Glow-to-arc transition in mercury-free HID lamps: Cathode phenomena and salt evaporation model

Summary form only given. The time scale for development of high-intensity discharge (HID) plasmas ranges from 100s ns at the breakdown phase to 10s ms in the arc regime, and to seconds for fully quasi-steady state operation. The breakdown phase is dominated by space charge effects, while the high density plasma in the arc regime is essentially quasi-neutral where ambipolar transport dominates. Coincident to the formation of the arc, heating of the dose of metal halides such as NaI, ScI₃ and ZnI₂, adds their vapor to the system which also adds additional complexity to the startup sequence of the lamp.In this talk we report on the results from a computational investigation of the smooth transition from breakdown to arc to quasi-steady state operation of an HID lamp having a metal halide dose. The model used in this work, nonPDPSIM, is a plasma hydrodynamics model in which continuity, momentum and energy equations are solved for charged and neutral spe cies with solution of Poisson´s equation for the electric potential. Algorithms were developed to represent the transition of the lamp from a Poisson regime during breakdown to an am bipolar regime as the arc begins to form. Heating of the lamp, evaporation of the salts, and the production of metal-halide vapors are included in the model. We directly model the heating of deposits of salts and, based on their saturated vapor pressure, allow diffusion of vapor from the salts into the volume of the lamp. To facilitate these calculations, a thermodynamic data base was developed for Nal, Scl₃ and Znl₂. Results from the simulation will be discussed for break down of Xe filled Hg-free HID lamps with doses of mixtures of Nal, Scl₃ and Znl₂. The breakdown phase is characterized by the time rate of change of applied voltage, dV/dt. The on set of the arc-mode depends critically on the thermionic emis sion properties of the cathode. We found that in the experimental- y observed time for arc-formation, the uniform heating of the cathode is not sufficient to produce a high enough current to heat the lamp and sustain the current. As a result, we allow non-uniform cathode properties, as might occur in thoriated-tungsten electrodes, that preferentially heat small regions of the cathode.