Experimental investigation of electrodes for high pressure discharge lamps

To support modelling of the electrodes in high intensity discharge lamps a model lamp was realized for precise caloric and electrical measurements at the electrodes. In this model lamp the properties of electrodes made of pure and modified tungsten were investigated in a current range between 1-10 A in dependence on the electrode geometry. The electrodes were operated in Ar or Xe in a pressure range between 0.1-1 MPa. By a determination of the radiation power emitted by the electrode from spatially resolved temperature measurements and by caloric measurements of the power removed by heat conduction the power fed from the arc into the electrode was determined. From these measurements the cathode fall was evaluated assuming that the electrical power invested into the cathode sheath is mainly transfered to the cathode by the ion current. The cathode tails are found to he in agreement with values which are determined by probe measurements. It confirms the power transfer to the cathode by the ion current. The outcomes are separate voltage current characteristics of the cathode and anode falls in dependence on the electrode properties and operating conditions. Complications result from two modes of current transfer at the cathode: a spot mode of high current density and a diffuse mode of low current density but with a higher cathode fall. It is shown that the spot mode is favoured by a low current, a strong cooling of the cathode and a high gas pressure, the diffuse mode by the opposite conditions. In a transition region the cathode changes statistically under the influence of the surface structure between the two modes.