formed. We think that this clean surface was one of the main reasons for the extended stable operation. The measured temperature at the downstream end of the insert under the discharge condition was 1,120 °C in the additional experiment using a similar cathode. In addition, a heat calculation showed the temperature varied between both ends of the insert inside was 77 K. This clean surface can be secured due to the low operating temperature and small temperature difference. Through a life test and destructive analyses, we confirmed that this hollow cathode had sufficient surplus life capability.